1.5. Source coverage analysis with gnatcov coverage

Source coverage analysis computes metrics focused on source programming language entities such as high level statements or decisions (DO-178C parlance for boolean expressions), which translate as Source Coverage Obligations, or SCOs, in GNATcoverage terms.

Once you have produced coverage traces, actual analysis is performed with gnatcov coverage to generate coverage reports. Source coverage is queried by passing a specific --level argument to gnatcov coverage.

The general structure of this command line is always like:

gnatcov coverage --level=<criterion> --annotate=<format>
                 <units-of-interest> ... <traces-or-checkpoints>

The next sections describe the available report formats, then provide more details regarding Statement Coverage analysis, Decision Coverage analysis, and Modified Condition/Decision Coverage analysis.

The Specifying Units Of Interest chapter describes how application units to be considered for coverage assessment are to be specified.

1.5.1. gnatcov coverage command line

Coverage analysis with GNATcoverage is performed by invoking gnatcov coverage for a set of critera queried via the --level command line option. The general interface synopsis is available from gnatcov --help:

gnatcov coverage OPTIONS TRACE_FILES

The available options are as follows:

-c, --level (mandatory):

Tell the set of coverage criteria to be assessed. The possible values for source coverage analysis are stmt, stmt+decision, stmt+mcdc, and stmt+uc_mcdc, all explained later in this chapter.

-a, --annotate (mandatory):

Request one or more output report formats. All the criteria support cobertura, html,, report, xcov[+], xml formats, with interpretations that vary depending on the assessed criteria. See the corresponding documentation later in this chapter for more details. This option accepts comma separated values and/or can be specified multiple times on the command line, in which case there will be one report produced for each of the requested formats.

-o :

Request that the synthetic report produced by --annotate=report be output in the provided filname instead of standard output by default. This is just ignored for other output formats.

--timezone :

Select the timezone to use when displaying trace creation dates in reports, which could be either local or utc. The local timezone is used by default.

--output-dir :

Request that the report files (index and annotated sources for the xcov and html output formats) be output in the provided directory. If not specified, the default is the root project’s object directory if using projects, and the current directory if not. If more than one of the above annotation formats is requested, then each report will be placed in a subdirectory named accordingly.

--report-title :

Request that generated HTML documents (index for the html output format) are assigned a customized title. For instance, passing --report-title="Project ABC" will yield titles such as: Project ABC - GNATcoverage Report. If passed multiple times, passing an empty string last will restore the default behavior. This option is ignored is the selected output format does not support titles.

-T, --trace (mandatory), possibly repeated and accepting @listfile arguments :

Provide the set of execution traces for which a report is to be produced. When multiple traces are provided, gnatcov produces a consolidated result, as described in detail in the Coverage Consolidation chapter of this manual.

--exec:

Override executable from traces. Trace files contain an indication of the executable used to generate them. This option causes the named executable to be loaded for coverage analysis, and to override the indication contained in any trace specified after it on the command line. An empty executable name may be specified to restore the default behaviour of using the indication contained in each trace file. Note that --exec may appear last on the command line, in which case it applies to no trace file, but still causes the indicated executable to be included in the coverage analysis. This ensures that any code in that executable that is not exercised by some trace file will be reported as not covered.

-t, --target :

State the target toolchain configuration used to build the analyzed programs, as provided to gprbuild. For cross or 32bit native configurations, this switch together with its possible --RTS companion is required for all commands using project files unless the root project provides the information with Target/Runtime attributes.

--non-coverable:

Report about language statements for which no object code could be found in the surrounding subprogram (typically out of optimization).

-P:

Use the indicated project file as the root project to select the units of interest for the analysis and find default options. Default options are taken only from this project. In absence of --projects and of --no-subprojects, the units of interest are those designated by this project and all it’s transitive dependencies, minus those advertised as externally-built. The target/runtime/scenario contextual options that would be passed to build the project should also be passed for proper interpretation of the project files. See the Specifying Units Of Interest chapter of this manual for more details.

--projects, possibly repeated and accepting @listfile arguments:

When using -P, use the provided projects to select units of interest, together with their transitive dependencies unless --no-subprojects is also provided. The projects designated by this option must all be part of the import transitive closure reachable from the root project designated by -P.

--no-subprojects:

Consider only the projects (and units) encompassed by the -P / --projects options as of-interest. Don’t include any project imported from there.

--units, possibly repeated and accepting @listfile arguments:

When using project files, override the list of units of interest for source coverage with those provided.

--subdirs:

When using project files, look for Library Information files in the indicated subdirectory of each project’s object directory.

--scos, possibly repeated and accepting @listfile arguments:

Provide the set of Library Information files from which Source Coverage Obligations (SCOs) should be loaded to process binary traces. This low-level switch effectively overrides the project based units of interest selection. It only has effect if used with binary traces.

--sid, possibly repeated and accepting @listfile arguments:

Provide the set of Source Instrumentation Data files from which Source Coverage Obligations (SCOs) should be loaded to process source traces. This low level switch effectively overrides the project based units of interest selection. It only has effect if used with source traces.

--ignore-source-files, possibly repeated and accepting @listfile arguments:

Provide a list of globbing patterns (as in Unix shells) of source files to be excluded from the analysis and from the output report. See the Conveying source files to ignore / handling Ada subunits section for more information.

--dump-units-to:

For source coverage analysis specifically, output the names of units that are considered of-interest to the requested assessment, that is, for which a report or checkpoint is going to be produced. This also outputs, for each unit of interest, the list of files that were individually ignored using the Ignored_Source_Files project attribute or corresponding command-line option. Ignored source files listed with --dump-units-to will be either marked as always ignored, if they were ignored in all the inputs (traces or checkpoints) that were used to produce this report, or as sometimes ignored, if the source files were ignored in at least one of the inputs of this report, but not all of them. The argument may be either the name of a file, clobbered it if it already exists, or ‘-’ to request displaying the list on standard output. In the latter case, when a report output is also requested, the list of units is displayed as an additional report section.

--save-checkpoint:

Save the resulting coverage analysis to the named checkpoint file.

--checkpoint, possibly repeated and accepting @listfile arguments:

Load previously saved coverage analysis checkpoint(s), and continue coverage analysis from that initial state.

--source-rebase, possibly repeated and accepting @listfile arguments:

Specify alternate absolute path prefixes to locate source files when producing annotated source outputs. See Handling source relocation for annotated sources output formats

--source-search, possibly repeated and accepting @listfile arguments:

Specify a directory in which missing source files will be searched when producing annotated source outputs. See Handling source relocation for annotated sources output formats

A lot of options are available to control the set of units for which coverage is to be assessed. They may be combined in multiple ways and attributed within the project files are available to refine the set of units to include or exclude from each designated project. See Specifying Units Of Interest for extra details, and Using project files for a general overview of how the project facilities operate.

Saving coverage analysis state checkpoints allows the production of consolidated results from successive runs of the coverage command. In particular this allows coverage results to be computed incrementally, and allows consolidation with different sets of units of interest, in order to avoid incidental coverage. See Consolidation from coverage checkpoints for a discussion of these use cases.

Positional arguments on the command line (not tied to a particular option) are considered as trace file arguments. At least one trace file is required for the coverage command to operate, which may but need not be introduced with -T or --trace. Here are a few examples of valid command lines to illustrate. Other examples will be exposed along the course of the following sections:

gnatcov coverage --level=stmt -P ut.gpr --annotate=report --trace=prog.srctrace
#                      (a)       (b)            (c)             (d)
# (a) Request Statement coverage assessment,
# (b) for units belonging to the "ut.gpr" project,
# (c) producing a synthetic text report on standard output (no -o option),
# (d) out of a single execution trace "prog.trace".

gnatcov coverage --level=stmt+decision -P ut.gpr --annotate=html t1 t2
# Statement and Decision coverage assessments for two traces "t1" and "t2",
# producing an html report in the current directory.

gnatcov coverage --level=stmt+decision -P ut.gpr --annotate=html @mytraces
# Same report, with t1 and t2 listed in the "mytraces" text file

gnatcov coverage --level=stmt -Papp.gpr --annotate=html @mytraces
# Same kind of report, for Statement coverage only, on source units owned
# by "app.gpr" and its transitive closure of project dependencies.

gnatcov coverage --level=stmt -Papp.gpr --no-subprojects --annotate=html @mytraces
# Likewise, considering only the units owned by app.gpr

1.5.2. Output report formats (--annotate)

Source coverage reports may be produced in various formats, as requested with the --annotate command line argument of gnatcov coverage. The xcov and html formats produce a set of annotated source files, in the directory where gnatcov is launched unless overriden with a --output-dir option. The report output consists in a synthetic text report of coverage violations with respect to the requested criteria, produced on standard output by default or in the file specified by the -o command line option. The cobertura produce a single xml file containing a coverage report in the Cobertura format. Its name is by default cobertura.xml and can be altered using the -o command line option.

Later in this chapter we name output formats by the text to add to --annotate on the command line. For example, we use “the =report outputs” to mean “the coverage reports produced with --annotate=report”.

We will illustrate the various formats with samples extracted from outputs obtained by performing coverage analysis of the following example Ada application unit:

function Between (X1, X2, V : Integer) return Boolean;
--  Is V between X1 and X2, inclusive and regardless of their ordering?

function Between (X1, X2, V : Integer) return Boolean is
begin
   if X1 < X2 then
      return V >= X1 and then V <= X2;
   else
      return V >= X2 and then V <= X1;
   end if;
end Between;

1.5.2.1. Cobertura report (=cobertura)

--annotate=cobertura produces a coverage report in the Cobertura format, as specified per the cobertura.dtd (document type description) that is generated alongside.

This format specification is not maintained by gnatcov, and it does not thus provide all of the information that other report formats do, notably MC/DC coverage information, violation messages. Decision violations are output using the “branch” terminology of Cobertura (one decision being two branches, and either 0, 1 or 2 of those branches are covered).

It is mainly provided for integration with external tools, such as continuous integration systems, e.g. gitlab, which supports integration of coverage reports into merge requests using this format.

File names in this coverage report are absolute. Depending on the use context of this coverage report, it can also make sense to strip a given prefix from the absolute paths to make them relative to, e.g. a project root. The --source-root command line option accepts a string prefix that will be removed from absolute path references in the report.

1.5.2.2. Annotated sources, html (=html)

For source coverage criteria, gnatcov coverage --annotate=html produces an index-based report under the HTML format (the other command names are aliases for backward compatibility).

To navigate the report, open the index.html file using the browser of your choice. This index file contains a summary of the assessment context (assessed criteria, trace files involved, …) and of the coverage results for all the units, with links to their annotated sources. If the -P was used to designate the source units of interest, sources are indexed per-project.

Note that some dynamic filtering / sorting can be applied:

  • Filter by kind of coverage obligations: either reporting on lines, or on statement / decision / MCDC (one or several) obligations, depending on the coverage level. See Synthetic metrics for more information.

  • Sort indexes by clicking on column headers, allowing for example sorts keyed on unit names or on relative coverage achievement.

See our sample html index appendix for an example index page, which embeds a self-description of all the items it contains.

The user can browse through an annotated version of the sources from the index. Each annotated source page contains a summary of the assessment results. This summary can be expanded to print subprogram metrics: the user can click on a subprogram’s metrics to access it in the annotated source immediately. This summary is followed by the original source lines, all numbered and marked with a coverage annotation as in the --annotate=xcov case. Lines with obligations are colorized in green, orange or red for +, ! or - coverage respectively.

See the sample annotated source appendix for a sample of html annotated source.

1.5.2.3. SARIF report (=sarif)

For source coverage criteria, gnatcov coverage --annotate=sarif produces a JSON-based report under the Static Analysis Results Interchange Format (SARIF).

The generation of SARIF reports is mainly provided to allow viewing gnatcov coverage reports with external IDEs that supports viewing them (for example, Visual Studio Code with Microsoft’s SARIF Viewer extension). To navigate this report, open the coverage.sarif file using an integrated development environment which supports viewing SARIF results.

The reports contain information on the violations observed by gnatcov as “Results” and the enabled coverage levels as “Rules”. Each violation is registered as “error”. Exempted violations are reported as “note” and undetermined coverage results as “warning”.

1.5.2.4. Violations summary, text (=report)

For source coverage criteria, gnatcov coverage --annotate=report produces a summary that lists all the coverage violations (failure to satisfy some aspect of a coverage criterion) relevant to the set of assessed criteria.

The report features explicit start/end of report notifications and at least three sections in between: Assessment Context, Coverage Violations, and Analysis Summary. Should gnatcov be unable to determine the coverage state of some coverage obligations, those will be reported in a dedicated Undetermined Coverage Items section, with a description of why the tool was unable to determine the coverage state for each obligation. A few variations are introduced when exemption regions are in scope. See the Coverage Exemptions section for more details on their use and effect on the output reports.

If --dump-units-to - is also on the command line, a UNITS OF INTEREST section is produced, which contains the list of units considered of-interest for the reported assessment, as well as the list of source files individually ignored with the Ignored_Source_Files project attribute and corresponding command-line option.

1.5.2.5. Annotated sources, text (=xcov[+])

For source coverage criteria, gnatcov coverage --annotate=xcov produces an annotated version of each source file, in text format, named after the original source with an extra .xcov extension at the end (x.ext.xcov for a source named x.ext).

Each annotated source contains a global summary of the assessment results followed by the original source lines, all numbered and marked with a coverage annotation next to the line number. The annotation on a line always consists in a single character, which may be one of the following:

Annotation

Meaning

.

No coverage obligation is attached to the line

-

Coverage obligations attached to the line, none satisfied

!

Coverage obligations attached to the line, some satisfied

?

Coverage obligations attached to the line, undetermined coverage state (*)

+

Coverage obligations attached to the line, all satisfied

(*) The Undetermined Coverage state (?) is only shown on the line in the absence of other known violations for that same line.

Here is, to illustrate, the full statement coverage report produced for our example unit when the Between function was called so that the if control evaluated True only. The function is actually part of an Ada package, called Ranges, with an original body source file named ranges.adb:

examples/src/ranges.adb:
67% of 3 lines covered
Coverage level: stmt
  1 .: package body Ranges is
  2 .:    function Between (X1, X2, V : Integer) return Boolean is
  3 .:    begin
  4 +:       if X1 < X2 then
  5 +:          return V >= X1 and then V <= X2;
  6 .:       else
  7 -:          return V >= X2 and then V <= X1;
  8 .:       end if;
  9 .:    end;
 10 .: end;

--annotate=xcov+ (with a trailing +) works the same, only providing extra details below lines with improperly satisfied obligations. The available details consists in the list of coverage violations diagnosed for the line, which depends on the coverage criteria involved. Here is an excerpt for our previous example, where the only improperly satisfied obligation is an uncovered statement on line 7:

7 -:          return V >= X2 and then V <= X1;
STATEMENT "return V ..." at 7:10 not executed

1.5.2.6. XML report, xml (=xml)

--annotate=xml produces a coverage report in the XML format, as specified per the gnatcov-xml-report.xsd (XML schema description) that is generated alongside.

This report format is on par with the HTML report in terms of features, and it is the preferred choice for programmatically accessing the coverage results.

1.5.2.6.1. Assessment Context

The Assessment Context report section exposes the following information items:

  • Date & time when the report was produced

  • Command line and Version of GNATcoverage that produced the report. The set of units that the report is about is conveyed by the command line switches summarized there (--projects, --units, --scos).

  • Coverage level requested to be analyzed

  • Details on the input trace files, such as the path to the binary program exercised (as recorded in the trace header) or the trace production time stamp.

Here is a example excerpt:

===========================
== 1. ASSESSMENT CONTEXT ==
===========================

Date and time of execution: 2011-11-24 16:33:44.00
Tool version: GNATcoverage 1.0.0w (20111119)

Command line:

gnatcov coverage -Pmytest.gpr --level=stmt+mcdc --annotate=report test_x1x2.trace

Coverage level: stmt+mcdc

Trace files:

test_x1x2.trace
  program: obj/test_x1x2
  date   : 2011-11-24 15:33:44
  tag    : sample run

1.5.2.6.2. Coverage Violations

The Coverage Violations report section lists and counts the coverage violations that relate to source lines not part of an exemption region. The violations are grouped in subsections, one per assessed criterion according to the --level option:

--level=

Assessed criteria / Report subsections

stmt

Statement Coverage

stmt+decision

Statement and Decision Coverage

stmt+mcdc

Statement, Decision and MCDC Coverage

All the violations are reported using a consistent format, as follows:

ranges.adb:7:10: statement not executed
  source  :sloc: violation description

source and sloc are the source file basename and the precise line:column location within that source where the violation was detected.

The following table summarizes the list of violation items that might be emitted together for each criterion:

Criterion

Possible violations

Statement Coverage

statement not executed

Decision Coverage

decision outcome TRUE not covered

decision outcome FALSE not covered

one decision outcome not covered

MCDC Coverage

all the decision coverage items, plus …

condition has no independent influence pair

When multiple violations apply someplace, only the most basic diagnostic is emitted, not the more precise ones corresponding to stricter criteria. For instance, if an Ada statement like X := A and then B; is not covered at all, a statement not executed violation is always emitted alone, even when assessing --level=stmt+mcdc and we also have improper decision and conditions coverage.

Here is an output excerpt for our example with --level=stmt+mcdc, producing one subsection for each of the three criteria requested at that level:

============================
== 2. COVERAGE VIOLATIONS ==
============================

2.1. STMT COVERAGE
------------------

ranges.adb:7:10: statement not executed

1 violation.

2.2. DECISION COVERAGE
----------------------

ranges.adb:4:10: decision outcome FALSE never exercised

1 violation.

2.3. MCDC COVERAGE
------------------

ranges.adb:5:17: condition has no independent influence pair, MC/DC not achieved

1 violation.

1.5.2.6.3. Analysis Summary

The Analysis Summary report section summarizes just the counts reported in each of the previous sections. For our example report so far, this would be:

=========================
== 3. ANALYSIS SUMMARY ==
=========================

1 non-exempted STMT violation.
1 non-exempted DECISION violation.
1 non-exempted MCDC violations.

This section provides a quick way to determine whether the requested coverage level is fully satisfied, with details available from the per criterion sections that precede.

1.5.3. Statement Coverage analysis

1.5.3.1. Core notions and Reporting (--level=stmt)

gnatcov performs Statement Coverage assessments with the --level=stmt command line option. The assessment determines the status of statement coverage obligations out of the tests execution, considering that:

  • A statement is covered, and the obligation discharged, as soon as the control flow has reached the statement.

  • A statement is uncovered otherwise.

There is no notion of partial coverage for statements, even when a statement execution is interrupted somehow, for example by an exception occurrence. For instance, the statement below will be reported as covered as soon as the statement is reached, even if the expression evaluation never really terminates:

X := Function_That_Raises_Exception (Y) + Z;

In synthetic =report outputs, unexecuted source statements are listed as Statement Coverage violations in the report section dedicated to these.

In annotated source outputs, the coverage annotations convey the following indications:

Annotation

Meaning

-

At least one statement on the line, none covered

!

More than one statement on the line, some covered

?

At least one statement on this line, some with undetermined coverage state (*)

+

At least one statement on the line, all covered

(*) The Undetermined Coverage state (?) is only shown on the line in the absence of other known violations for that same line.

When a single statement spans multiple lines, the coverage annotation is present on all the lines, as the two + signs for the single assignment in the following excerpt:

3 .:  -- A single assignment spanning two lines:
4 +:  Result :=
5 +:     Input1 * Input2;

For compound statements, the coverage status of the compound construct per se is reported only on the parts that embed flow control expressions. For an Ada if statement, for example, coverage is reported on the if or elsif lines only, not on the else, or end if; lines, and not on lines where inner statements reside. The lines where inner statements reside are annotated in accordance with the nature and coverage status of those statements only. For example, see the . annotations on lines 4 and 6 in:

2 +:  if This_Might_Not_Be_True then
3 -:     Result := -1;
4 .:  else
5 +:     Result := 12;
6 .:  end if;

Declarations are generally considered as statements, so are reported covered/uncovered when they have initialization code associated with them.

Note that if no executable code for a given unit can be found in any of the executables submitted to gnatcov, then all statements in the unit will be conservatively reported as uncovered. This ensures that if tests for an entire unit have been omitted from a test campaign, a violation will be properly reported. Such violations can be suppressed either using exemptions, or by removing the unit from the list of units of interest.

1.5.3.2. Example program and assessments

Let us consider the example functional unit below, with the spec and body stored in source files named div_with_check.ads and div_with_check.adb:

function Div_With_Check (X, Y : Integer) return Integer;
--  return X / Y if Y /= 0. Raise Program_Error otherwise

function Div_With_Check (X, Y : Integer) return Integer is
begin
   if Y = 0 then
      raise Program_Error;
   else
      return X / Y;
   end if;
end;

We first exercise the function for Y = 1 only, using the following test driver in test_div1.adb:

procedure Test_Div1  is
   X : constant Integer := 4;
begin
   Assert (Div_With_Check (X, 1) = X);
end;

From a test_div1.srctrace obtained by building the instrumented source produced with gnatcov instrument, we analyze for the Statement Coverage criterion using the following gnatcov coverage invocation:

gnatcov coverage --level=stmt -P ut.gpr --annotate=xcov test_div1.srctrace

We get an =xcov annotated source result in text format for the functional unit on which the analysis is focused, in div_with_check.adb.xcov:

examples/src/div_with_check.adb:
67% of 3 lines covered
Coverage level: stmt
  1 .: function Div_With_Check (X, Y : Integer) return Integer is
  2 .: begin
  3 +:    if Y = 0 then
  4 -:       raise Program_Error;
  5 .:    else
  6 +:       return X / Y;
  7 .:    end if;
  8 .: end;

We can observe that:

  • Only the if line of the compound if statement is annotated, as covered since the function was called.

  • The inner raise and return statements are marked uncovered and covered respectively, as expected since the function was only called with arguments for which the if controlling decision evaluates False.

As a second experiment, we exercise the function for Y = 0 only, using:

procedure Test_Div0  is
   Result : Integer
     := Div_With_Check (4, 0);
begin
   Put_Line ("R = " & Integer'Image (Result));
end;

We request results on the test driver as well this time, as it features constructs of relevance for our purposes:

gnatcov coverage --level=stmt -Pmytest.gpr --annotate=xcov test_div0.trace

The =xcov outputs follow. First, for the functional unit, with the if statement coverage reversed compared to the previous testcase:

1 .: function Div_With_Check (X, Y : Integer) return Integer is
2 .: begin
3 +:    if Y = 0 then
4 +:       raise Program_Error;
5 .:    else
6 -:       return X / Y;
7 .:    end if;
8 .: end;
9 .:

Then, for the test driver where we can note that

  • The two lines of the local Result definition are annotated,

  • This definition is marked covered even though it was evaluated only once with an initialization expression that raised an exception, and

  • The driver body is reported uncovered, as expected since an exception triggered during the elaboration of the subprogram declarative part.

67% of 3 lines covered
Coverage level: stmt
   1 .: with Div_With_Check, Ada.Text_IO; use Ada.Text_IO;
   2 .:
   3 .: procedure Test_Div0  is
   4 +:    Result : Integer
   5 +:      := Div_With_Check (4, 0);
   6 .: begin
   7 -:    Put_Line ("R = " & Integer'Image (Result));
   8 .: end;

The corresponding synthetic report is simply obtained by running gnatcov coverage again with --annotate=report instead of --annotate=xcov:

===========================
== 1. ASSESSMENT CONTEXT ==
===========================
...

============================
== 2. COVERAGE VIOLATIONS ==
============================

2.1. STMT COVERAGE
------------------
div_with_check.adb:6:7: statement not executed
test_div0.adb:7:4: statement not executed

2 violations.

=========================
== 3. ANALYSIS SUMMARY ==
=========================

2 STMT violations.

We can see here that the two lines marked - in the =xcov outputs are properly reported as violations in the STMT COVERAGE section of this report, and that this section is the only one presented in the COVERAGE VIOLATIONS part, as only this criterion was to be analyzed per the --level=stmt argument.

1.5.4. Decision Coverage analysis

1.5.4.1. Core notions and Reporting (--level=stmt+decision)

With the --level=stmt+decision command line option, gnatcov performs Statement and Decision Coverage assessments combined together. Decisions in this context are defined as:

  • Any Boolean expression used to influence the control flow via explicit constructs in the source program, such as if statements or while loops;

  • The controlling predicate of Ada if-expressions or C conditional-expressions, in any kind of context except Ada contracts.

  • The iteration predicate of Ada quantified expressions, in the same contexts as for if-expressions.

  • The iterator filter predicate optionally present in a loop parameter specification or in an iterator specification, in the same context as if-expressions.

The expression may be of essentially any type in C. In Ada, this may be the standard Boolean type, or subtypes of it, or derived types thereof. Here are a few examples show-casing decisions in a variety of contexts:

while (z++) ...  /* "z++", controlling a while-loop.  */

if X > 0 then ...  -- "X > 0", controlling an if-statement

q = z-- ? x : y;  /* "z++", conditional-expression predicate.  */

Q := (X if Z < 2 else Y); -- "Z < 2", if-expression predicate

Z := (for all X of Container => P(X));  -- "P(X)", quantified-expression predicate
T := (for some X of Container => P(X));

for I in 1 .. 10 when (I mod 2) = 0 loop ... -- "(I mod 2) = 0", iterator filter predicate
F := (for Elt of Container when Pred (Elt) => Elt); -- "Pred (Elt)", iterator filter predicate

The coverage status of a decision obligation is determined as follows:

  • A decision is said fully covered, or just covered, and the obligation discharged, as soon as the decision has been evaluated at least once True and once False during program execution.

  • A decision is said uncovered when none of the possible outcomes was exercised, either because the enclosing statement was not executed at all or when all the attempted evaluations were interrupted e.g. because of exceptions.

  • If only one of the two possible outcomes was exercised, the decision is said partially covered, and the obligation only partially discharged.

When a decision is part of a statement and the statement is not executed at all, only the statement level violation is reported. The nested decision level violations are implicit in this case and diagnosing them as well would only add redundancy.

The =report synthetic output lists the statement and decision coverage violations in the STMT and DECISION coverage report section respectively. For the =xcov and =html annotated-source oriented formats, the single annotation produced on each source line combines the statement and decision coverage indications. The following table summarizes the meaning of the possible annotations:

Annotation

Meaning

-

At least one statement on the line, none executed.

!

Unless multiple statements are involved, decision partially covered on the line.

?

At least one statement or decision on the line with undetermined coverage state. (*)

+

All the statements and decisions on the line are covered.

(*) The Undetermined Coverage state (?) is only shown on the line in the absence of other known violations for that same line.

When a trailing + is added to the format passed to --annotate (=xcov+), a precise description of the actual violations is available for each line in addition to the annotation. The =html provides it by default.

1.5.4.2. Example program and assessments

Consider the example functional Ada unit below, with the spec and body stored in source files named divmod.ads and divmod.adb:

procedure Divmod
  (X, Y : Integer; Value : out Integer;
   Divides : out Boolean; Tell : Boolean);
--  Compute X / Y into VALUE and set DIVIDES to indicate whether
--  Y divides X. Output a note to this effect when requested to TELL.

procedure Divmod
  (X, Y : Integer; Value : out Integer;
   Divides : out Boolean; Tell : Boolean) is
begin
   if X mod Y = 0 then
      Divides := True;
      if Tell then
         Put_Line (Integer'Image(Y) & " divides " & Integer'Image(X));
      end if;
   else
      Divides := False;
   end if;

   Value := X / Y;
end Divmod;

We first experiment with the following test driver:

procedure Test_Divmod2  is
   Value : Integer;
   Divides : Boolean;
begin
   Divmod (X => 5, Y => 2, Value => Value,
           Divides => Divides, Tell => True);
   Assert (Divides = False);

   Divmod (X => 6, Y => 2, Value => Value,
           Divides => Divides, Tell => True);
   Assert (Divides = True);
end Test_Divmod2;

This exercises the Divmod function twice. The outer if construct executes both ways and the if Tell then test runs once only for Tell True. As a result, the only stmt+decision violation by our driver is the Tell decision coverage, only partially achieved since we have only exercised the True case. This is confirmed by =report excerpt below, where we find the two violations sections in accordance with the requested set of criteria:

2.1. STMT COVERAGE
------------------

No violation.

2.2. DECISION COVERAGE
----------------------

divmod.adb:14:10: decision outcome FALSE never exercised

1 violation.

For --annotate=xcov, this translates as a single partial coverage annotation on the inner if control line:

 8 .: procedure Divmod
 9 .:   (X, Y : Integer; Value : out Integer;
10 .:    Divides : out Boolean; Tell : Boolean) is
11 .: begin
12 +:    if X mod Y = 0 then
13 +:       Divides := True;
14 !:       if Tell then
15 +:          Put_Line (Integer'Image (Y) & " divides " & Integer'Image (X));
16 .:       end if;
17 .:    else
18 +:       Divides := False;
19 .:    end if;
20 .:
21 +:    Value := X / Y;
22 .: end Divmod;

Now we exercise with another test driver:

procedure Test_Divmod0  is
   Value : Integer;
   Divides : Boolean;
begin
   Divmod (X => 5, Y => 0, Value => Value,
           Divides => Divides, Tell => True);
end Test_Divmod0;

Here we issue a single call passing 0 for the Y argument, which triggers a check failure for the mod operation.

This results in the following =xcov output:

 8 .: procedure Divmod
 9 .:   (X, Y : Integer; Value : out Integer;
10 .:    Divides : out Boolean; Tell : Boolean) is
11 .: begin
12 !:    if X mod Y = 0 then
13 -:       Divides := True;
14 -:       if Tell then
15 -:          Put_Line (Integer'Image (Y) & " divides " & Integer'Image (X));
16 .:       end if;
17 .:    else
18 -:       Divides := False;
19 .:    end if;
20 .:
21 -:    Value := X / Y;
22 .: end Divmod;

We have an interesting situation here, where

  • The outer if statement is reached and covered (as a statement),

  • No evaluation of the X mod Y = 0 decision terminates, because the only attempted computation is interrupted by an exception, so none of the other statements is ever reached.

This gets all confirmed by the =report output below, on which we also notice that the only diagnostic emitted for the uncovered inner if on line 14 is the statement coverage violation:

2.1. STMT COVERAGE
------------------
divmod.adb:13:7: statement not executed
divmod.adb:14:7: statement not executed
divmod.adb:15:10: statement not executed
divmod.adb:18:7: statement not executed
divmod.adb:21:4: statement not executed

5 violations.

2.2. DECISION COVERAGE
----------------------
divmod.adb:12:7: decision never evaluated

1 violation.

1.5.5. Modified Condition/Decision Coverage analysis

1.5.5.1. Core notions and Reporting (--level=stmt+mcdc)

Combined Statement and Modified Condition/Decision Coverage (MCDC) analysis is performed by passing the --level=stmt+mcdc option to gnatcov coverage commands. Decisions in this context are defined as:

  • All the expressions considered as decisions for decision coverage, as well as:

  • All the Boolean expressions which combine operands with short-circuit logical operators (”&&” and “||” in C, “and then” and “or else” by default in Ada), in any kind of context except Ada contracts.

Then for all the decisions in the sources of interest:

  • Testing shall exercise both the True and False outcomes of the expression as a whole and demonstrate the independent influence of conditions in DO-178C parlance, where …

  • Separate conditions in a decision are identified as the operands of short-circuit operators.

Regarding coverage status definitions:

  • A condition is covered, and the obligation discharged, when the independent effect on the enclosing decision was demonstrated by the tests,

  • A condition is said uncovered otherwise.

The Example decisions and conditions section that follows provides a few examples to illustrate the identification of decisions and conditions. Handling non short-circuit operators focuses on the handling of computational Boolean operators, then MCDC variants expands on the notion of independent influence and on possible variations of the MCDC criterion definition.

Output-wise, the source annotations for the =xcov or =html formats are the same as for decision coverage, with condition specific cases marked with a ! as well:

Annotation

Meaning

-

At least one statement associated with this line, none executed.

!

For a single statement line, decision partially covered or condition not covered on the line.

?

At least one statment or decision on the line with undetermined coverage state. (*)

+

All the statements, decisions and conditions on the line are covered.

(*) The Undetermined Coverage state (?) is only shown on the line in the absence of other known violations for that same line.

The =report outputs feature an extra MCDC section in the Coverage Violations segment, which holds:

  • The condition specific diagnostics (independent influence not demonstrated), as well as

  • Decision level diagnostics (such as decision outcome True not covered messages) for the Complex Boolean Expressions not directing a control-flow oriented statement and which we treat as decisions nevertheless.

There again, condition or decision related messages are only emitted when no more general diagnostic applies on the associated entity. Condition specific diagnostics, for example, are only produced in absence of enclosing statement or decision level violation.

See the Example program and assessments section of this chapter for a few illustrations of these points.

1.5.5.2. Example decisions and conditions

As described in the previous section, the set of expressions considered as decisions for MCDC is wider than for decision coverage analysis, which encompasses all the expressions used as conditionals in explicit control-flow construct, for example:

if Check(X) then -- 1 decision (if-stmt control)

while (x > 12) { // 1 decision (while loop control)

MCDC also includes the conditionals of Ada if-expressions or they C conditional-expression counterpart, as in:

Val := (if X > 0 then X else -X); -- 1 decision (if-expr control)

return (x > y) ? 1 : -1; // 1 decision (cond-expr control)

Then we have logical expressions combining operands with short-circuit operators in other contexts, such as:

X := A and then B; -- 1 decision (short-circuit and-then)
Check (C or else D); -- 1 decision (short-circuit or-else)

return (x < low || y > high); // 1 decision (short-circuit ||)

This can raise lead to various cases of multiple decisions within a single expression, for example:

if Check (Arg1 => A and then B, Arg2 => C or else D) then
-- 3 decisions here:
-- * A and then B (short circuit and-then)
-- * C or else D (short circuit or-else)
-- * Check (...)  (if-stmt control)

In C as in Ada, logical negation is allowed anywhere and influences neither decision boundaries nor the split into conditions:

if (x && !y) { // 1 decision (if-stmt control)
               // 2 conditions (short-circuit &&)

if not (A or else B) then -- 1 decision (if-stmt control)
                          -- 2 conditions (short-circuit or-else)

1.5.5.3. Handling non short-circuit operators

Short-circuit operators are key to instating a decision at all in some contexts, and to direct the split of decisions into conditions in all contexts. By default, non short-circuit binary operators are taken as regular computational devices:

if A or B then -- 1 decision (if-statement control)
               -- single condition (no short-circuit by default)

if A or else B then -- 1 decision (if-statement control)
                    -- 2 conditions (short-circuit or-else)

X := A and B;  -- no decision (no short-circuit by default)

X := A and then B;  -- 1 decision (short-circuit and-then)
                    -- 2 conditions (short-circuit and-then)

This computational view for non short-circuit operators in Ada is similar to the C model where bitwise operations on non purely Boolean types, yet used as a Boolean result, are extremely common as in:

return (x & 0x3) && (y & 0x3);  // 1 decision (short-circuit &&)
                                // 2 conditions, each a & on int

It can also result in splits introducing the possibility of mulitple decisions in arguments, as in:

if ((A and then not B) == (C or else (D and then E))) then
-- 3 decisions here:
-- * The toplevel expression as a whole (... == ...), if-stmt control,
--   single condition.
-- * (A and then not B), short-circuit and-then in the first operand of ==,
--   2 conditions.
-- * (C or else (D and then E)), short-circuit tree in the second operand,
--   3 conditions.

GNAT compilers offer two devices to mitigate possible issues caused by this behavior on non short-circuit Boolean operators for Ada:

  • To make sure operators of this kind are not used, e.g. as a coding standard rule, a No_Direct_Boolean_Operator restriction pragma can be setup in the development environment to trigger compilation errors on such uses; Alternatively:

  • The Short_Circuit_And_Or configuration pragma requests that the non-short circuit and and or operators on standard Booleans are compiled as as their short-circuit counterparts, then processed as such for coverage analysis.

There is no equivalent in C, where the allowed operand types are much more varied and where the restriction would make the language really much harder to use.

1.5.5.4. MCDC variants

Compared to Decision Coverage, achieving MCDC requires tests that demonstrate the independent influence of conditions in decisions. Several variants of the criterion exist.

Unique Cause MCDC is the original criterion described in the DO-178C reference guidelines, where independent influence of a specific condition must be demonstrated by a pair of tests where only that condition changes and the decision value toggles.

Consider the following table which exposes the 4 possible condition/decision vectors for the A and then B expression, where T stands for True, F stands for False, and the italics indicate that the condition evaluation is short-circuited:

#

A

B

A and then B

1

T

T

T

2

T

F

F

3

F

T

F

4

F

F

F

Each line in such a table is called an evaluation vector, and the pairs that demonstrate the independent effect of conditions are known as independence pairs.

Evaluations 1 + 3 constitute a Unique Cause independence pair for A, where A changes, B does not, and the expression value toggles. 1 + 2 constitues a pair for B.

The closest criterion supported by GNATcoverage is a very minor variation where conditions that are not evaluated due to short-circuit semantics are allowed to differ as well in a pair. Indeed, their value change cannot possibly have influenced the decision toggle (since they are not even considered in the computation), so they can never invalidate the effect of another condition.

We call this variation Unique Cause + Short-Circuit MCDC, activated with --level=stmt+uc_mcdc on the command line. From the A and then B table just introduced, 4 + 1 becomes another valid independence pair for A, as B is not evaluated at all when A is False so the change on B is irrelevant in the decision switch.

--level=stmt+mcdc actually implements another variant, known as Masking MCDC, accepted as a sound alternative and offering improved support for coupled conditions.

Masking MCDC allows even further flexibility in the possible variations of conditions in an independence pair. Indeed, as soon as only short-circuit operators are involved, all the conditions that appear on the left of a given condition in the expression text are allowed to change without invalidating the said condition influence demonstration by a pair.

1.5.5.5. Example program and assessments

We reuse one of our previous examples to illustrate, with a simple functional unit to exercise:

function Between (X1, X2, V : Integer) return Boolean;
--  Whether V is between X1 and X2, inclusive and however they are ordered

function Between (X1, X2, V : Integer) return Boolean is
begin
   if X1 < X2 then
      return V >= X1 and then V <= X2;
   else
      return V >= X2 and then V <= X1;
   end if;
end Between;

First consider the following test driver, which exercises only a single case where X1 < V < X2:

procedure Test_X1VX2 is
begin
   Assert (Between (X1 => 2, X2 => 5, V => 3)); -- X1 < V < X2
end Test_X1VX2;

After instrumentation for MCDC and execution, we can then request, say, an =xcov+ report to get a first set of results in the ranges.adb.xcov annotated source:

gnatcov coverage --level=stmt+mcdc -Pmytest.gpr --annotate=xcov+ test_x1vx2.srctrace

...
   8 .:    function Between (X1, X2, V : Integer) return Boolean is
   9 .:    begin
  10 !:       if X1 < X2 then
DECISION "X1 < X2" at 10:10 outcome FALSE never exercised
  11 !:          return V >= X1 and then V <= X2;
DECISION "V >= X1 a..." at 11:17 outcome FALSE never exercised
  12 .:       else
  13 -:          return V >= X2 and then V <= X1;
STATEMENT "return V ..." at 13:10 not executed
  14 .:       end if;
  15 .:    end Between;

This is all as expected from what the driver does, with a few points of note:

  • The diagnostic on line 11 confirms that Complex Boolean Expression are treated as decisions even when not used to direct a conditional control-flow statement. The expression is indeed used here as a straight, unconditional return statement value;

  • Only the decision level violations are emitted for lines 10 and 11. The independant influence of the conditions is not demonstrated but this is implicit from the decision partial coverage so is not notified;

  • Similarily, only the statement level violation is emitted for line 13, eventhough there are decision and condition level violations as well.

Another aspect of interest is that we have partial decision coverage on two kinds of decisions (one control-flow decision controling the if, and another one used a straight return value), and this distinction places the two decision outcome FALSE never exercised violations in distinct sections of the =report output:

2.1. STMT COVERAGE
------------------
ranges.adb:13:10: statement not executed

2.2. DECISION COVERAGE
----------------------
ranges.adb:10:10: decision outcome FALSE never exercised

2.3. MCDC COVERAGE
------------------
ranges.adb:11:17: decision outcome FALSE never exercised

Now running another test driver which exercises two cases where X1 < X2:

procedure Test_X1VX2V is
begin
   Assert (Between (X1 => 2, X2 => 5, V => 3)); -- X1 < V < X2
   Assert (not Between (X1 => 2, X2 => 5, V => 8)); -- X1 < X2 < V
end;

The first return expression is valued both ways so we get an example of condition specific diagnostic on line 11:

   8 .:    function Between (X1, X2, V : Integer) return Boolean is
   9 .:    begin
  10 !:       if X1 < X2 then
DECISION "X1 < X2" at 10:10 outcome FALSE never exercised
  11 !:          return V >= X1 and then V <= X2;
CONDITION "V >= X1" at 11:17 has no independent influence pair, MC/DC not achieved
  ...

Indeed, looking at an evaluation table for the first return decision:

#

A: V >= X1

B: V <= X2

A and then B

Case

1

T

T

T

X1 < V < X2

2

T

F

F

X1 < X2 < V

3

F

T

F

4

F

F

F

We observe that our driver exercises vectors 1 and 2 only, where:

  • The two evaluations toggle the decision and the second condition only, so achieve decision coverage and demonstrate that condition’s independant influence;

  • The first condition (V >= X1) never varies so the independant influence of this condition isn’t demonstrated.

As we mentioned in the discussion on MCDC variants, adding vector 3 would achieve MCDC for this decision. Just looking at the table, adding vector 4 instead would achieve MCDC as well since the second condition is short-circuited so its value change is not relevant. The condition expressions are such that running vector 4 is not possible, however, since we can’t have V both < X1 (condition 1 False) and V > X2 (condition 2 False) at the same time when X1 < X2.

1.5.6. Assertion True Coverage (ATC) analysis (experimental)

Along with the previous coverage levels, gnatcov can provide different strictness levels of assertion coverage for source traces for Ada.

Assertion coverage warrants defining separate criteria to perform coverage analysis since contrary to all other regular decisions, those contained in assertions are expected to never be evaluated to False. Assertion decisions and conditions are defined the same way as for Decision and MCDC but, the coverage is computed differently.

If such coverage analysis is needed, it should always be activated along one of the previously described coverage levels, as an “addition”. In the following sections on assertion coverage, their associated command line options will be written as –level=…+<assertion coverage level> where … is one of stmt, stmt+decision, stmt+mcdc and stmt+uc_mcdc.

Using assertion coverage levels allows to compute the coverage of the pragma statements Assert, Assert_And_Cut, Assume, Check, Loop_Invariant, and the aspects Type_Invariant, Precondition and Postcondition.

1.5.6.1. Core notions and Reporting (--level=...+atc)

gnatcov performs Assertion True Coverage assessments with the --level=...+atc command line option. The assessment determines the status of assertion true coverage obligations out of the tests execution, considering that:

  • An assertion is covered, and the obligation discharged, as soon as the assertion’s decision has been evaluated once to True.

  • An assertion is uncovered otherwise.

In synthetic =report outputs, uncovered source assertions are listed as ATC Coverage violations in the report section dedicated to these.

In annotated source outputs, the coverage annotations convey the following indications:

Annotation

Meaning

-

At least one assertion on the line, none covered

!

More than one assertion on the line, some covered

?

At least one assertion on this line, some with undetermined coverage state (*)

+

At least one assertion on the line, all covered

(*) The Undetermined Coverage state (?) is only shown on the line in the absence of other known violations for that same line.

1.5.7. Assertion True Condition Coverage (ATCC) analysis (experimental)

1.5.7.1. Core notions and Reporting (--level=...+atcc)

The coverage status of an “ATCC” obligation is determined as follows:

  • An assertion is said fully covered, or just covered, and the obligation discharged, as soon as all conditions have been evaluated to True or False at least once accross all evaluations to True of the whole decision.

  • An assertion is said uncovered when the decision was never evaluated to True, either because the enclosing assertion statement was not executed at all or when all the attempted evaluations were interrupted e.g. because of exceptions.

  • An assertion is said partially covered when at least one of the conditions of the assertion’s decision was never evaluated to either True or False in the context of an evaluation to True of the whole decision. In this case, the obligation is partially discharged.

The =report synthetic output lists the ATC and ATCC coverage violations in the ATC and ATCC coverage report section respectively. For the =xcov and =html annotated-source oriented formats, the single annotation produced on each source line combines the statement and ATC coverage indications. The following table summarizes the meaning of the possible annotations:

Annotation

Meaning

-

At least one statement on the line, none executed.

!

Unless multiple statements are involved, assertion partially covered on the line.

?

At least one statement or assertion on the line with undetermined coverage state. (*)

+

All the statements and assertions on the line are covered.

(*) The Undetermined Coverage state (?) is only shown on the line in the absence of other known violations for that same line.

When a trailing + is added to the format passed to --annotate (=xcov+), a precise description of the actual violations is available for each line in addition to the annotation. The =html provides it by default.

1.5.8. Function and Call Coverage (FUN_CALL) analysis (experimental)

gnatcov can also provide an analysis of the coverage of subprograms and calls for Ada, C, and C++ source traces.

If such coverage analysis is needed, it should always be activated along one of the non-assertion coverage levels previously described. In this section on function and call coverage, the associated command line option will be written as –level=…+fun_call where … is one of stmt, stmt+decision, stmt+mcdc and stmt+uc_mcdc. Assertion coverage can also be activated at the same time.

1.5.8.1. Core notions and Reporting (--level=...+fun_call)

gnatcov performs Function and Call Coverage assessments with the --level=...+fun_call command line option. The assessment determines the status of subprograms and call coverage obligations out of the tests execution, considering that:

  • A subprogram is covered, and the obligation discharged, as soon as the execution as entered it at least once.

  • A subprogram is uncovered otherwise.

  • A call is covered, and the obligation discharged, if it has been executed at least once.

  • A call is uncovered otherwise.

In synthetic =report outputs, uncovered source subprograms and calls are listed as FUN_CALL Coverage violations in the dedicated report section.

It is important to note that for an uncovered statement which happens to be a call statement, only a statement violation will be emitted in coverage reports.

The limitations of this coverage level are detailed in the gnatcov instrument limitations section.

1.5.9. Guarded Expression Coverage (GEXPR) analysis (experimental)

For the Ada language, gnatcov can provide an analysis on coverage of guarded expressions.

1.5.9.1. Core notions and Reporting (--level=...+gexpr)

gnatcov the following syntaxes to be guarded expressions:

  • Then and Else dependent expressions of if expressions

procedure Foo (A : Boolean) is
   Var : constant String :=
     (if A
      then "True expression"
      else "False expression");
begin
   null;
end Foo;

  • Dependent expressions of case expressions

type Animal is (Cat, Dog, Cow);

procedure Foo (A : Animal) is
   Var : constant String :=
     (case A
      when Cat    => "Expression Cat",
      when Dog    => "Expression Dog",
      when others => "Expression other");
begin
   null;
end Foo;

  • Predicate expression of quantified expressions

function Prime_In_Range (L, R : Natural) return Boolean is
begin

   --  Is_Prime (I) is the child expression that will be analysized.

   return (for some I in (L .. R) => Is_Prime (I));
end Foo;

For each of these, we consider the expression to be covered and the obligation to be discharged if the execution flow evaluated it at least once.

The limitations of this coverage level are detailed in the gnatcov instrument limitations section.

1.5.10. Synthetic metrics

A number of output formats feature synthetic metrics (essentially, coverage percentages) for each source unit, and for the entire analysis when an index summary is also produced. We first introduce the metrics definition for the =xcov format then describe how they are presented in other outputs.

For the =xcov format, the synthetic metrics are summary numbers at the top the annotated source for each unit. For example:

.../metrics/main.adb:
50% of 4 lines covered
75% statement coverage (3 out of 4)
0% decision coverage (0 out of 1)

Coverage level: stmt+decision
 1 .: with Ada.Text_IO; use Ada.Text_IO;
 2 .: with Types; use Types;
 3 .:
 4 .: procedure main is
 5 +:    X : My_Int := 12 with Volatile;
 6 .: begin
 7 !:    if X < 2 then
 8 -:       Put_Line ("X LT 2");
 9 .:    else
10 +:       Put_Line ("X GE 2");
11 .:    end if;
12 .: end;

The metric displayed first is:

50% of 4 lines covered

This is line count based, where the count includes all the lines to which is attached at least one coverage obligation of the asserted level (here, stmt+decision). In the provided example, these are lines 5, 7, 8 and 10, for a total of 4 lines, where line 7 encompasses two obligations (one statement and one decision).

The metric conveys the total number of lines and the percentage of such lines for which all the obligations are fully covered, that is, those marked with a ‘+’ annotation.

The two other metrics in this example are:

75% statement coverage (3 out of 4)
0% decision coverage (0 out of 1)

These are coverage obligation count based, one for each obligation kind involved in the assessed level (here, for --level=stmt+decision). As for lines, each metric displays the total number of obligations of the given kind and the ratio of such obligations which have been fully discharged. The xcov obligation metrics don’t distinguish partially covered from uncovered items. This information is available from the html report.

For the html output, synthetic metrics for all the units are first displayed on the index page, together with metrics for the entire analysis and for sets of units grouped by GPR project.

Initially, the line count based metrics are displayed, as illustrated by Fig. 1.7. The main procedure source used in our xcov report example is the main.adb source here, single source encompassed by the main.gpr project:

Switching to coverage obligation count based metrics is simply achieved by clicking the button(s) at the top of the page labelled with obligation kind names (“Stmt”, “Decision” or “Mcdc”).

Each kind of obligation can be selected alone. Selecting multiple kinds is also allowed and just sums the individual counts. Fig. 1.8 illustrates the results we get with the Stmt and Decision kinds selected together for our previous example:

../_images/html-index-lines.png

Fig. 1.7 Html index with line count based synthetic metrics

../_images/html-index-obligations.png

Fig. 1.8 Html index with obligations count based synthetic metrics (stmt+decision)

1.5.11. Handling source relocation for annotated sources output formats

For all annotated sources output formats (--annotate=xcov[+]|html), gnatcov needs access to the sources of the units of interest to generate the output. The tool searches for sources in some default locations which are usually correct when the whole coverage analysis process is done on the same machine.

When coverage reports are produced on a different machine than the one where the coverage run was done, or when sources are moved in the interim, gnatcov will likely not be able to automatically find the source files. The tool offers two options to help in those situations:

  • The option --source-rebase=<old_prefix>=<new_prefix>

    This option allows the specification of alternate path prefixes for source files.

    If the location of a source file, when built on a first machine, was /work/build_1234/src/main.adb but on the machine where the report is to be generated, the same file is located at /some/path/project/src/main.adb, then in order for gnatcov to find this file, the option --source-rebase=/work/build_1234/=/some/path/project/ can be passed. Note that the prefixes apply to absolute paths.

    This option supports globbing expressions for the old_prefix part of the option, so in the example above, if the build number varies from run to run, the option passed can be --source-rebase=/work/build_*=/some/path/project/.

    This option also allows for response-files (--source-rebase=@rebase_file) in which each line must be of the form old_prefix=new_prefix.

    This option can be passed multiple times on the command line, and gnatcov will try each pair in the order in which they were passed until the source file is found.

  • The option --source-search=directory_path

    This option allows the specification of directories in which to search for missing source files.

    If the location of a source file, when built on a first machine, was /work/build_1234/src/main.adb but its location on the machine where the report is generated is /some/path/project/src/main.adb then passing --source-search=/some/path/project/src will enable gnatcov to find the missing source file. This option also accepts response files.

    This option can appear multiple times on the command line. Note that gnatcov will first try the --source-rebase prefix pairs, if any, and if the source file was not found, it will then look for the file in all the directories passed with --source-search in the order in which they appear on the command line.

These two options perform very similar functions and can sometimes be used interchangeably, however the second option is less selective and can lead to unexpected results in some circumstances. Since --source-search only specifies an additional directory in which to search for missing sources, it does not guarantee that if a file is found then it is the correct one, but only that the file found has the same basename than the searched file. For Ada projects this should not be an issue, since all files should have different names, but for C projects where there can be homonyms, this can be problematic:

If a project is built with the files /work/build_1234/src1/foo.h and /work/build_1234/src2/foo.h among its sources, and --source-search=/some/path/src1 --source-search=/some/path/src2 is passed to gnatcov to add the two source directories to the list of locations where sources can be found, then the resulting location for both foo.h files will be /some/path/src1/foo.h. In this case, passing --source-rebase=/work/build=/some/path/ removes the ambiguity.

--source-rebase is also more convenient when the project has multiple source directories, provided that their structure is the same when building/instrumenting the project and generating the coverage report. In the example above, it is necessary to pass --source-search twice to include all the source directories in the search path, but only one instance of --source-rebase is needed. For very large projects, since the number of required instances of --source-search is equal to the number of source directories, it is clearly more advantageous to use --source-rebase which only needs to be passed once.

1.5.12. Inlining & Ada generic units

In the vast majority of situations, inlining is just transparent to source coverage metrics: calls are treated as regular statements, and coverage of the inlined bodies is reported on the corresponding sources regardless of their actual inlining status. See the Optimization and non-coverable items section for a description of effects that might show up on rare occasions.

Generic units have a different status, where each instantiation produces a distinct, potentially unique instance of the generic code working on a different types, objects, with different helper subprograms or packages.

GNATcoverage offers two main kinds of coverage strategies for generic units: one where the generic source is considered as the entity of interest, to which all the instances contribute, and one where each instance is considered as a separate entity of interest.

1.5.12.1. Combined coverage on generics

Ada generic units are also uniformly treated as single source entities, with the coverage achieved by all the instances combined and reported against the generic source only, not for each individual instance.

Consider the following functional Ada generic unit for example. It provides a simple vector type abstraction on which two operations are available; Inc adds some amount to each element of a vector, and Mult multiplies each element by some amount. The exposed type is of fixed size, provided as a parameter:

generic                               -- vops.ads
   Size : in Integer;
package Vops is
   type Vector_Type is array (1 .. Size) of Integer;

   procedure Inc (V : in out Vector_Type; Amount : Integer);
   procedure Mult (V : in out Vector_Type; Amount : Integer);
end;

package body Vops is                  -- vops.adb

   procedure Inc (V : in out Vector_Type; Amount : Integer) is
   begin
      for I in V'Range loop
         V(I) := V(I) + Amount;
      end loop;
   end;

   procedure Mult (V : in out Vector_Type; Amount : Integer) is
   begin
      for I in V'Range loop
         V(I) := V(I) * Amount;
      end loop;
   end;
end;

Now consider this test, checking operations on vectors of different sizes, from two instances of the Vops unit:

with Vops;                            -- v5.ads
package V5 is new Vops (Size => 5);

with Vops;                            -- v8.ads
package V8 is new Vops (Size => 8);

with V5, V8;                          -- test_5inc_8mult.adb
procedure Test_5inc_8mult is
   V5o : V5.Vector_Type := (others => 1);
   V8o : V8.Vector_Type := (others => 2);
begin
   V5.Inc (V5o, 3);
   V8.Mult (V8o, 2);
end;

Only the Inc subprogram is called through the V5 instance and only the Mult subprogram is called through the V8 instance. Both suprograms are nevertheless called overall, so the Vops package body is claimed fully covered by default:

gnatcov coverage -Pvops.gpr --level=stmt --annotate=xcov test_5inc_8mult.trace
...
100% of 4 lines covered
Coverage level: stmt
  1 .: package body Vops is
  2 .:
  3 .:    procedure Inc (V : in out Vector_Type; Amount : Integer) is
  4 .:    begin
  5 +:       for I in V'Range loop
  6 +:          V(I) := V(I) + Amount;
  7 .:       end loop;
  8 .:    end;
  9 .:
 10 .:    procedure Mult (V : in out Vector_Type; Amount : Integer) is
 11 .:    begin
 12 +:       for I in V'Range loop
 13 +:          V(I) := V(I) * Amount;
 14 .:       end loop;
 15 .:    end;
 16 .: end;

1.5.13. Handling Ada assertions and contracts

Ada 2012 introduced sophisticated programming by contract mechanisms which generalize the base pragma Assert construct originally offered by the language. All these facilities allow asserting various properties of the program state at specific points of its execution, for example as type invariants or as Pre/Post-conditions on subprograms.

Each property is expressed a Boolean expression expected to hold True, triggering an exception otherwise. When the current assertion policy activates a given assertion construct, the associated Boolean expression is treated by gnatcov as a decision for MCDC purposes.

As assertions are by construction designed never to evaluate False, reaching proper coverage for them is non-trivial, if not entirely meaningless, for post-conditions in particular.

The simple way out consists in disabling the relevant constructs in builds intended for coverage analysis, by setting the corresponding Assertion_Policy to Disable with GNAT Pro toolchains.

1.5.14. Processing of C macros

For source coverage purposes, Source Coverage Obligations for C are produced after the preprocessing of sources, with two consequences of note:

  • Macro expansions leading to code with conditionals will trigger coverage violations, and multiple “calls” to the same macro just multiply these as they yield distinct expansions.

  • The source locations output by gnatcov for coverage violations within macro expansions designate preprocessed tokens at the macro expansion site, typically on the line of the macro invocation but with column numbers unrelated to what is visible in the source on this line.

Consider this C code for example:

 1#define COND_INC(cond,x,y) \
 2  do {                     \
 3    if (cond)              \
 4      (x)++;               \
 5    else                   \
 6      (y)++;               \
 7  } while(0)
 8
 9int main ()
10{
11  volatile x = 0, y = 0;
12
13  COND_INC(x == 0, x, y);
14  COND_INC(x == 0, x, y);
15}

The two macro invocations actually expand as:

13    do { if (x == 0) (x)++; else (y)++; } while(0);
14    do { if (x == 0) (x)++; else (y)++; } while(0);

The expanded version is the basis of SCO identification process, so we have one decision and two conditioned statements on line 13, likewise on line 14. Only one of each is exercised at execution time, and a stmt+decision analysis on this program yields:

2.1. STMT COVERAGE
------------------

t.c:13:32: statement not executed
t.c:14:20: statement not executed

2 violations.

2.2. DECISION COVERAGE
----------------------

t.c:13:12: decision outcome FALSE never exercised
t.c:14:12: decision outcome TRUE never exercised

2 violations.

We do see one statement and one decision coverage violation per invocation, different in the two cases since the x == 0 test is True on the first call and False on the second one. We also observe column numbers unrelated to what the original source lines contain on line 13 and 14.

1.5.15. Optimization and non-coverable items

GNATcoverage essentially operates by relating execution traces to source entities of interest thanks to debug information mapping machine code addresses to source locations. With optimization enabled, there sometimes is no machine code attached to a given statement, for example when the statement is determined to be redundant or when the machine code for it can be factorized with the machine code for another statement. When the coverage status of a code-less statement cannot be be inferred from that of other statements around, GNATcoverage categorizes the statement as non-coverable.

By default, nothing is said about non-coverable statements in the =report outputs and the corresponding lines are marked with a ‘.’ in annnotated sources, as for any other line to which no machine code is attached. Below is an example source annotated for statement coverage, where absence of code for a couple of Ada statments was triggered by constant propagation and inlining. The local Pos function is called only once, with a constant argument such that only one alternative of the if statement is taken. With -O1 -gnatn, the compiler sees that the else part can never be entered and no code is emitted at all for this alternative:

 4 .: procedure Test_Pos1 is
 5 .:    function Pos (X : Integer) return Boolean;
 6 .:    pragma Inline (Pos);
 7 .:
 8 .:    function Pos (X : Integer) return Boolean is
 9 .:    begin
10 +:       if X > 0 then
11 +:          Put_Line ("X is positive");
12 +:          return True;
13 .:       else
14 .:          Put_Line ("X is not positive");
15 .:          return False;
16 .:       end if;
17 .:    end Pos;
18 .: begin
19 +:    Assert (Pos (1) = True);
20 .: end Test_Pos1;

A common similar case is that of debugging code inhibited on purpose for regular operation, for example with constructs like:

Debug_Mode : constant Boolean := False;    or     #define DEBUG_MODE 0
...                                               ...
if Debug_Mode then                                #if DEBUG_MODE

Another way to get this in Ada is with generic instanciations where constant parameters turn what appears to be conditional in the source into a constant value in some instances.

Back to our Test_Pos1 example, no code is emitted for the test on line 10 either. gnatcov is however able to infer the if coverage status by looking at the status of statements controlled by the decision, and the Decision coverage report remains accurate:

   8 .:    function Pos (X : Integer) return Boolean is
   9 .:    begin
  10 !:       if X > 0 then
decision "X > 0" at 10:10 outcome FALSE never exercised
  11 +:          Put_Line ("X is positive");
  12 +:          return True;

gnatcov coverage features the --non-coverable command line option to expose the non-coverable statements if needed. They are listed in an additional “NON COVERABLE ITEMS” section of the =report outputs and the corresponding lines are flagged with a ‘0’ mark in annotated sources, as well as a specific color in the html formats. For our example, this yields:

10 !:       if X > 0 then
11 +:          Put_Line ("X is positive");
12 +:          return True;
13 .:       else
14 0:          Put_Line ("X is not positive");
15 0:          return False;
16 .:       end if;