gcov.texi

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The file @file{tmp.c.gcov} contains output from @command{gcov}.
Here is a sample:

@smallexample
        -:    0:Source:tmp.c
        -:    0:Graph:tmp.gcno
        -:    0:Data:tmp.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        -:    1:#include <stdio.h>
        -:    2:
        -:    3:int main (void)
        1:    4:@{
        1:    5:  int i, total;
        -:    6:
        1:    7:  total = 0;
        -:    8:
       11:    9:  for (i = 0; i < 10; i++)
       10:   10:    total += i;
        -:   11:
        1:   12:  if (total != 45)
    #####:   13:    printf ("Failure\n");
        -:   14:  else
        1:   15:    printf ("Success\n");
        1:   16:  return 0;
        -:   17:@}
@end smallexample

When you use the @option{-a} option, you will get individual block
counts, and the output looks like this:

@smallexample
        -:    0:Source:tmp.c
        -:    0:Graph:tmp.gcno
        -:    0:Data:tmp.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        -:    1:#include <stdio.h>
        -:    2:
        -:    3:int main (void)
        1:    4:@{
        1:    4-block  0
        1:    5:  int i, total;
        -:    6:
        1:    7:  total = 0;
        -:    8:
       11:    9:  for (i = 0; i < 10; i++)
       11:    9-block  0
       10:   10:    total += i;
       10:   10-block  0
        -:   11:
        1:   12:  if (total != 45)
        1:   12-block  0
    #####:   13:    printf ("Failure\n");
    $$$$$:   13-block  0
        -:   14:  else
        1:   15:    printf ("Success\n");
        1:   15-block  0
        1:   16:  return 0;
        1:   16-block  0
        -:   17:@}
@end smallexample

In this mode, each basic block is only shown on one line -- the last
line of the block.  A multi-line block will only contribute to the
execution count of that last line, and other lines will not be shown
to contain code, unless previous blocks end on those lines.
The total execution count of a line is shown and subsequent lines show
the execution counts for individual blocks that end on that line.  After each
block, the branch and call counts of the block will be shown, if the
@option{-b} option is given.

Because of the way GCC instruments calls, a call count can be shown
after a line with no individual blocks.
As you can see, line 13 contains a basic block that was not executed.

@need 450
When you use the @option{-b} option, your output looks like this:

@smallexample
$ gcov -b tmp.c
90.00% of 10 source lines executed in file tmp.c
80.00% of 5 branches executed in file tmp.c
80.00% of 5 branches taken at least once in file tmp.c
50.00% of 2 calls executed in file tmp.c
Creating tmp.c.gcov.
@end smallexample

Here is a sample of a resulting @file{tmp.c.gcov} file:

@smallexample
        -:    0:Source:tmp.c
        -:    0:Graph:tmp.gcno
        -:    0:Data:tmp.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        -:    1:#include <stdio.h>
        -:    2:
        -:    3:int main (void)
function main called 1 returned 1 blocks executed 75%
        1:    4:@{
        1:    5:  int i, total;
        -:    6:
        1:    7:  total = 0;
        -:    8:
       11:    9:  for (i = 0; i < 10; i++)
branch  0 taken 91% (fallthrough)
branch  1 taken 9%
       10:   10:    total += i;
        -:   11:
        1:   12:  if (total != 45)
branch  0 taken 0% (fallthrough)
branch  1 taken 100%
    #####:   13:    printf ("Failure\n");
call    0 never executed
        -:   14:  else
        1:   15:    printf ("Success\n");
call    0 called 1 returned 100%
        1:   16:  return 0;
        -:   17:@}
@end smallexample

For each function, a line is printed showing how many times the function
is called, how many times it returns and what percentage of the
function's blocks were executed.

For each basic block, a line is printed after the last line of the basic
block describing the branch or call that ends the basic block.  There can
be multiple branches and calls listed for a single source line if there
are multiple basic blocks that end on that line.  In this case, the
branches and calls are each given a number.  There is no simple way to map
these branches and calls back to source constructs.  In general, though,
the lowest numbered branch or call will correspond to the leftmost construct
on the source line.

For a branch, if it was executed at least once, then a percentage
indicating the number of times the branch was taken divided by the
number of times the branch was executed will be printed.  Otherwise, the
message ``never executed'' is printed.

For a call, if it was executed at least once, then a percentage
indicating the number of times the call returned divided by the number
of times the call was executed will be printed.  This will usually be
100%, but may be less for functions that call @code{exit} or @code{longjmp},
and thus may not return every time they are called.

The execution counts are cumulative.  If the example program were
executed again without removing the @file{.gcda} file, the count for the
number of times each line in the source was executed would be added to
the results of the previous run(s).  This is potentially useful in
several ways.  For example, it could be used to accumulate data over a
number of program runs as part of a test verification suite, or to
provide more accurate long-term information over a large number of
program runs.

The data in the @file{.gcda} files is saved immediately before the program
exits.  For each source file compiled with @option{-fprofile-arcs}, the
profiling code first attempts to read in an existing @file{.gcda} file; if
the file doesn't match the executable (differing number of basic block
counts) it will ignore the contents of the file.  It then adds in the
new execution counts and finally writes the data to the file.

@node Gcov and Optimization
@section Using @command{gcov} with GCC Optimization

If you plan to use @command{gcov} to help optimize your code, you must
first compile your program with two special GCC options:
@samp{-fprofile-arcs -ftest-coverage}.  Aside from that, you can use any
other GCC options; but if you want to prove that every single line
in your program was executed, you should not compile with optimization
at the same time.  On some machines the optimizer can eliminate some
simple code lines by combining them with other lines.  For example, code
like this:

@smallexample
if (a != b)
  c = 1;
else
  c = 0;
@end smallexample

@noindent
can be compiled into one instruction on some machines.  In this case,
there is no way for @command{gcov} to calculate separate execution counts
for each line because there isn't separate code for each line.  Hence
the @command{gcov} output looks like this if you compiled the program with
optimization:

@smallexample
      100:   12:if (a != b)
      100:   13:  c = 1;
      100:   14:else
      100:   15:  c = 0;
@end smallexample

The output shows that this block of code, combined by optimization,
executed 100 times.  In one sense this result is correct, because there
was only one instruction representing all four of these lines.  However,
the output does not indicate how many times the result was 0 and how
many times the result was 1.

Inlineable functions can create unexpected line counts.  Line counts are
shown for the source code of the inlineable function, but what is shown
depends on where the function is inlined, or if it is not inlined at all.

If the function is not inlined, the compiler must emit an out of line
copy of the function, in any object file that needs it.  If
@file{fileA.o} and @file{fileB.o} both contain out of line bodies of a
particular inlineable function, they will also both contain coverage
counts for that function.  When @file{fileA.o} and @file{fileB.o} are
linked together, the linker will, on many systems, select one of those
out of line bodies for all calls to that function, and remove or ignore
the other.  Unfortunately, it will not remove the coverage counters for
the unused function body.  Hence when instrumented, all but one use of
that function will show zero counts.

If the function is inlined in several places, the block structure in
each location might not be the same.  For instance, a condition might
now be calculable at compile time in some instances.  Because the
coverage of all the uses of the inline function will be shown for the
same source lines, the line counts themselves might seem inconsistent.

@c man end

@node Gcov Data Files
@section Brief description of @command{gcov} data files

@command{gcov} uses two files for profiling.  The names of these files
are derived from the original @emph{object} file by substituting the
file suffix with either @file{.gcno}, or @file{.gcda}.  All of these files
are placed in the same directory as the object file, and contain data
stored in a platform-independent format.

The @file{.gcno} file is generated when the source file is compiled with
the GCC @option{-ftest-coverage} option.  It contains information to
reconstruct the basic block graphs and assign source line numbers to
blocks.

The @file{.gcda} file is generated when a program containing object files
built with the GCC @option{-fprofile-arcs} option is executed.  A
separate @file{.gcda} file is created for each object file compiled with
this option.  It contains arc transition counts, and some summary
information.

The full details of the file format is specified in @file{gcov-io.h},
and functions provided in that header file should be used to access the
coverage files.

@node Cross-profiling
@section Data file relocation to support cross-profiling

Running the program will cause profile output to be generated.  For each 
source file compiled with @option{-fprofile-arcs}, an accompanying @file{.gcda} 
file will be placed in the object file directory. That implicitly requires 
running the program on the same system as it was built or having the same 
absolute directory structure on the target system. The program will try
to create the needed directory structure, if it is not already present.

To support cross-profiling, a program compiled with @option{-fprofile-arcs}
can relocate the data files based on two environment variables: 

@itemize @bullet
@item
GCOV_PREFIX contains the prefix to add to the absolute paths 
in the object file. Prefix must be absolute as well, otherwise its 
value is ignored. The default is no prefix.

@item
GCOV_PREFIX_STRIP indicates the how many initial directory names to strip off
the hardwired absolute paths. Default value is 0.

@emph{Note:} GCOV_PREFIX_STRIP has no effect if GCOV_PREFIX is undefined, empty
or non-absolute.
@end itemize

For example, if the object file @file{/user/build/foo.o} was built with
@option{-fprofile-arcs}, the final executable will try to create the data file
@file{/user/build/foo.gcda} when running on the target system.  This will
fail if the corresponding directory does not exist and it is unable to create
it.  This can be overcome by, for example, setting the environment as
@samp{GCOV_PREFIX=/target/run} and @samp{GCOV_PREFIX_STRIP=1}.  Such a
setting will name the data file @file{/target/run/build/foo.gcda}.

You must move the data files to the expected directory tree in order to
use them for profile directed optimizations (@option{--use-profile}), or to
use the @command{gcov} tool.