gcov
coverage testing tool
see also :
gcc
Synopsis
gcov
[-v|--version]
[-h|--help]
[-a|--all-blocks]
[-b|--branch-probabilities]
[-c|--branch-counts]
[-u|--unconditional-branches]
[-n|--no-output]
[-l|--long-file-names]
[-p|--preserve-paths]
[-r|--relative-only]
[-f|--function-summaries]
[-o|--object-directory
directory|file]
[-s|--source-prefix
directory]
[-d|--display-progress]
files
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description
gcov
is a test coverage program. Use it in concert with
GCC to analyze your programs to help create
more efficient, faster running code and to discover untested
parts of your program. You can use gcov as a
profiling tool to help discover where your optimization
efforts will best affect your code. You can also use
gcov along with the other profiling tool,
gprof, to assess which parts of your code use the
greatest amount of computing time.
Profiling tools
help you analyze your code’s performance. Using a
profiler such as gcov or gprof, you can find
out some basic performance statistics, such as:
•
how often each line of code executes
•
what lines of code are actually executed
•
how much computing time each section of code uses
Once you know
these things about how your code works when compiled, you
can look at each module to see which modules should be
optimized. gcov helps you determine where to work on
optimization.
Software
developers also use coverage testing in concert with
testsuites, to make sure software is actually good enough
for a release. Testsuites can verify that a program works as
expected; a coverage program tests to see how much of the
program is exercised by the testsuite. Developers can then
determine what kinds of test cases need to be added to the
testsuites to create both better testing and a better final
product.
You should
compile your code without optimization if you plan to use
gcov because the optimization, by combining some
lines of code into one function, may not give you as much
information as you need to look for ’hot spots’
where the code is using a great deal of computer time.
Likewise, because gcov accumulates statistics by line
(at the lowest resolution), it works best with a programming
style that places only one statement on each line. If you
use complicated macros that expand to loops or to other
control structures, the statistics are less
helpful---they only report on the line
where the macro call appears. If your complex macros behave
like functions, you can replace them with inline functions
to solve this problem.
gcov
creates a logfile called sourcefile.gcov which
indicates how many times each line of a source file
sourcefile.c has executed. You can use these logfiles
along with gprof to aid in fine-tuning the
performance of your programs. gprof gives timing
information you can use along with the information you get
from gcov.
gcov
works only on code compiled with GCC . It is
not compatible with any other profiling or test coverage
mechanism.
options
--help
Display help about using
gcov (on the standard output), and exit without doing
any further processing.
--version
Display the gcov version
number (on the standard output), and exit without doing any
further processing.
--all-blocks
Write individual execution
counts for every basic block. Normally gcov outputs
execution counts only for the main blocks of a line. With
this option you can determine if blocks within a single line
are not being executed.
--branch-probabilities
Write branch frequencies to the
output file, and write branch summary info to the standard
output. This option allows you to see how often each branch
in your program was taken. Unconditional branches will not
be shown, unless the -u option is given.
--branch-counts
Write branch frequencies as the
number of branches taken, rather than the percentage of
branches taken.
--no-output
Do not create the gcov
output file.
--long-file-names
Create long file names for
included source files. For example, if the header file
x.h contains code, and was included in the file
a.c, then running gcov on the file a.c
will produce an output file called a.c##x.h.gcov
instead of x.h.gcov. This can be useful if x.h
is included in multiple source files and you want to see the
individual contributions. If you use the -p
option, both the including and included file names will be
complete path names.
--preserve-paths
Preserve complete path
information in the names of generated .gcov files.
Without this option, just the filename component is used.
With this option, all directories are used, with /
characters translated to # characters, .
directory components removed and unremoveable ..
components renamed to ^. This is useful if
sourcefiles are in several different directories.
--relative-only
Only output information about
source files with a relative pathname (after source prefix
elision). Absolute paths are usually system header files and
coverage of any inline functions therein is normally
uninteresting.
--function-summaries
Output summaries for each
function in addition to the file level summary.
-o
directory|file
--object-directory directory
--object-file file
Specify either the directory
containing the gcov data files, or the object path name. The
.gcno, and .gcda data files are searched for
using this option. If a directory is specified, the data
files are in that directory and named after the input file
name, without its extension. If a file is specified here,
the data files are named after that file, without its
extension.
-s
directory
--source-prefix directory
A prefix for source file names
to remove when generating the output coverage files. This
option is useful when building in a separate directory, and
the pathname to the source directory is not wanted when
determining the output file names. Note that this prefix
detection is applied before determining whether the source
file is absolute.
--unconditional-branches
When branch probabilities are
given, include those of unconditional branches.
Unconditional branches are normally not interesting.
--display-progress
Display the progress on the
standard output.
gcov
should be run with the current directory the same as that
when you invoked the compiler. Otherwise it will not be able
to locate the source files. gcov produces files
called mangledname.gcov in the current directory.
These contain the coverage information of the source file
they correspond to. One .gcov file is produced for
each source (or header) file containing code, which was
compiled to produce the data files. The mangledname
part of the output file name is usually simply the source
file name, but can be something more complicated if the
-l or -p options are given. Refer
to those options for details.
If you invoke
gcov with multiple input files, the contributions
from each input file are summed. Typically you would invoke
it with the same list of files as the final link of your
executable.
The
.gcov files contain the : separated fields
along with program source code. The format is
<execution_count>:<line_number>:<source line text>
Additional
block information may succeed each line, when requested by
command line option. The execution_count is
- for lines containing no code. Unexecuted
lines are marked ##### or ====, depending on
whether they are reachable by non-exceptional paths or only
exceptional paths such as C ++ exception
handlers, respectively.
Some lines of
information at the start have line_number of zero.
These preamble lines are of the form
-:0:<tag>:<value>
The ordering
and number of these preamble lines will be augmented as
gcov development progresses --- do
not rely on them remaining unchanged. Use tag to
locate a particular preamble line.
The additional
block information is of the form
<tag> <information>
The
information is human readable, but designed to be
simple enough for machine parsing too.
When printing
percentages, 0% and 100% are only printed when the values
are exactly 0% and 100% respectively. Other values
which would conventionally be rounded to 0% or 100% are
instead printed as the nearest non-boundary value.
When using
gcov, you must first compile your program with two
special GCC options:
-fprofile-arcs
-ftest-coverage. This tells the compiler to
generate additional information needed by gcov (basically a
flow graph of the program) and also includes additional code
in the object files for generating the extra profiling
information needed by gcov. These additional files are
placed in the directory where the object file is
located.
Running the
program will cause profile output to be generated. For each
source file compiled with -fprofile-arcs,
an accompanying .gcda file will be placed in the
object file directory.
Running
gcov with your program’s source file names as
arguments will now produce a listing of the code along with
frequency of execution for each line. For example, if your
program is called tmp.c, this is what you see when
you use the basic gcov facility:
$ gcc -fprofile-arcs -ftest-coverage tmp.c
$ a.out
$ gcov tmp.c
90.00% of 10 source lines executed in file tmp.c
Creating tmp.c.gcov.
The file
tmp.c.gcov contains output from gcov. Here is
a sample:
-: 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:}
When you use
the -a option, you will get individual block
counts, and the output looks like this:
-: 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:}
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 -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.
When you use
the -b option, your output looks like this:
$ 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.
Here is a
sample of a resulting tmp.c.gcov file:
-: 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:}
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
"exit" or "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 .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
.gcda files is saved immediately before the program
exits. For each source file compiled with
-fprofile-arcs, the profiling code first
attempts to read in an existing .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.
Using gcov
with GCC Optimization
If you plan to
use gcov to help optimize your code, you must first
compile your program with two special GCC
options: -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:
if (a != b)
c = 1;
else
c = 0;
can be compiled
into one instruction on some machines. In this case, there
is no way for gcov to calculate separate execution
counts for each line because there isn’t separate code
for each line. Hence the gcov output looks like this
if you compiled the program with optimization:
100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;
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
fileA.o and fileB.o both contain out of line
bodies of a particular inlineable function, they will also
both contain coverage counts for that function. When
fileA.o and 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.
copyright
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2005, 2008, 2010 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free
Documentation License, Version 1.3 or any later version published
by the Free Software Foundation; with the Invariant Sections
being " GNU General Public License" and "Funding
Free Software", the Front-Cover texts being (a) (see below), and
with the Back-Cover Texts being (b) (see below). A copy of the
license is included in the gfdl(7) man page.
(a) The FSF ’s Front-Cover Text is:
A GNU Manual
(b) The FSF ’s Back-Cover Text is:
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funds for GNU development.
see also
gpl,
gfdl, fsf-funding, gcc
and the Info entry for gcc.