gcov - test coverage program
gcov [-b] [-v] [-n] [-l] [-f] [-o directory] sourcefile
The gcov utility is a test coverage program. Use it in concert with
gcc(1) to analyze programs to help create more efficient,
faster running
code. gcov can be used as a profiling tool to help discover
where optimization
efforts will best affect the code. gcov can also
be used along
with the other profiling tool gprof(1), to assess which
parts of the code
use the greatest amount of computing time.
Profiling tools help analyze the code's performance. Using
a profiler
such as gcov or gprof(1), basic performance statistics can
be obtained,
such as:
+o how often each line of code executes
+o what lines of code are actually executed
+o 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 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.
Code should be compiled without optimization when using gcov
because the
optimization, by combining some lines of code into one function, may not
give as much information as necessary 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
complicated macros that expand to loops or to other control
structures
are used, the statistics are less helpful - they only report
on the line
where the macro call appears. If complex macros behave like
functions,
they can be replaced 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. These logfiles
can then be used along with gprof(1) to aid in finetuning the performance
of the programs. gprof(1) gives timing information
which can be
used along with the information you obtained from gcov.
gcov works only on code compiled with GNU CC. It is not
compatible with
any other profiling or test coverage mechanism.
gcov accepts the following options:
-b Write branch frequencies to the output file, and
write branch
summary info to the standard output. This option
indicates how
often each branch in the program was taken.
-v Display the gcov version number (on the standard error stream).
-n Do not create the gcov output file.
-l 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.
-f Output summaries for each function in addition to
the file level
summary.
-o directory
The directory where the object files live. gcov
will search for
.bb, .bbg, and .da files in this directory.
When using gcov, programs must first be compiled with two
special GNU CC
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 source
code is located.
Running the program will cause profile output to be generated. For each
source file compiled with -fprofile-arcs, an accompanying
.da file will
be placed in the source directory.
Running gcov with the 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 the program is called tmp.c, this is
what is displayed
when using the basic gcov facility:
$ gcc -fprofile-arcs -ftest-coverage tmp.c
$ a.out
$ gcov tmp.c
87.50% of 8 source lines executed in file tmp.c
Creating tmp.c.gcov.
The file tmp.c.gcov contains output from gcov. Here is a
sample:
main()
{
1 int i, total;
1 total = 0;
11 for (i = 0; i < 10; i++)
10 total += i;
1 if (total != 45)
###### printf ("Failure0);
else
1 printf ("Success0);
1 }
When the -b option is used, output looks like this:
$ gcov -b tmp.c
87.50% of 8 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:
main()
{
1 int i, total;
1 total = 0;
11 for (i = 0; i < 10; i++)
branch 0 taken = 91%
branch 1 taken = 100%
branch 2 taken = 100%
10 total += i;
1 if (total != 45)
branch 0 taken = 100%
###### printf ("Failure0);
call 0 never executed
branch 1 never executed
else
1 printf ("Success0);
call 0 returns = 100%
1 }
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 which 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 .da 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 .da 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 .da 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 [Toc] [Back] If gcov is to be used to help optimize code, programs must
be compiled
with two special GNU CC options: -fprofile-arcs
-ftest-coverage. Aside
from that, any other GNU CC options can be used; 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 the program is compiled with optimization:
100 if (a != b)
100 c = 1;
100 else
100 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.
BRIEF DESCRIPTION OF GCOV DATA FILES [Toc] [Back] gcov uses three files for doing profiling. The names of
these files are
derived from the original _source_ file by substituting the
file suffix
with either .bb, .bbg, or .da. All of these files are
placed in the same
directory as the source file, and contain data stored in a
platform-independent
method.
The .bb and .bbg files are generated when the source file is
compiled
with the GNU CC -ftest-coverage option. The .bb file contains a list of
source files (including headers), functions within those
files, and line
numbers corresponding to each basic block in the source
file.
The .bb file format consists of several lists of 4-byte integers which
correspond to the line numbers of each basic block in the
file. Each
list is terminated by a line number of 0. A line number of
-1 is used to
designate that the source file name (padded to a 4-byte
boundary and followed
by another -1) follows. In addition, a line number of
-2 is used
to designate that the name of a function (also padded to a
4-byte boundary
and followed by a -2) follows.
The .bbg file is used to reconstruct the program flow graph
for the
source file. It contains a list of the program flow arcs
(possible
branches taken from one basic block to another) for each
function which,
in combination with the .bb file, enables gcov to reconstruct the program
flow.
In the .bbg file, the format is:
number of basic blocks for function #0 (4-byte number)
total number of arcs for function #0 (4-byte number)
count of arcs in basic block #0 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
destination basic block of arc #1 (4-byte number)
flag bits (4-byte number)
...
destination basic block of arc #N (4-byte number)
flag bits (4-byte number)
count of arcs in basic block #1 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
...
A -1 (stored as a 4-byte number) is used to separate each
function's list
of basic blocks, and to verify that the file has been read
correctly.
The .da file is generated when a program containing object
files built
with the GNU CC -fprofile-arcs option is executed. A separate .da file
is created for each source file compiled with this option,
and the name
of the .da file is stored as an absolute pathname in the resulting object
file. This path name is derived from the source file name
by substituting
a .da suffix.
The format of the .da file is fairly simple. The first
8-byte number is
the number of counts in the file, followed by the counts
(stored as
8-byte numbers). Each count corresponds to the number of
times each arc
in the program is executed. The counts are cumulative; each
time the
program is executed, it attempts to combine the existing .da
files with
the new counts for this invocation of the program. It ignores the contents
of any .da files whose number of arcs doesn't correspond to the
current program, and merely overwrites them instead.
All three of these files use the functions in gcov-io.h to
store integers;
the functions in this header provide a machine-independent mechanism
for storing and retrieving data from a stream.
gcc(1), gcc-local(1), gprof(1)
This man page describes version 1.5 of gcov.
OpenBSD 3.6 February 15, 2003
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