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gcov
---a Test Coverage Program
gcov
is a tool you can use in conjunction with GCC to
test code coverage in your programs.
9.1 Introduction to gcov
Introduction to gcov. 9.2 Invoking gcov How to use gcov. 9.3 Using gcov
with GCC OptimizationUsing gcov with GCC optimization. 9.4 Brief description of gcov
data filesThe files used by gcov.
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gcov
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:
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.
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gcov [options] sourcefile |
gcov
accepts the following options:
-h
--help
gcov
(on the standard output), and
exit without doing any further processing.
-v
--version
gcov
version number (on the standard output),
and exit without doing any further processing.
-a
--all-blocks
-b
--branch-probabilities
-c
--branch-counts
-n
--no-output
gcov
output file.
-l
--long-file-names
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. If you uses the `-p' option, both the including and
included file names will be complete path names.
-p
--preserve-paths
-f
--function-summaries
-o directory|file
--object-directory directory
--object-file file
-u
--unconditional-branches
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 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.
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 and `#####' for lines which were never executed. Some lines of information at the start have line_number of zero.
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) 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++) 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) function main called 1 returned 1 blocks executed 75% 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 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 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.
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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.
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gcov
data files
gcov
uses two files for profiling. The names of these files
are derived from the original object file by substituting the
file suffix with either `.gcno', or `.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 `.gcno' file is generated when the source file is compiled with the GCC `-ftest-coverage' option. It contains information to reconstruct the basic block graphs and assign source line numbers to blocks.
The `.gcda' file is generated when a program containing object files built with the GCC `-fprofile-arcs' option is executed. A separate `.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 `gcov-io.h', and functions provided in that header file should be used to access the coverage files.
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