gcov - Unix, Linux Command

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:


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.


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.


The 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.




        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.




Below are the assumptions made
1. Programs must be compiled using gcc only to use gcov
2. gcov must be invoked in the same directory as that where the source was compiled.
#include 
int main()
{
        int cond = 1;
        if ( cond ) {
                printf("Condition is true...!\n");
        } else {
                printf("Condition is false...!\n");
                return(1);
        }
        printf("End of program\n");
        return(0);
}

$ gcc -fprofile-arcs -ftest-coverage -o Example Example.c

$ ls
Example  Example.c  Example.gcno

$ ./Example
Condition is true...!
End of program

$ ls
Example  Example.c  Example.gcda  Example.gcno

$ gcov -b -c Example.c
File 'Example.c'
Lines executed:75.00% of 8
Branches executed:100.00% of 2
Taken at least once:50.00% of 2
Calls executed:66.67% of 3
Example.c:creating 'Example.c.gcov'

$ cat Example.c.gcov
        -:    0:Source:Example.c
        -:    0:Graph:Example.gcno
        -:    0:Data:Example.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        -:    1:#include 
        -:    2:
        -:    3:int main()
function main called 1 returned 100% blocks executed 71%
        1:    4:{
        1:    5:        int cond = 1;
        -:    6:
        1:    7:        if ( cond ) {
branch  0 taken 1 (fallthrough)
branch  1 taken 0
        1:    8:                printf("Condition is true...!\n");
call    0 returned 1
        -:    9:        } else {
    #####:   10:                printf("Condition is false...!\n");
call    0 never executed
    #####:   11:                return(1);
        -:   12:        }
        -:   13:
        1:   14:        printf("End of program\n");
call    0 returned 1
        -:   15:
        1:   16:        return(0);
        -:   17:}


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