2 Compiling a Program for Profiling

The first step in generating profile information for your program is to compile and link it with profiling enabled.

To compile a source file for profiling, specify the ‘-pg’ option when you run the compiler. (This is in addition to the options you normally use.)

To link the program for profiling, if you use a compiler such as cc to do the linking, simply specify ‘-pg’ in addition to your usual options. The same option, ‘-pg’, alters either compilation or linking to do what is necessary for profiling. Here are examples:

cc -g -c myprog.c utils.c -pg
cc -o myprog myprog.o utils.o -pg

The ‘-pg’ option also works with a command that both compiles and links:

cc -o myprog myprog.c utils.c -g -pg

Note: The ‘-pg’ option must be part of your compilation options as well as your link options. If it is not then no call-graph data will be gathered and when you run gprof you will get an error message like this:

gprof: gmon.out file is missing call-graph data

If you add the ‘-Q’ switch to suppress the printing of the call graph data you will still be able to see the time samples:

Flat profile:
Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  Ts/call  Ts/call  name
 44.12      0.07     0.07                             zazLoop
 35.29      0.14     0.06                             main
 20.59      0.17     0.04                             bazMillion

If you run the linker ld directly instead of through a compiler such as cc, you may have to specify a profiling startup file gcrt0.o as the first input file instead of the usual startup file crt0.o. In addition, you would probably want to specify the profiling C library, libc_p.a, by writing ‘-lc_p’ instead of the usual ‘-lc’. This is not absolutely necessary, but doing this gives you number-of-calls information for standard library functions such as read and open. For example:

ld -o myprog /lib/gcrt0.o myprog.o utils.o -lc_p

If you are running the program on a system which supports shared libraries you may run into problems with the profiling support code in a shared library being called before that library has been fully initialised. This is usually detected by the program encountering a segmentation fault as soon as it is run. The solution is to link against a static version of the library containing the profiling support code, which for gcc users can be done via the ‘-static’ or ‘-static-libgcc’ command line option. For example:

gcc -g -pg -static-libgcc myprog.c utils.c -o myprog

If you compile only some of the modules of the program with ‘-pg’, you can still profile the program, but you won’t get complete information about the modules that were compiled without ‘-pg’. The only information you get for the functions in those modules is the total time spent in them; there is no record of how many times they were called, or from where. This will not affect the flat profile (except that the calls field for the functions will be blank), but will greatly reduce the usefulness of the call graph.

If you wish to perform line-by-line profiling you should use the gcov tool instead of gprof. See that tool’s manual or info pages for more details of how to do this.

Note, older versions of gcc produce line-by-line profiling information that works with gprof rather than gcov so there is still support for displaying this kind of information in gprof. See Line-by-line Profiling.

It also worth noting that gcc implements a ‘-finstrument-functions’ command line option which will insert calls to special user supplied instrumentation routines at the entry and exit of every function in their program. This can be used to implement an alternative profiling scheme.