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
     
      %         the percentage of the total running time of the

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 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 will also need to specify the -g option, instructing the compiler to insert debugging symbols into the program that match program addresses to source code lines. See Line-by-line.

In addition to the -pg and -g options, older versions of GCC required you to specify the -a option when compiling in order to instrument it to perform basic-block counting. Newer versions do not require this option and will not accept it; basic-block counting is always enabled when -pg is on.

When basic-block counting is enabled, as the program runs it will count how many times it executed each branch of each if statement, each iteration of each do loop, etc. This will enable gprof to construct an annotated source code listing showing how many times each line of code was executed.

It also worth noting that GCC supports a different profiling method which is enabled by the -fprofile-arcs, -ftest-coverage and -fprofile-values switches. These switches do not produce data which is useful to gprof however, so they are not discussed further here. There is also the -finstrument-functions switch which will cause GCC to 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.