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

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 section 5.3 Line-by-line Profiling.

In addition to the `-pg' and `-g' options, you may also wish to specify the `-a' option when compiling. This will instrument the program to perform basic-block counting. 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.