Sampling and Call Count instrumentation profiling

This type of profiling is a combination of sampling mode and Call Count instrumentation data, and it provides per line statistical coverage (as well as a call graph at the same time), with relatively small overhead.

To instrument a binary or library in this mode, use the -p option for both compiling and linking. The -p option for the compiler prepares the binary for profiling (the compiler will then insert code before each function to gather call information); however, it won't cause the profiling versions of the libraries to be linked in. To link in the profiling versions from the libc library, use the -p option for the linker.

If you compile and link with either the -pg or -p option, when the executable program runs, either gprof or prof monitors the program and produces a report file called gmon.out. The gprof utility can't report information about program calls to routines from a precompiled library (such as libc) that weren't compiled with the -pg option. Consequently, the resulting profiling information won't include data about calls made to those routines (for example printf()).

If most of the execution time occurs in various library routines, then this fact will likely reduce the value of the profiling results, since there is no indication in the results of where the call was made. In this case, you can use Function Instrumentation profiling, which causes this additional time to be charged to the higher-level routine that called the library function.