To collect profiling information, a program must be compiled and linked with profiling enabled. To accomplish this, the command-line option -pg is passed to the compiler. For example, if our source program was ss.cxx , we would use the following command:
linux$ g++ ss.cxx -pg -o ss
If your program consists of separate source files, which are compiled individually, each intermediate compilation should be passed the -pg option if you want to profile the functions found in the source file. If the debug command-line option -g is also specified, the gprof utility will generate additional line-by-line profiling information. This technique is discussed in detail at the end of this appendix.
Once successfully compiled, the program is run using its normal arguments, input files, and so on. The profiling data generated by the program's execution is stored in a file called gmon.out in the current directory. Should there be an existing gmon.out file, it is overwritten. Note that the gmon.out file will not be produced if the program does not exit normally (i.e., does not return from main or call exit ).  The gmon.out file is a data file not a plain text file and as such should not be directly printed or displayed on the screen.
 In this case calling _exit would not work, as it does not perform the cleanup activities that a call to exit does.