1. Manual according to Gprof
2. GLAP p.619

Both compiling an linking phase of GNU GCC must be telled to generate extra code to write profile information -pg:

gcc -o foo foo.c -pg
# or
gcc -c foo.c -pg
gcc -o foo foo.o -pg

What if external libs are used?

According to GLAP p.619 the profile data file gmon.out is only written upon normal exit of the application, not eg on exit with SIGKILL