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. See also Terminal Ctrl characters

In order to exit gracefully for a programm with an infintite main loop, eg a signal handler for TERM could be installed. See Linux Signals

gprof foo gmon.out

Writes ACII to stdout for review.
It shows that different gmon.out produces from the same binary produce different results. GLAP p.628 proposes to produce multiple gmon.out's, build the average and review this:

gprof --sum foo gmon.out.?
gprof foo gmon.sum > gmon.sum.txt