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
Note: The ‘-pg’ option must be part of your compilation options
as well as your link options. If it is not then no call-graph data
will be gathered and when you run gprof you will get an error
message like this:
gprof: gmon.out file is missing call-graph data
If you add the ‘-Q’ switch to suppress the printing of the call graph data you will still be able to see the time samples:
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls Ts/call Ts/call name
44.12 0.07 0.07 zazLoop
35.29 0.14 0.06 main
20.59 0.17 0.04 bazMillion
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 are running the program on a system which supports shared
libraries you may run into problems with the profiling support code in
a shared library being called before that library has been fully
initialised. This is usually detected by the program encountering a
segmentation fault as soon as it is run. The solution is to link
against a static version of the library containing the profiling
support code, which for gcc users can be done via the
‘-static’ or ‘-static-libgcc’ command line option. For
example:
gcc -g -pg -static-libgcc myprog.c utils.c -o myprog
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 should use the
gcov tool instead of gprof. See that tool's manual or
info pages for more details of how to do this.
Note, older versions of gcc produce line-by-line profiling
information that works with gprof rather than gcov so
there is still support for displaying this kind of information in
gprof. See Line-by-line Profiling.
It also worth noting that gcc implements a
‘-finstrument-functions’ command line option which will insert
calls to special user supplied instrumentation routines at the entry
and exit of every function in their program. This can be used to
implement an alternative profiling scheme.