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 2 Compiling a Program for Profiling
 ***********************************
 
 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'. 
 Line-by-line Profiling Line-by-line.
 
    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.
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