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Implementation of Profiling

   Profiling works by changing how every function in your program is
compiled so that when it is called, it will stash away some information
about where it was called from.  From this, the profiler can figure out
what function called it, and can count how many times it was called.
This change is made by the compiler when your program is compiled with
the `-pg' option.

   Profiling also involves watching your program as it runs, and
keeping a histogram of where the program counter happens to be every
now and then.  Typically the program counter is looked at around 100
times per second of run time, but the exact frequency may vary from
system to system.

   A special startup routine allocates memory for the histogram and
sets up a clock signal handler to make entries in it.  Use of this
special startup routine is one of the effects of using `gcc ... -pg' to
link.  The startup file also includes an `exit' function which is
responsible for writing the file `gmon.out'.

   Number-of-calls information for library routines is collected by
using a special version of the C library.  The programs in it are the
same as in the usual C library, but they were compiled with `-pg'.  If
you link your program with `gcc ... -pg', it automatically uses the
profiling version of the library.

   The output from `gprof' gives no indication of parts of your program
that are limited by I/O or swapping bandwidth.  This is because samples
of the program counter are taken at fixed intervals of run time.
Therefore, the time measurements in `gprof' output say nothing about
time that your program was not running.  For example, a part of the
program that creates so much data that it cannot all fit in physical
memory at once may run very slowly due to thrashing, but `gprof' will
say it uses little time.  On the other hand, sampling by run time has
the advantage that the amount of load due to other users won't directly
affect the output you get.

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