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 32.1 General Capacity Limits
 ============================
 
 The POSIX.1 and POSIX.2 standards specify a number of parameters that
 describe capacity limitations of the system.  These limits can be fixed
 constants for a given operating system, or they can vary from machine to
 machine.  For example, some limit values may be configurable by the
 system administrator, either at run time or by rebuilding the kernel,
 and this should not require recompiling application programs.
 
    Each of the following limit parameters has a macro that is defined in
 'limits.h' only if the system has a fixed, uniform limit for the
 parameter in question.  If the system allows different file systems or
 files to have different limits, then the macro is undefined; use
 'sysconf' to find out the limit that applies at a particular time on a
 particular machine.   Sysconf.
 
    Each of these parameters also has another macro, with a name starting
 with '_POSIX', which gives the lowest value that the limit is allowed to
 have on _any_ POSIX system.   Minimums.
 
  -- Macro: int ARG_MAX
      If defined, the unvarying maximum combined length of the ARGV and
      ENVIRON arguments that can be passed to the 'exec' functions.
 
  -- Macro: int CHILD_MAX
      If defined, the unvarying maximum number of processes that can
      exist with the same real user ID at any one time.  In BSD and GNU,
      this is controlled by the 'RLIMIT_NPROC' resource limit; 
      Limits on Resources.
 
  -- Macro: int OPEN_MAX
      If defined, the unvarying maximum number of files that a single
      process can have open simultaneously.  In BSD and GNU, this is
      controlled by the 'RLIMIT_NOFILE' resource limit;  Limits on
      Resources.
 
  -- Macro: int STREAM_MAX
      If defined, the unvarying maximum number of streams that a single
      process can have open simultaneously.   Opening Streams.
 
  -- Macro: int TZNAME_MAX
      If defined, the unvarying maximum length of a time zone name.
       Time Zone Functions.
 
    These limit macros are always defined in 'limits.h'.
 
  -- Macro: int NGROUPS_MAX
      The maximum number of supplementary group IDs that one process can
      have.
 
      The value of this macro is actually a lower bound for the maximum.
      That is, you can count on being able to have that many
      supplementary group IDs, but a particular machine might let you
      have even more.  You can use 'sysconf' to see whether a particular
      machine will let you have more ( Sysconf).
 
  -- Macro: ssize_t SSIZE_MAX
      The largest value that can fit in an object of type 'ssize_t'.
      Effectively, this is the limit on the number of bytes that can be
      read or written in a single operation.
 
      This macro is defined in all POSIX systems because this limit is
      never configurable.
 
  -- Macro: int RE_DUP_MAX
      The largest number of repetitions you are guaranteed is allowed in
      the construct '\{MIN,MAX\}' in a regular expression.
 
      The value of this macro is actually a lower bound for the maximum.
      That is, you can count on being able to have that many repetitions,
      but a particular machine might let you have even more.  You can use
      'sysconf' to see whether a particular machine will let you have
      more ( Sysconf).  And even the value that 'sysconf' tells
      you is just a lower bound--larger values might work.
 
      This macro is defined in all POSIX.2 systems, because POSIX.2 says
      it should always be defined even if there is no specific imposed
      limit.
 

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