code.delx.au - gnu-emacs/blob - admin/notes/exit-value

   1 ttn 2004-05-09
   2
   3 The exit value of a program returning to the shell on unixoid systems is
   4 typically 0 for success, and non-0 (such as 1) for failure.  For vms it is
   5 odd (1,3,5...) for success, even (0,2,4...) for failure.
   6
   7 This holds from the point of view of the "shell" (in quotes because vms has a
   8 different dispatch model that is not explained further here).
   9
  10 From the point of view of the program, nowadays stdlib.h on both type of
  11 systems provides macros `EXIT_SUCCESS' and `EXIT_FAILURE' that should DTRT.
  12
  13 NB: The numerical values of these macros DO NOT need to fulfill the exit
  14 value requirements outlined in the first paragraph!  That is the job of the
  15 `exit' function.  Thus, this kind of construct shows misunderstanding:
  16
  17    #ifdef VMS
  18       exit (1);
  19    #else
  20       exit (0);
  21    #endif
  22
  23 Values aside from EXIT_SUCCESS and EXIT_FAILURE are tricky.
  24
  25
  26
  27 ttn 2004-05-12
  28
  29 Values aside from EXIT_SUCCESS and EXIT_FAILURE can be used to indicate
  30 finer gradations of failure.  If this is the only information available
  31 to the caller, clamping such values to EXIT_FAILURE loses information.
  32 If there are other ways to indicate the problem to the caller (such as
  33 a message to stderr) it may be ok to clamp.  In all cases, it is the
  34 relationship between the program and its caller that must be examined.
  35 [Insert ZAMM quote here.]