varargs(3) — bsd

VARARGS(3)                           BSD                            VARARGS(3)



NAME
     varargs - variable argument list

SYNOPSIS
     #include <varargs.h>

     function(va_alist)
     va_dcl
     va_list pvar;
     va_start(pvar);
     f = va_arg(pvar, type);
     va_end(pvar);

DESCRIPTION
     This set of macros provides a means of writing portable procedures that
     accept variable argument lists.  Routines having variable argument lists
     (such as printf(3)) that do not use varargs are inherently nonportable,
     since different machines use different argument passing conventions.

     va_alist is used in a function header to declare a variable argument
     list.

     va_dcl is a declaration for va_alist.  Note that there is no semicolon
     after va_dcl.

     va_list is a type which can be used for the variable pvar, which is used
     to traverse the list.  One such variable must always be declared.

     va_start(pvar) is called to initialize pvar to the beginning of the list.

     va_arg(pvar, type) will return the next argument in the list pointed to
     by pvar.  Type is the type to which the expected argument will be
     converted when passed as an argument.  In standard C, arguments that are
     char or short should be accessed as int, unsigned char or unsigned short
     are converted to unsigned int, and float arguments are converted to
     double.  Different types can be mixed, but it is up to the routine to
     know what type of argument is expected, since it cannot be determined at
     runtime.

     va_end(pvar) is used to finish up.

     Multiple traversals, each bracketed by va_start ...  va_end, are
     possible.

EXAMPLE
          #include <varargs.h>
          execl(va_alist)
          va_dcl
          {
               va_list ap;
               char *file;
               char *args[100];
               int argno = 0;

               va_start(ap);
               file = va_arg(ap, char *);
               while (args[argno++] = va_arg(ap, char *))
                    ;
               va_end(ap);
               return execv(file, args);
          }

BUGS
     It is up to the calling routine to determine how many arguments there
     are, since it is not possible to determine this from the stack frame.
     For example, execl passes a 0 to signal the end of the list.  printf can
     tell how many arguments are supposed to be there by the format.

     The macros va_start and va_end may be arbitrarily complex; for example,
     va_start might contain an opening brace, which is closed by a matching
     brace in va_end.  Thus, they should only be used where they could be
     placed within a single complex statement.

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