varargs - variable argument list
f = va_arg(pvar, type);
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.
PLEASE NOTE: varargs is being supplanted by stdarg(5). Users should
reference that man page for the recommended method of passing variable
va_alist is used in a function header to declare a variable argument
va_dcl is a declaration for va_alist. Note that there is no semicolon
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
va_end(pvar) is used to finish up.
Multiple traversals, each bracketed by va_start ... va_end, are
int argno = 0;
file = va_arg(ap, char *);
while (args[argno++] = va_arg(ap, char *))
return execv(file, args);
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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