varargs(5)varargs(5)NAMEvarargs - handle variable argument list
SYNOPSISDESCRIPTION
This set of macros enables programmers to write portable procedures
that accept variable argument lists. Routines that have variable argu‐
ment lists (such as but do not use are inherently nonportable, because
different machines use different argument-passing conventions (see
printf(3S)).
is used as the parameter list in a function header.
is a declaration for No semicolon should follow
is a type defined for the variable used to traverse the list.
is called to initialize pvar to the beginning of the list. The type of
argN should be the same as the argument to the function just before the
variable portion of the argument list.
returns the next argument in the list pointed to by type is the type
the argument is expected to be. Different types can be mixed, but it
is up to the routine to know what type of argument is expected, because
it cannot be determined at runtime.
is used to clean up.
Multiple traversals, each bracketed by are possible.
NOTE: The header file is provided for compatibility with pre-ANSI com‐
pilers and earlier releases of HP C/HP-UX. It is superceded by which
includes all of the macros.
EXAMPLE
The following example shows a possible implementation of (see exec(2)):
The next example illustrates how a function that receives variable
arguments can pass these arguments down to other functions. To accom‐
plish this, the first routine in this example) which receives the vari‐
able argument list must pass the address pointer resulting from a call
to on to any subsequent calls that need to access this same variable
argument list. All routines that receive this address pointer in this
example) need only to use to access the original variable argument list
just as if they were the original routine to be passed the variable
arguments.
In this example, one can imagine that there are a series of other rou‐
tines (such as a and that also call the function.
#include <stdio.h>
#include <varargs.h>
#include <unistd.h>
int error_count;
/* VARARGS4 -- for lint */
int
log_errors(log_fp, func_name, err_num, msg_fmt, va_alist)
FILE *log_fp;
char *func_name;
int err_num;
char *msg_fmt;
va_dcl
{
va_list ap;
/* Print error header information */
(void) fprintf(log_fp, "\nERROR in process %d\n", getpid());
(void) fprintf(log_fp, " function \"%s\": ", func_name);
switch(err_num)
{
case ILLEGAL_OPTION:
(void) fprintf(log_fp, "illegal option\n");
break;
case CANNOT_PARSE:
(void) fprintf(log_fp, "cannot parse input file\n");
break;
...
}
/*
* Get pointer to first variable argument so that we can
* pass it on to v_print_log(). We do this so that
* v_print_log() can access the variable arguments passed
* to this routine.
*/
va_start(ap);
v_print_log(log_fp, msg_fmt, ap);
va_end(ap);
}
/* VARARGS2 -- for lint */
int
v_print_log(log_fp, fmt, ap)
FILE *log_fp;
char *fmt;
va_list ap;
{
/*
* If "%Y" is the first two characters in the format string,
* a second file pointer has been passed in to print general
* message information to. The rest of the format string is
* a standard printf(3S) format string.
*/
if ((*fmt == '%') && (*(fmt + 1) == 'Y'))
{
FILE *other_fp;
fmt += 2;
other_fp = (FILE *) va_arg(ap, char *);
if (other_fp != (FILE *) NULL)
{
/*
* Print general message information to additional stream.
*/
(void) vfprintf(other_fp, fmt, ap);
(void) fflush(other_fp);
}
}
/*
* Now print it to the log file.
*/
(void) vfprintf(log_fp, fmt, ap);
}
WARNINGS
It is up to the calling routine to specify how many arguments there
are, because it is not always possible to determine this from the stack
frame. For example, is passed a zero pointer to signal the end of the
list. can determine how many arguments are present by the format.
It is non-portable to specify a second argument of or to because argu‐
ments seen by the called function are not or C converts and arguments
to and converts arguments to before passing them to a function.
SEE ALSOexec(2), vprintf(3S), stdarg(5).
STANDARDS CONFORMANCEvarargs(5)
