Content-type: text/html Man page of stdarg

stdarg

Section: C Library Functions (3)
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NAME

stdarg - Handles a variable-length parameter list  

LIBRARY

Standard C Library (libc.so, libc.a)  

SYNOPSIS

#include <stdarg.h>

va_list void va_start (        va_list argp, parmN );
type va_arg (        va_list argp,
       type );
void va_end (        va_list argp );
 

PARAMETERS

argp
Specifies a variable that the stdarg macros use to keep track of the current location in the parameter list. Do not modify this variable.
parmN
Specifies the last named parameter (the one just before the
"..." in the execl() definition in the Example section). There must be at least one named parameter.
type
Specifies the type to which the expected argument will be converted when passed as an argument. Unsigned char or short arguments 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 because the type cannot be determined at run time.
 

DESCRIPTION

The stdarg set of macros allows you to write portable functions that accept a variable number of parameters. Subroutines that have variable-length parameter lists (such as the printf() function), but that do not use the stdarg macros, are inherently nonportable because different systems use different parameter-passing conventions.

The stdarg macros are as follows:

v_alist
Defines the type of the variable used to traverse the list.
va_start()
Initializes argp to point to the first unnamed argument. The va_start() macro will be invoked before any access to the unnamed arguments.
va_arg()
Returns the next parameter in the list pointed to by argp.
va_end()
Cleans up at the end.

Your function can traverse, or scan, the parameter list more than once. Start each traversal with a call to va_start() and end it with va_end().  

EXAMPLE

The following example is a possible implementation of the execl() function: #include <stdarg.h> #define MAXargS 100 /* ** execl is called by ** execl(file, arg1, arg2, . . . , (char *) 0); */ execl(char * file, . . .) { va_list ap;
   char *file;
   char *args[MAXargS];
   int argno = 0;
   va_start(ap, file);
   while ((args[argno++] = va_arg(ap, char *)) != (char *) 0)
      ;   /* Empty loop body */
   va_end(ap);
   return (execv(file, args)); }

 

NOTES

The calling routine is responsible for specifying the number of parameters because it is not always possible to determine this from the stack frame. For example, the execl() function is passed a null pointer to signal the end of the list. The printf() function determines the number of parameters from its fmt parameter.

AES Support Level: Temporary use  

RELATED INFORMATION

Functions: exec(2), printf(3), varargs(3), vprintf(3) delim off


 

Index

NAME
LIBRARY
SYNOPSIS
PARAMETERS
DESCRIPTION
EXAMPLE
NOTES
RELATED INFORMATION

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Time: 02:41:18 GMT, October 02, 2010