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stdarg(3) Library Functions Manual stdarg(3) 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 con- verted when passed as an argument. Unsigned char or short argu- ments 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 un- named 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() func- tion: #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 parame- ters 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 stdarg(3)