Linux and UNIX Man Pages

Linux & Unix Commands - Search Man Pages

stdarg(3) [minix man page]

STDARG(3)						     Library Functions Manual							 STDARG(3)

NAME
stdarg - variable argument list SYNOPSIS
#include <stdarg.h> void va_start(va_list ap, argtypeN parmN) type va_arg(va_list ap, type) void va_end(va_list ap) 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 stdarg are inherently nonportable, since different machines use different argument passing con- ventions. A function that accepts a variable argument list is declared with "..." at the end of its parameter list. It must have at least one normal argument before the "...". For example: int printf(const char *format, ...) { /* code */ } int fprintf(FILE *stream, const char *format, ...) { /* code */ } va_list is a type which is used for the variable ap within the body of a variable argument function which is used to traverse the list. va_start(ap, parmN) is called to initialize ap to the beginning of the list. The last true parameter of the function, parmN, must be sup- plied to allow va_start to compute the address of the first variable parameter. va_arg(ap, type) will return the next argument in the list pointed to by ap. Type is the type to which the expected argument will be con- verted when passed as an argument. 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 run- time. va_end(ap) must be used to finish up. Multiple traversals, each bracketed by va_start ... va_end, are possible. EXAMPLE
#include <stdarg.h> execl(const char *path, ...) { va_list ap; char *args[100]; int argno = 0; va_start(ap, path); while ((args[argno++] = va_arg(ap, char *)) != NULL) {} va_end(ap); return execv(path, args); } NOTES
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 null pointer 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. BUGS
It is impossible to properly show the macros as C declarations as is done in the synopsis. They can never be coded as C functions, because all three macros use their arguments by address, and the type field is certainly impossible. Just look at them as being part of the C lan- guage, like sizeof. 7th Edition May 15, 1986 STDARG(3)

Check Out this Related Man Page

VARARGS(3)						     Library Functions Manual							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 con- ventions. 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 rou- tine 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. 7th Edition May 15, 1986 VARARGS(3)
Man Page