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<TITLE>va_start</TITLE> <body bgcolor="#ffffcc"> <hr> <pre> <h3>STDARG(3) Linux Programmer's Manual STDARG(3) </h3> <h3>NAME </h3> stdarg - variable argument lists <h3>SYNOPSIS </h3> #include <stdarg.h> void va_start( va_list ap, last); typeva_arg( va_list ap, type); void va_end( va_list ap); <h3>DESCRIPTION </h3> A function may be called with a varying number of argu- ments of varying types. The include file stdarg.h declares a type va_list and defines three macros for step- ping through a list of arguments whose number and types are not known to the called function. The called function must declare an object of type va_list which is used by the macros va_start, va_arg, and va_end. The va_start macro initializes ap for subsequent use by va_arg and va_end, and must be called first. The parameter last is the name of the last parameter before the variable argument list, i.e., the last parame- ter of which the calling function knows the type. Because the address of this parameter is used in the va_start macro, it should not be declared as a register variable, or as a function or an array type. The va_start macro returns no value. The va_arg macro expands to an expression that has the type and value of the next argument in the call. The parameter ap is the va_list ap initialized by va_start. Each call to va_arg modifies ap so that the next call returns the next argument. The parameter type is a type name specified so that the type of a pointer to an object that has the specified type can be obtained simply by adding a * to type. If there is no next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), random errors will occur. The first use of the va_arg macro after that of the va_start macro returns the argument after last. Succes- sive invocations return the values of the remaining argu- ments. The va_end macro handles a normal return from the function whose variable argument list was initialized by va_start. <h3>BSD MANPAGE 29 November 1993 1 </h3> <h3>STDARG(3) Linux Programmer's Manual STDARG(3) </h3> The va_end macro returns no value. <h3>EXAMPLES </h3> The function foo takes a string of format characters and prints out the argument associated with each format char- acter based on the type. void foo(char *fmt, ...) { va_list ap; int d; char c, *p, *s; va_start(ap, fmt); while (*fmt) switch(*fmt++) { case 's': /* string */ s = va_arg(ap, char *); printf("string %s\n", s); break; case 'd': /* int */ d = va_arg(ap, int); printf("int %d\n", d); break; case 'c': /* char */ c = va_arg(ap, char); printf("char %c\n", c); break; } va_end(ap); } <h3>STANDARDS </h3> The va_start, va_arg, and va_end macros conform to ANSI C3.159-1989 (``ANSI C''). <h3>COMPATIBILITY </h3> These macros are not compatible with the historic macros they replace. A backward compatible version can be found in the include file varargs.h. <h3>BUGS </h3> Unlike the varargs macros, the stdarg macros do not permit programmers to code a function with no fixed arguments. This problem generates work mainly when converting varargs code to stdarg code, but it also creates difficulties for variadic functions that wish to pass all of their argu- ments on to a function that takes a va_list argument, such as vfprintf(3). <h3>BSD MANPAGE 29 November 1993 2 </h3> </pre> <P> <hr> <p> <center> <table border=2 width=80%> <tr align=center> <td width=25%> <a href=../index.htm>Top</a> </td><td width=25%> <a href=../master_index.html>Master Index</a> </td><td width=25%> <a href=../SYNTAX/keywords.html>Keywords</a> </td><td width=25%> <a href=../FUNCTIONS/index.htm>Functions</a> </td> </tr> </table> </center> <p> <hr> This manual page was brought to you by <i>mjl_man V-2.0</i>

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