The processing of variable parameters of C-language function

/************************************************************************* > File name:va_list.c > Author:zs hh0604 > Mail: [email protected] > Created time:2014 October 14 Tuesday 15:16 09 seconds ********************************** /#include <stdio.h>/*** uses variable parameters in c, and C provides a variable number of va_list types to store Functions. 1. The variable parameter method for defining a function in the C language is as follows: one: the reference must be at the End.
Two: the last parameter must declare the type of the variable Parameter. For example, The following three: the ability to use a three-point Representation.
Four: the use of the function calls through the FMT this character parameter for the parameter type and the number of parameters Set. Such as: "%s%d%c%f" void Test (char *fmt,...); 2. Assume the use of variable Parameters.
C provides a set of functions such as the following: void Va_start (va_list ap, last), type va_arg (va_list ap, type), void va_end (va_list ap), void Va_copy (va_ List dest, Va_list src), which operates on a variable number of Parameters. function Prototypes. void Va_start (va_list ap,last); function function: points the AP to a variable number in Turn. function Parameters: ap: points to each variable parameter in Turn.
Last: is the fmt, and its role is to specify the number of variable parameters of the Parameter. and Types. function return Value: no return Value. You must first call the Va_start () function before calling the group Function. Formats the characters in the FMT format. To get the value of the parameter at Once.
Char *s;int d;char c;while (*fmt) {switch (*fmt++) {case ' s ': s = va_arg (ap, char *); break;case ' d ':d = va_arg (ap, int); Case ' C ': C = va_arg (ap, char); Va_end (ap); the Va_end () function must be called before the function ends.
Use vsnprintf () directly with ap.vsnprintf (): function prototypes such as the following: int vsnprintf (char *str, size_t size, const char *format, va_list ap); Use for example the Following://va_list function://header file <stdarg.h> provides functionality for traversing function tables of unknown data and Types. then, declare a variable ap of type va_list within function F. It will point to each actual argument in Turn. Va_list Ap;////you must initialize the AP once by using the Va_start macro, regardless of the unnamed number of References.
Va_start (va_list Ap,lastarg);////thereafter Each run of the macro va_arg will produce a value with the same type and value as the next Non-command parameter.
It also changes the AP at the same time so that the next parameter is returned the next time the va_arg is Run. Type Va_arg (va_list ap, type),////void ve_end (va_list ap),////after The processing of all processed parameters is Complete. And before exiting F. You must call the macro Va_end once.
void Va_end (va_list ap); **//************************************************************************* > Fil E name:snprintf.c > author:zshh0604 > Mail: [email protected] > Created time:2014 year October 14 Tuesday 13:21 13 sec * * * /#include <stdlib.h> #include < stdio.h> #include <stdarg.h>char * make_message (const char *fmt, ...) The {int n;/* is used to record the return of vsnprintf ().
Assuming this value is less than or equal to-1, the Lifespan. There is not enough space allocated.
Assuming that the value is greater than-1, but n is not less than size, then the value should be added 1, (size+1), otherwise space will be increased by one ***/int size = 100; /* Guess We need no more than bytes.       */char *p, *np; Va_list ap;if ((p = malloc (size)) = = NULL) return null;while (1) {/* Try to print in the allocated SPACE. */VA_START (ap, FM       t); This function allows the AP to point to each of the parameters in the variable parameter.  n = vsnprintf (p, size, fmt, ap); Formats the number of APS that the AP points to in P.
The maximum size of P is sizes. print in FMT format.
Va_end (ap); After the use is finished, call Va_end (ap); Function.
/* If that worked, return the String. */if (n >-1 && N < size) return p;/* Else try again with more space. */if (n >-1)/* glibc 2.1 */size = n+1;  /* precisely what is needed */else/* glibc 2.0 */size *= 2; /* twice the old size */if ((NP = realloc (p, Size) = = NULL) {free (p); return NULL;} Else {p = np;}}}

The processing of variable parameters of C-language function