Variable output parameter (a)

Linux C on the output function definition:

int printf(const char *format,…);
int vprintf(const char * format,va_list ap);
int vfprintf(FILE *stream,cosnt char *format,va_list ap);
int vsprintf(char *str,const char *format,va_list ap);
int vsnprintf(char *str,size_t size,const char*format,va_list ap);

 

va_list ap; // first define a variable of type va_list

void va_start (va_list ap, last); a first variable // va_start va_list parameter, the second parameter is a function of the last fixed parameter

void va_end (va_list ap); // end of the iteration with va_end

type va_arg（va_list ap,type）;

 

Use int_vsnprintf (char * str, size_t size, const char * format, va_list ap); format conversion function output variable parameters of the function.

vsnprintf () c is one of the library functions, are variable parameters, for the character string, and print data and user-defined data format and the like.

 

Header file: #include <stdio.h>

Function declaration: int_vsnprintf (char * str, size_t size, const char * format, va_list ap);

Parameter Description:

char * str [out]: generating the formatted string stored here.

size_t size [in]: maximum number of bytes str prevent pharmaceutically array bounds.

const char * format: [in] string specifying the output format, which determines the type of the variable parameter, you need to provide a sequence number.

va_list ap [in], va_list variable, va: variable-argument: variable parameter. (... it is the content included in this variable is stored in the variable parameters were to go);

 

Variable length data reading method

va_start(args,fmt);
size_t buf_len = vsnprintf(buffer,MAX_LOG_LEN,str_format,args);
va_len(args);

Examples of variable parameters:

/***
vsnprintf.c
***/
#include<stdio.h>
#include<stdarg.h>

void my_print(char *fmt,...)
{
    va_list args;
    va_start(args,fmt);
    char buff[1024];
    vsnprintf(buff,1023,fmt,args);
    printf("%s\n",buff);
    va_end(args);
}

int main()
{
    int age = 78;
    my_print("hello world");
    my_print("hello %d",2345);
    my_print("hello my age is : %d",age);
    return 0;
}

Output:

Bot @ ubuntu: ~ / wangqinghe / C / 20190703 $ gcc vsnprintf.c -that vsnprintf

exbot @ ubuntu: ~ / wangqinghe / C / 20190703 $ ./vsnprintf

hello world

hello 2345

hello my age is : 78