函数参数，如C中的printf

Question

I want to implement a function Myprintf() that takes arguments like printf() . Now I am doing this by:

sprintf(demoString, "Num=%d String=%s", num, str);
Myprintf(demoString);

I want to replace this function call as:

Myprintf("Num=%d String=%s", num, str);

How is this possible?

Answer 1

#include <stdio.h>
#include <stdarg.h>

extern int Myprintf(const char *fmt, ...);

int Myprintf(const char *fmt, ...)
{
    char buffer[4096];
    va_list args;
    va_start(args, fmt);
    int rc = vsnprintf(buffer, sizeof(buffer), fmt, args);
    va_end(args);
    ...print the formatted buffer...
    return rc;
}

It isn't clear from your question exactly how the output is done; your existing Myprintf() presumably outputs it somewhere, maybe with fprintf() . If that's the case, you might write instead:

int Myprintf(const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    int rc = vfprintf(debug_fp, fmt, args);
    va_end(args);
    return rc;
}

If you don't want to use the return value, declare the function as void and don't bother with the variable rc .

This is a fairly common pattern for ' printf() cover functions'.

Answer 2

您需要使用变量参数定义函数 ，并使用vsprintf构建字符串。

Answer 3

The printf and its relatives are in a family of functions called variadic functions and there are macros/functions in the <stddef.h> header in the C standard library to manipulate variadic argument lists. Look at the GNU docs for examples: How Variadic Functions are Defined and Used .