forward args of variadic function in C-CodePudding

I want to forward args of variadic function, I have already find the some topic.

Forward an invocation of a variadic function in C

When I start to practice, I found a problem.

#include <stdio.h>
#include <stdarg.h>
void fun1(const char *msg, ...) // try to forward printf
{
    va_list arg_list;
    va_start(arg_list, msg);
    vprintf(msg, arg_list);
    va_end(arg_list);
}

void fun2(const char *msg, ...) // try to forward fun1
{
    va_list arg_list;
    va_start(arg_list, msg);
    fun1(msg, arg_list);
    va_end(arg_list);
}

int main()
{
    fun1("this is int %d, float %f\n", 1, 2.3);
    fun2("this is int %d, float %f\n", 1, 2.3);
    return 0;
}

I compile code with gcc main.c and the output shown that

this is int 1, float 2.300000
this is int 6684168, float 2.300000

I can not understand why the fun2 not forward the args of fun1 correctly. Why the int 1 goes to another number but 2.3 still good. How can I modify my code to implement the forward?

Thanks for your time.

CodePudding user response：

fun1 needs a list of arguments to match its format, but when call it from fun2 you give it a va_list. To call it that way you need to rewrite it to take a va_list rather than a ...:

void fun1(const char *fmt, va_list args) {
    vfprintf(fmt, args);
}

CodePudding user response：

As suggtested by StoryTeller - Unslander Monica and Chris Dodd, My finial code looks like follows.

// before modify
//   vprintf  <-- fun1  <-- fun2


//    after modify

//    vprintf  <-- vfun1 <--- fun1
//                         |- fun2

#include <stdio.h>
#include <stdarg.h>
void vfun1(const char *msg, va_list arg_list)
{
    printf("this is vfun1\n");
    vprintf(msg, arg_list);
}

void fun1(const char *msg, ...) // try to forward vprintf, but implemented by call vfun1
{
    printf("this is fun1\n");
    va_list arg_list;
    va_start(arg_list, msg);
    vfun1(msg, arg_list);
    va_end(arg_list);
}

void fun2(const char *msg, ...) // try to forward fun1, but implemented by call vfun1
{
    printf("this is fun2\n");
    va_list arg_list;
    va_start(arg_list, msg);
    vfun1(msg, arg_list);
    va_end(arg_list);
}

int main()
{
    printf("------------------------------------\n");
    fun1("this is int %d, float %f\n", 5, 2.3);
    
    printf("------------------------------------\n");
    fun2("this is int %d, float %f\n", 5, 2.3);
    return 0;
}