Forward an invocation of a variadic function in C

Question

In C, is it possible to forward the invocation of a variadic function? As in,

int my_printf(char *fmt, ...) {
    fprintf(stderr, \"Calling printf with fmt %         


        

Answer 1

Not sure if this helps to answer OP's question since I do not know why the restriction for using a helper function akin to vfprintf in the wrapper function applies. I think the key problem here is that forwarding the variadic argument list without interpreting them is difficult. What is possible, is to perform the formatting (using a helper function akin to vfprintf: vsnprintf) and forward the formatted output to the wrapped function with variadic arguments (i.e. not modifying the definition of the wrapped function). So, here we go:

#include 
#include 

int my_printf(char *fmt, ...)
{
    if (fmt == NULL) {
        /* Invalid format pointer */
        return -1;
    } else {
        va_list args;
        int len;

        /* Initialize a variable argument list */
        va_start(args, fmt);

        /* Get length of format including arguments */
        len = vsnprintf(NULL, 0, fmt, args);

        /* End using variable argument list */
        va_end(args);
        
        if (len < 0) {
            /* vsnprintf failed */
            return -1;
        } else {
            /* Declare a character buffer for the formatted string */
            char formatted[len + 1];

            /* Initialize a variable argument list */
            va_start(args, fmt);
            
            /* Write the formatted output */
            vsnprintf(formatted, sizeof(formatted), fmt, args);
            
            /* End using variable argument list */
            va_end(args);

            /* Call the wrapped function using the formatted output and return */
            fprintf(stderr, "Calling printf with fmt %s", fmt);
            return printf("%s", formatted);
        }
    }
}

int main()
{
    /* Expected output: Test
     * Expected error: Calling printf with fmt Test
     */
    my_printf("Test\n");
    //printf("Test\n");

    /* Expected output: Test
     * Expected error: Calling printf with fmt %s
     */
    my_printf("%s\n", "Test");
    //printf("%s\n", "Test");

    /* Expected output: %s
     * Expected error: Calling printf with fmt %s
     */
    my_printf("%s\n", "%s");
    //printf("%s\n", "%s");

    return 0;
}

I came across this solution here.

Editted: fixed mistakes pointed out by egmont