How can I add a new argument to the existing variable argument list?

Question

In a multi-threaded program I'm writing a custom print function which accepts a variable argument list.

void t_printf(char * str, ...)
{
    if(file_ptr != NULL)
    {
            va_list ap;
            va_start(ap, str);

            vfprintf(file_ptr, str, ap);

            va_end(ap);

            fflush(file_ptr);
    }
}

Inside this function I want to add the current thread id (using pthread_self()) to the message getting printed. How can I do it? Is there a way to add it to the existing va_list?

Answer 1

With a variadic macro:

With a variadic macro you can call the function with an argument prepended or appended:

#define t_printf(format, args...) \
    _t_printf(format, thread_id, __VA_ARGS__);

This prepends the thread_id before other arguments. (Note that on the _t_printf() function you have to modify the format string too.)

If you do this:

t_printf("some format string", a, b, c);

This will expand do this:

_t_printf("some format string", thread_id, a, b, c);

If t_printf() is called without other argument that format, you will have a trailing comma. GCC has a ## extension that takes care of adding the comma as needed:

#define t_printf(format, args...) \
    _t_printf(format, thread_id ##__VA_ARGS__);

Complete solution with the macro:

#define t_printf(format, args...) \
    _t_printf(format, thread_id, __VA_ARGS__);

void _t_printf(char * str, ...)
{
    if(file_ptr != NULL)
    {
            char format[1024];

            /* safely prefix the format string with [thread_id: %x] */
            snprintf(format, sizeof(format), "%s%s", "[thread_id: %x] ", str);

            va_list ap;
            va_start(ap, str);

            vfprintf(file_ptr, format, ap);

            va_end(ap);

            fflush(file_ptr);
    }
}

---

Without modifying the arguments

An other solution is to do two printf()s:

vsnprintf(buffer, bufsize, str, ap);
vfprintf(file_ptr, "[thread_id: %x] %s", thread_id, buffer);

Complete solution:

void _t_printf(char * str, ...)
{
    if(file_ptr != NULL)
    {
            char buffer[1024];

            va_list ap;
            va_start(ap, str);

            vsnprintf(buffer, sizeof(buffer), str, ap);
            vfprintf(file_ptr, "[thread_id: %x] %s", thread_id, buffer);

            va_end(ap);

            fflush(file_ptr);
    }
}

Answer 2

I believe there's no standard way to manipulate a va_list. The stdarg.h header defines macros to declare, initialize, copy, terminate lists and fetch an argument (discerning type is up to caller).

Here's the suggestion of an alternative to accomplish the same result: Note that this imposes a limitation on the format of the string. Depending on what you want, it may not be a problem:

#define MAXLEN 256
void t_printf(char * str, ...)
{
    if(file_ptr != NULL)
    {
        va_list ap;
        va_start(ap, str);
        char msg[MAXLEN];

        vsnprintf( msg , MAXLEN , str , ap ); /* msg is guaranteed 
                                               * to be NULL terminated
                                               */

        /* Now that we have the message printed into a string,
         * print the message, along with the thread_id into the
         * console
         */
        fprintf( file_ptr, "thread % 6d: %s", pthread_self() , msg );

        va_end(ap);

        fflush(file_ptr);
    }
}