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);
    }
}