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cvariadic-functions
Andrew Cottrell
Andrew Cottrell

Reputation: 3413

How to prepend a string or integer to a variadic printf function?

I have a debug function that takes a format and variable number of additional arguments (just like printf()), converts them to a formatted string stored in a buffer buf, and sends that buffer to a secondary function that actually prints it:

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

    void debug_printf(const char *fmt, ...)
    {
        char buf[100];
        va_list va;

        va_start(va, fmt);

        /* Format the string and store it in buf */
        vsnprintf(buf, sizeof(buf), fmt, va);

        /* Actually print the buf. */
        actual_print(buf);

        va_end(va);
    }

How can I modify this function to prepend a string to the resulting output? For example, a header like DBG: so that if I called debug_printf("test1") the result would print DBG: test1.

Alternatively, how can I modify this function take a variable integer (the return value of a function) and somehow prepend that as a string to the resulting output? For example, if I had a function rng() that returned a random integer, I might call debug_printf("test2") and the result would print something like 3572 test2, assuming rng() returned integer value 3572.

For both cases, ideally the solution will modify the body of debug_printf() rather than wrap it in another function and/or a preprocessor macro.

EDIT: It appears I forgot an important point. For performance reasons, I would greatly prefer to only call actual_printf() once within debug_printf(). Otherwise yes, it would be a fairly easy solution to call it twice: Once with the header and again with the actual formatted string. Sorry about that!

Upvotes: 1

Views: 515

Answers (2)

Ricardo Branco
Ricardo Branco

Reputation: 6079

Print into buf whatever you want to prepend and that's it.

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

void debug_printf(const char *fmt, ...)
{
    char buf[100];
    va_list va;

    va_start(va, fmt);

    int n = snprintf(buf, sizeof(buf), "DBG: ");

    /* Format the string and store it in buf */
    vsnprintf(buf + n, sizeof(buf) - n, fmt, va);

    /* Actually print the buf. */
    actual_print(buf);

    va_end(va);
}

There's no need to use a fixed size. The manpage has an example to calculate the right size for the buffer to allocate with malloc():

http://man7.org/linux/man-pages/man3/printf.3.html

Upvotes: 2

Aganju
Aganju

Reputation: 6405

Instead of finding a complicated way to insert another parameter in the variadic list, you can just add a simple printf in the beginning of your code:
printf("%d ",rng());

Upvotes: 0

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