call printf using va_list

Question

void TestPrint(char* format, ...)
{
    va_list argList;

    va_start(argList, format);
    printf(format, argList);
    va_end(argList);
}


int main()
{
    TestPrint("Test print %s %d\n", "string", 55);
    return 0;
}

I need to get:

Test print string 55

Actually, I get garbage output. What is wrong in this code?

score 82 · Accepted Answer · answered May 12 '11 at 11:36

82

Use vprintf() instead.

answered May 12 '11 at 11:36

Oliver Charlesworth

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5

@PingwinTux: Yup. This kind of response was acceptable back in 2011 ;) – Oliver Charlesworth Oct 18 '14 at 19:03
yeah, nowadays people want their hands held. Probably also a better, and more pedadogical way, and more friendly to newcomers (on StackOverflow, and in programming in general). – alexpanter Apr 06 '19 at 14:02

score 68 · Answer 2 · edited Feb 23 '21 at 10:10

68

Instead of printf, I recommend you try vprintf instead, which was created for this specific purpose:

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

void errmsg( const char* format, ... )
{
    va_list arglist;

    printf( "Error: " );
    va_start( arglist, format );
    vprintf( format, arglist );
    va_end( arglist );
}

int main( void )
{
    errmsg( "%s %d %s", "Failed", 100, "times" );
    return EXIT_SUCCESS;
}

Source

edited Feb 23 '21 at 10:10

Neuron - Freedom for Ukraine

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answered May 12 '11 at 11:39

onteria_

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1

this example made me happy! – Gewure Apr 19 '17 at 15:09

score 24 · Answer 3 · edited Apr 19 '16 at 15:58

24

As others have pointed out already: In this case you should use vprintf instead.

But if you really want to wrap printf, or want to wrap a function that does not have a v... version, you can do that in GCC using the non-standard __builtin_apply feature:

int myfunction(char *fmt, ...)
{
    void *arg = __builtin_apply_args();
    void *ret = __builtin_apply((void*)printf, arg, 100);
    __builtin_return(ret);
}

The last argument to __builtin_apply is the max. total size of the arguments in bytes. Make sure that you use a value here that is large enough.

edited Apr 19 '16 at 15:58

answered May 21 '14 at 17:13

CliffordVienna

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thanks for that answer, which gives a straight solution to OP's problem. For interception purpose, this code saved my life! (or at least avoided me to defer to assembly :) – Kevin Oct 27 '15 at 09:58
2

Can there EVER be a maximum for this value? The `__builtin_apply()` should parse the call stack frames in order to find out the value on its own. But it depends on the architecture if and how good this would work. I once grafted a similar solution for myself. It was horrible. – glglgl May 14 '18 at 08:03

score 4 · Answer 4 · edited Feb 25 '21 at 15:24

4

This is not how you use printf(). If you want to use va_lists, use vprintf() instead. Look here for reference.

edited Feb 25 '21 at 15:24

Neuron - Freedom for Ukraine

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answered May 12 '11 at 11:37

Constantinius

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call printf using va_list

4 Answers4

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