LinuxQuestions.org - vsprintf and va_list

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vsprintf and va_list - string length?

Ok, so how to explain this easy?

Well I have a function that takes a char* and a va_list as arguments.
It will then send the complete, processed string to another function.

Example:

Code:

void function(char *txt, va_list args)

{

    /* the char way

      * problem: don't know the length of txt + arguments

      */

    char *string1 = malloc(???);

    vsprintf(string1, txt, args);

    another_function(string1);



    /* the string way

      * problem: don't know how to use it with vsprintf()

      */

    string string2;

    ???

    another_function(string2.c_str());

}



void another_function(char *txt)

{

    printf("I got: %s\n", txt);

}

So, two ways to go:

1) Use char* with malloc. I need to know how long the complete string will be to avoid having to chop it.

2) Use string. But can I use it with a va_list?

Hi

I don't think there is a standard C/C++ way of handling this. But GNU has some functions that are not standard, but avaiable in Linux: vasprintf and asprintf. They work like sprintf, but you don't pass a string pointer, but the address of one. The string is then allocated for you, so you don't need to worry about size.

Example:

Code:

#define GNU_SOURCE



#include <stdio.h>

#include <stdlib.h>

#include <stdarg.h>

#include <string.h>



void myprintf(const char *format, ...)

{

  char *ptr;

  va_list list;

  va_start(list,format);

  vasprintf(&ptr,format,list);

  printf("Formated: %s\nLength=%d\n", ptr, strlen(ptr));

  free(ptr);

  va_end(list);

}



int main(int argc, const char *argv[])

{

  myprintf("Hello %s", "world");

  return 0;

}

Too bad those functions are not in the C/C++ standard...

Does the standard vsnprintf with a small size argument (1 for example) return the size minus one required by your malloc? If so it could avoid you to use nonstandard functions, but it could reduce a lot the speed of your code too because of a double call to vsnprintf.