Thread: Stdarg :(

Code:

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

typedef struct
{
  unsigned char **phrase;
} Greetings;

Greetings phrases;

void InitGreeting(Greetings *, int, ...);

unsigned char hello[] = "hello";
unsigned char goodbye[] = "goodbye";

int main()
{
// Pass the number of args (3) also
  InitGreeting(&phrases, 3, hello, goodbye, "salami");
  getchar();
  return 0;
}

void InitGreeting(Greetings *tempGreets, int ngreets, ...)
{
  int loop;
  unsigned char *s;
  va_list ap;

  tempGreets->phrase = malloc(sizeof(unsigned char *)*ngreets);

  va_start(ap, ngreets);

  for(loop = 0;loop < ngreets;loop++)
  {
// s becomes pretty pointless here. Remove it if you want
    s = va_arg(ap, unsigned char *);
    tempGreets->phrase[loop] = s;
    printf("%s\n", s);
  }

  va_end(ap);
}

Try that one. You can't reliably find the end of the arglist using va_arg()'s return value. I threw in "salami" just to show you that you don't need to pass variables. I wasn't sure if you knew that, but if you didn't you do now

From the man page:

type va_arg( va_list ap, type);

If there is no next argument, or if type is not compatible
with the type of the actual next argument (as promoted
according to the default argument promotions), random
errors will occur.

I have no idea what you were doing here:

Code:

tempGreets->phrase[loop]*s;

Did you forget an = sign? You wouldn't want to dereference s in this situation if that's the case.

Also, try indenting your code a bit to make it easier to read.

Originally Posted by itsme86

Code:

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

typedef struct
{
  unsigned char **phrase;
} Greetings;

Greetings phrases;

void InitGreeting(Greetings *, int, ...);

unsigned char hello[] = "hello";
unsigned char goodbye[] = "goodbye";

int main()
{
// Pass the number of args (3) also
  InitGreeting(&phrases, 3, hello, goodbye, "salami");
  getchar();
  return 0;
}

void InitGreeting(Greetings *tempGreets, int ngreets, ...)
{
  int loop;
  unsigned char *s;
  va_list ap;

  tempGreets->phrase = malloc(sizeof(unsigned char *)*ngreets);

  va_start(ap, ngreets);

  for(loop = 0;loop < ngreets;loop++)
  {
// s becomes pretty pointless here. Remove it if you want
    s = va_arg(ap, unsigned char *);
    tempGreets->phrase[loop] = s;
    printf("%s\n", s);
  }

  va_end(ap);
}

Try that one. You can't reliably find the end of the arglist using va_arg()'s return value. I threw in "salami" just to show you that you don't need to pass variables. I wasn't sure if you knew that, but if you didn't you do now

From the man page:


I have no idea what you were doing here:

Code:

tempGreets->phrase[loop]*s;

Did you forget an = sign? You wouldn't want to dereference s in this situation if that's the case.

Also, try indenting your code a bit to make it easier to read.

Err I changed some of my original program so that my code wouldn't take up the whole page. It was suppose to be taken out. Also thank you. It works !. If anyone can figure out a way to not have to send in the number of strings let me know.

It's really just not possible to not tell InitGreeting() how many arguments you're passing to it in some way. Check this page out: http://www.mcsr.olemiss.edu/cgi-bin/man-cgi?stdarg+5

This paragraph specifically:

It is up to the calling routine to determine how many arguments there
are, since it is not possible to determine this from the stack frame.
For example, execl passes a 0 to signal the end of the list. Printf can
tell how many arguments are supposed to be there by the format.

So you have to tell InitGreeting() in some way where the end of the list is. I suppose you could try calling it like this:

Code:

InitGreeting(&phrases, hello, goodbye, NULL);

Then when s == NULL you know you've reached the end of the list. Telling it exactly how many arguments you're passing it just makes it simpler to allocate the memory you need for yours. If you used the method I just introduced you'd have to either loop twice through the arg list (once to count how many there are) and then allocate memory for your phrases array, or you'd have to keep realloc()'ing the memory for it every time through the loop.

EDIT: No, it doesn't automatically tag a NULL at the end of the list. The calling function is responsible for that as the webpage I posted points out.

Here's a modification to your code that does it by marking the end of the arg list with NULL:

Code:

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

typedef struct
{
  unsigned char **phrase;
} Greetings;

Greetings phrases;

void InitGreeting(Greetings *, ...);

unsigned char hello[] = "hello";
unsigned char goodbye[] = "goodbye";

int main()
{
  InitGreeting(&phrases, hello, goodbye, NULL);
  getchar();
  return 0;
}

void InitGreeting(Greetings *tempGreets, ...)
{
  int loop, ngreets;
  unsigned char *s;
  va_list ap;

// Count the number of args
  va_start(ap, tempGreets);
  for(loop = 0;;loop++)
    if(va_arg(ap, unsigned char *) == NULL)
      break;
  va_end(ap);
  ngreets = loop;

// Allocate the memory
  tempGreets->phrase = malloc(sizeof(unsigned char *)*ngreets);

// Loop again, this time storing the args in the array
  va_start(ap, tempGreets);
  for(loop = 0;loop < ngreets;loop++)
  {
    s = va_arg(ap, unsigned char *);
    tempGreets->phrase[loop] = s;
    printf("%s\n", s);
  }
  va_end(ap);
}

EDIT (again): Passing ngreets directly to InitGreeting() (like I did in my original reply) has the added benefit of causing the compiler (most compilers anyway) to barf or at least throw up a warning if you forget to supply that argument. If the prototype looks like InitGreeting(Greetings *, int, ...); and you try calling it with InitGreeting(&phrases, hello, goodbye); then the compiler is going to see that the type for hello is not an int and complain about it. However, if you forget to add a NULL to the end of your agument list with the method presented in this reply, the compiler will most certainly happily compile it and then you'll just get runtime problems.

Originally Posted by itsme86

I suppose you could try calling it like this:

Code:

InitGreeting(&phrases, hello, goodbye, NULL);

Beware, the (char*) typecast is not an option with variadics.

Code:

InitGreeting(&phrases, hello, goodbye, (char*) NULL);

Originally Posted by itsme86

Sorry, you're right. But shouldn't the NULL be cast to unsigned char * instead of void *? I mean, it's already a void *.

NULL is not necesseraly a (void*), but you are correct, it must be typecasted to (char *) if a string is expected.

I fix my post.