How do I fill a va_list

Question

If I have a va_list I know how to extract all its elements:

void printInts(int n,...)
{
    va_list va;
    va_start(va, n);
    for(unsigned int i=0; i<n; i++)
    {
        int arg=va_arg(va, int);
        printf("%d",arg);
    }
    va_end(va);
}

So when I call printInts(3,1,2,3) the va_list get filled of all the parameters.
But how do I manually fill a va_list without using va_start? I mean that I want something like:

va_list va;
push_arg(va, int, 5); // And so on until I fill all parameters
...

I need this because there is a function that accept a va_list as argument, and I don't know how to fill that va_list of all its parameters.

The type `va_list` isn't actually a list as such, it's actually often a kind of a pointer to the stack, and `va_arg` modifies this pointer to point to the next argument on the stack. — Some programmer dude, Dec 04 '12 at 13:15
@JoachimPileborg: Sometimes it's not a pointer to the stack. — Dietrich Epp, Dec 04 '12 at 13:16
Because I need a raw va_list with all it's argument, I need to do the opposite thing than va_start-va_arg-va_end . — Ramy Al Zuhouri, Dec 04 '12 at 16:44

score 20 · Accepted Answer · answered Dec 04 '12 at 13:19

There's no ability to fill a va_list explicitly.

You should write a wrapper function. Say you need to call your function foo, instead of manually filling in a va_list, you define a new function like so:

void call_foo(int arg1, ...)
{
   va_list ap;
   va_start(ap, arg1);
   foo(arg1, ap);
   va_end(ap);
}

Now you can call foo, which takes a va_list, however you like, by doing e.g. call_foo(1,2,3,4);, call_foo(1, 1, "Hello"); etc.

This will only allow you to specify the arguments at compile time, you can't build the arguments at runtime.

glglgl · Answer 2 · 2012-12-04T15:52:57.163

Normally, these functions come in pairs. If you have a "va-accepting" function, it is easy to create another one:

void printInts_v(int n, va_list ap)
{
    unsigned int i=0;
    for(i=0; i<n; i++)
    {
        int arg=va_arg(ap, int);
        printf("%d", arg);
    }
}

This function can be called this way:

void printInts(int n,...)
{
    va_list ap;
    va_start(ap, n);
    printInts_v(n, ap);
    va_end(va);
}

But I don't think there is a way to portably pre-fill a va_list for later use.

If you work on one architecture and portability is not an issue, you could craft something on your own, however. How exactly to do that is platform-specific.

score 0 · Answer 3 · answered Dec 04 '12 at 13:14

0

I don't think there's a standardized way of doing this. On the other hand, such a pre-filled va_list would be pretty useless, since there's no way of "querying" it for the number and types of available arguments, either.

answered Dec 04 '12 at 13:14

unwind

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I think the question is like "how do I call `vprintf()`?" and the answer is "write `printf()` and call it from there". – Dietrich Epp Dec 04 '12 at 13:17
There is a function of a framework that accepts a va_list, how do I go around this? – Ramy Al Zuhouri Dec 04 '12 at 13:33
@RamyAlZuhouri As written in - up to now - two answers. – glglgl Dec 04 '12 at 14:22
@glglgl: (I know this is quite an old question but) none of those answers are usable. because they all assume you know the argument list at compile time. The real problem, is about building the va_list dynamically. – Camion May 04 '19 at 14:24
@Camion That cannot be done, at least not in portable standard C, which is what the answers are saying. – unwind May 07 '19 at 09:05
@unwind, what the answers say is that there is no portable way to build a va_list. Ramy AlZuhouri's question glglgl was replying was about the possibility of a work around solution. – Camion May 07 '19 at 11:33

score 0 · Answer 4 · answered May 07 '19 at 11:55

Since other replies tells it's impossible (which would be a good reason to add it in the design of the language for the next version), The only portable possibility would be

to check the documentation twice to investigate if you didn't miss or misunderstand something about the framework you're trying to use : I had the exact same problem here and it appeared in the answer that I was not using the right call for what I was trying to do;
to write yourself a function which would take an array of pointers as parameter instead of the va_list and split the problem in multiple calls (not always possible); or
assume the framework you're trying to use was poorly designed...

score 0 · Answer 5 · answered Oct 24 '21 at 18:13

Splitting the function into multiple calls works for the printf family - there is code for this in Kernighan & Ritchie, along the lines of:

void printf_OclFile(struct OclFile* self, char* f, void* sq[])
{ int n = length(sq);
  if (n == 0 || self->actualFile == NULL) 
  { return; }

  void* ap; 
  char* p = f; 
  int i = 0; 

  char* sval; 
  int ival; 
  double dval; 

  ap = sq[0]; 
  for ( ; *p != '\0' && i < n; p++)
  { if (*p != '%')
    { char* p0 = subString(p,1,1); 
      fputs(p0, self->actualFile); 
      continue; 
    } 

    p++; /* Points to flag after % */ 

    switch (*p)
    { case 'd' : 
        ival = *((int*) sq[i]); 
        i++; 
        fprintf(self->actualFile, "%d", ival);
        break; 
      case 'f' : 
        dval = *((double*) sq[i]); 
        i++; 
        fprintf(self->actualFile, "%f", dval); 
        break; 
      case 's' : 
        sval = ((char*) sq[i]); 
        i++; 
        fprintf(self->actualFile, "%s", sval); 
        break;
      default : 
        char* p0 = subString(p,1,1); 
        fputs(p0, self->actualFile);
        break; 
    }
  } 
}

How do I fill a va_list

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