Question

There is a function my_init(…) that uses a variable argument list. The length of this list is not known, but there is a maximum of 100 and the elements are always char-arrays (means plain C-strings).

That’s how I try to decode them:

void my_init(...)
{
    va_list vl; 
    int     tagCnt,tagLen=100; 
    char   *listTag,*listValue;

    va_start(vl,tagLen);
    for (tagCnt=0; tagCnt<50; tagCnt++)
    {
        listTag=va_arg(vl,char*);
        if (listTag==0) break;
        listValue=va_arg(vl,char*);

        ... // do some usefult things here
    }
    va_end(vl);
}

…and I call this function this way:

my_init("tag1","value1",
        "tag2","value2",
        0);

So there are always pairs and the end of a list is marked with a 0. Unfortunately my_init() fails, after calling va_start() vl contains some crap that has nothing to do with input parameters and the calls to va_arg() return invalid pointers. So what is wrong in my code?

The code given above is shortened a bit so may be it does not compile…

Thanks!

==================================================================================

EDIT:

I changed code and call this way:

void my_init(int dummy,...)
{
    va_list vl; 
    int     tagCnt,tagLen=100; 
    char   *listTag,*listValue;

    va_start(vl,dummy);
    for (tagCnt=0; tagCnt<50; tagCnt++)
    {
        listTag=va_arg(vl,char*);
        if (listTag==0) break;
        listValue=va_arg(vl,char*);

        ... // do some usefult things here
    }
    va_end(vl);
}

my_init(0,
        "tag1","value1",
        "tag2","value2",
        0);

Independent from the used header file I now get other crap in my vl, stdargs.h or varargs.h do not make a difference…

Answer 1

The second argument to va_start must be the last named function parameter. In other words, the canonical usage is this:

void my_func(int arg1, float arg2, ...) {

    va_list vl;

    va_start(vl, arg2);

    // Stuff
}