C中参数数量可变的函数的奇数行为

Question

I have the following C function with a variable number of arguments, which is supposed to search the char* word through a hashtable and write true or false in a file which, if specified, is the second parameter; otherwise, it is stdout .

It works fine if I specify the name of the file, the problem is when I don't (eg find("foo") ). In this case it writes the result in a file named foo instead of stdout .

What is the cause?

void find(char* word, ...)
{
va_list list;
char *fname = NULL;
va_start(list, word);
FILE* f;
fname = strdup(va_arg(list, char*));
va_end(list);
if (<condition>)    // condition suited for the case in which the file name is received 
    f = fopen(fname, "a");
else
    f = stdout;
if (member(word))
    fprintf(f, "True\n");
else
    fprintf(f, "False\n");
}

In place of <condition> I've tried fname != NULL and strlen(fname) > 0 but those don't apply and it keeps seeing fname as word when fname is not specified.

Thank you very much for any help you can provide.

Answer 1

From va_* 's man page:

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 .

If you want to use a variable parameter list, you need to devise some sort of terminator for the list (eg, always add a dummy NULL argument):

find (word, NULL);
find (word, filename, NULL);

or supply the number of parameter as parameter:

find (1, word);
find (2, word, filename);