Odd behavior of function with variable number of parameters in C

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 (e.g. 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.

varargs are not supposed be used this way. You are better off using void find(char* word, char* filename) and checking if filename is NULL and if so write to stdout. Also don't forget to close the file, you opened, if you did open a file. — Dan D.
– Dan D., Commented Feb 24, 2013 at 9:34

LSerni · Accepted Answer · 2013-02-24 09:56:12Z

5

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 (e.g., 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);

answered Feb 24, 2013 at 9:56

LSerni

57.7k10 gold badges70 silver badges110 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

thehousedude Over a year ago

OK, thanks for remembering me. It works with NULL at the end of the list. Initially I thought that NULL (or other behavior) was somehow implied for arguments that weren't received, but it doesn't know by itself when the list ends.

Collectives™ on Stack Overflow

Odd behavior of function with variable number of parameters in C

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related