Tag Archives: printf

Getting gcc to warn you when you mess up stdargs

This article was originally published on the LGP Blog on Wednesday, January 20th, 2010

Sometimes, you may write functions in C that do things in the same way as printf, using stdargs.

An example of this would be something like this short debug function

int ptf(const char *fmt,...)
{
  va_list varlist;
  FILE *fp=fopen("/tmp/debug","a");
  va_start(varlist,fmt);
  vfprintf(fp,fmt,varlist);
  va_end(varlist);
  fclose(fp);
}

This function isn’t rocket science, it just simply appends your string into a file. It is a simple time saver utility.

However, using it can be a problem. You can do something like this

int x=1;
ptf("Error %s\n",x);

And gcc will say ’sure, no problem’.

But running the code will always crash. It tries to interpret the integer as a string.

This is the kind of thing that should be picked up on by the compiler. And in fact it can be, quite easily.

In your prototype for the function, you would have something like

extern int ptf(const char *,...);

This is pretty standard, and no surprises there. However, gcc has the capability to be given a hint as to how this function should be handled. You can instead prototype the function using

extern int ptf(const char *,...) __attribute__ ((format (printf, 1, 2)));

This tells gcc to treat the parameters 1 and 2 as the parameters to printf (which it knows how to check for errors). It will then check parameter 1 (the format string) against what is passed in starting at parameter 2 (the …). If an incorrect data type is used, this will now be detected and flagged up as a warning, in exactly the same way as an incorrect type used in a printf.