Odd gcc warning behavior

c, gcc, linux

Solution

It seems it's a compiler option http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html:

-Wno-unused-result
    Do not warn if a caller of a function marked with attribute warn_unused_result (see Function Attributes) does not use its return value. The default is -Wunused-result. 

....

-Wtype-limits
    Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with ‘<’ or ‘>=’. This warning is also enabled by -Wextra.

Problem

I'm working on a linux driver, and I got this warning message: ``` /home/andrewm/pivot3_scsif/pivot3_scsif.c:1090: warning: ignoring return value of ‘copy_from_user’, declared with attribute warn_unused_result ``` The offending line is: ``` if (copy_from_user(tmp, buf, count) < 0) ``` After checking the declaration of `copy_from_user`, I found it returns an `unsigned long`, so obviously the comparison would always fail, so the return value wouldn't affect the comparison. That part makes sense, but why does gcc not also warn about the fact that it's a signed/unsigned comparison? Is that just a compiler peculiarity? Or does it avoid warning twice for the same expression? The function containing the line is: ``` int proc_write(struct file *f, const char __user *buf, unsigned long count, void *data) { char tmp[64]; long value; struct proc_entry *entry; if (count >= 64) count = 64; if (copy_from_user(tmp, buf, count) < 0) { printk(KERN_WARNING "pivot3_scsif: failed to read from user buffer %p\n", buf); return (int)count; } tmp[count - 1] = '\0'; if (tmp[count - 2] == '\n') tmp[count - 2] = '\0'; ... } ``` Using gcc 4.4.1 on 64-bit Red Hat (on a company server, I don't really have a choice on upgrading).

Original source