warning not treated as error with -Wall & -Werror on

Question

Here is the content of source file get.c :

#include <stdio.h>

int main(){
  //int i = 0;
  char b[10];
  gets(b);
  puts(b);
  return 0;
}

When I compile it with these command

gcc -o get get.c -Wall -Werror

The output is

/tmp/ccYEWZvx.o: In function `main':
get.c:(.text+0x10): warning: the `gets' function is dangerous and should not be used.

But when change the code to

#include <stdio.h>

int main(){
  int i = 0; // **this line just be uncommented**
  char b[10];
  gets(b);
  puts(b);
  return 0;
}

Using the same command, the output is

cc1: warnings being treated as errors  
get.c: In function 'main':  
get.c:4: error: unused variable 'i'  

So, why this unused variable warning be treated as error, while the use of gets() not?

Answer 1

The gets() warning is being issued by the linker not the compiler, so the compiler settings do not apply.

Only the linker is able to determine that the symbol is resolved with the standard library gets() rather than some other implementation with the same name.

To instruct the linker to treat warnings as errors you need to pass it the --fatal-warnings option. In turn when not invoking the linker directly, options are passed to the linker by gcc using the -Wl option in a comma separated list:

gcc -o get get.c -Wall -Werror -Wl,--fatal-warnings

Note that the GNU linker is documented separately from the compiler, as part of binutils. The linker options are described here.

Answer 2

If you look at the output from the first example, it says the "error" is in an object file, which means it is generated by the linker.

The second error is generated by the compiler, which means there is no object file being generated and so the linker will not be invoked.