Using errno with ftell() to distinguish error from success

Question

I need to figure out how does ftell() uses errno to distinguish between success and failure.

I know that the return value of ftell() would determine the success or failure, but it is possible to return the value of -1 (ftell() failure) even if on success, and the return is interpreted as a signed value.

So I check the return value of ftell() and if that is negative, I look at the errno, and if errno is non-zero, I determine the failure.

However, problem is that errno can be non-zero even from a previous error(doubtful about this). In this case how do I determine success or failure of ftell()?

Answer 1

As per Eugene Sh.'s arguments, it seems that it may be unneeded in this particular case, the return value of ftell should be enough.

Using errno, in general is done by setting its value to 0 before the function you expect to fail, and is documented as setting the value of errno.

Later you verify if its value was set, this value represents an implementation defined error code.

§7.21.9.4 - ftell()

Example:

#include <errno.h>
//...

FILE* f = fopen("fil.txt", "r"); // skipping error check

errno = 0;

long res = ftell(f)); // on failure ftell returns -1

if(errno > 0 && res == -1)
{
    // error occured
    perror("ftell"); // you can use perror to print error message related to the error code
}