fseek on a position beyond EOF does not trigger EOF using feof, how come?

Question

I'm reading data from a file to memory that is opened with:

FILE *f = fopen(path, "rb");

Before I start copying bytes from the file I seek to a start position using:

/**                                                                                                                                                    
 * Goes to the given position of the given file.                                                                                                       
 *                                                                                                                                                     
 * - Returns 0 on success                                                                                                                              
 * - Returns -1 on EOF                                                                                                                                 
 * - Returns -2 if an error occured, see errno for error code                                                                                          
 * - Returns -3 if none of the above applies. This should never happen!                                                                                
 */                                                                                                                                                    

static int8_t goto_pos(FILE *f, uint64_t pos)                                                                                                          
{                                                                                                                                                      
        int err = fseek(f, pos, SEEK_SET);                                                                                                             

        if (err != 0) {                                                                                                                                
                if (feof(f) != 0) return -1;                                                                                                           
                if (ferror(f) != 0) return -2;                                                                                                         
                return -3;                                                                                                                             
        }                                                                                                                                              

        return 0;                                                                                                                                      
}

The problem is that even though I seek to a position way beyond EOF, this function never returns -1.

According to the reference feof should return a non zero value when EOF is encountered.

Why is this? Is the feof function useless?

---

Note that I'm currently using the return value of fgetc to check for EOF.

Answer 1

Seeking simply does not test for the file's end.

The reason for this is that you perhaps might want to do an fwrite() where you sought to. fseek() can not know what your plans are for after it was called.

Do an fread() after seeking behind the file's end and you will have feof() returning a non-zero value.