System call exit status

Every system call has a return value - check them!

Usually -1 indicates error
On error, the variable 'errno' is set, for example
EBADF -- file descriptor not opened for writing
EACCES -- access (to file) denied
EPERM -- permission (for action) denied
EROFS -- read only file system
Error message for errno can be printed using perror()
      perror("Unable to open file foo: ");
      /* may yield "Unable to open file foo: no such file or directory" */
Or via sys_errlist[]
      fprintf(stderr, "Error: %s\n", sys_errlist[errno]);
Or via strerror() 
      fprintf(stderr, "Error: %s\n", strerror(errno));
errno is cleared after a sucessful system call