Errors From System Calls

• Errors are somewhat primitive but
  • Most system calls return an error flag.
  • Most system calls set a global variable errno
• Read man 2 open
  • Look at the function prototype
  • Look for RETURN VALUE it is quite a way down.
    • a non-negative return value means success
    • A value of -1 means error.
    • And errno is set.
  • Look at ERRORS
    • EACCESS : opening a file that is not allowed.
    • EINVAL: invalid flag, among others
    • Look at the rest, wow quite a few.
  • We will play with open now and investigate it fully soon.
• Explore further
  • Look at getpwent
  • Look at getpid note the errors here.
  • Look at getpriority: note -1 is a valid return value with no error, but could also be an error.
• The majority of calls return a -1 on error but you need to look.
  • Even if they return other values through pass by pointer. (This is c)
  • This violates one of the "EUP Programming Practices", which has sort of gone out of vogue with retirements.
• Our author warns us that we should check return values.
  • These are indications that something has gone wrong.
  • And we really shouldn't continue until it is fixed.
• Look at errorCode.cpp.
• errno
  • It is good to have : #include <errno.h>
  • Is a global variable.
  • Is set only when a system error occurs.
  • It is not reset when no error occurs.
  • Reading and writing, ie cin, cout, invoke system calls.
• void perror(const char *s);
  • Needs: #include <stdio.h>
  • This will print an error message to standard err (cerr).
  • It is based on the value of errno
  • User Message: standard error description.
• char *strerror(int errnum);
  • Needs: #include <string.h>
  • Returns a pointer to a string.
  • The string MUST NOT BE MODIFIED by the application.
  • The string could be modified by future calls to strerror, so save it if you need it later.
  • This is not your memory, don't free it.
  • This is a common pattern.
• const char *strerrordesc_np(int errnum);
  • Needs: #include <string.h>
  • Returns a pointer to a string containing the name of the error.
  • See restrictions above.