Handling Errors from System Calls

• Let us consider the system calls gethostname and sethostname
  • Look at the man page for these.
    • unistd.h is needed.
    • The function prototypes are given.
    • A quick description of each is provided.
    • Possible return value(s) are noted (0,-1)
    • A list of errors is provided.
    • And some other useful information, including notes and see also.
  • Important note: This is a c level interface, we can call it with c++ but it does use c++ style strings, just c strings.
    • What is a c string?
    • How do you know when a c string ends?
    • How do you allocate a static c string?
    • How do you allocate a dynamic c string?
  • There is a corresponding program hostname
  • a simple call might look like (Don't worry, I'll give you a full listing below)

  • const int BUF_SIZE = 256;
    
    int main() {
        char name[BUF_SIZE];
    
        if (-1 == gethostname(name, BUF_SIZE)) {
            cout << "Failed to get the host name " << endl;
        } else {
            cout << "The host name is " << name << endl;
        }
    	

  • The problem is, if it fails, we don't konw why.
• As we mentioned earlier the global variable errno is set when ther is a problem.
  • defined in errno.h, which you should include.
  • Any system call which fails sets errno
  • Any system call can change the value of errno
  • They note in the man page that this is bad...

    if (somecall() == -1) {
        printf("somecall() failed\n");
        if (errno == ...) { ... }
    }
    	

  • Why?
  • You should save the value

    if (somecall() == -1) {
        errorSave = errno;
        printf("somecall() failed\n");
        if (errorSave == ...) { ... }
    }
    	

• So in my code, I can force an error with gethostname

      if (-1 == gethostname(name, 0)) {
          cout << "Failed to get the host name " << endl;
      } else {
          cout << "The host name is " << name << endl;
      }
      cout << "errno is " << errno << endl;
  
      

• But that is fairly uninformative
• There are a number of functions which allow you to gain more information about the errors.
• perror()
  • Look at the man page. (section 3, not 1)
  • stdio.h
  • produces a message on stderr
  • Takes a string (c style)
  • Prints string parameter and then a standard system message.
  • They suggest including the name of the function that failed.

  •     if (-1 == gethostname(name, 0)) {
            perror("gethostbyename failed");
            strcpy(name, "UNKNOWN");
        }
    
        cout << "The host name is " << name << endl;
    	 

• strerror
  • Man page.
  • If a system call ends in _r it is thread safe.
    • more on this later.
  • This returns a pointer to string describing the error message.
  • You can not modify this string, you don't own it.
    • Make a copy if you wish to do anything other than print it.
    • Future calls to strerror* may modify this string.
    • Make a copy if you will need it long term.
  • Note that this may cause an error as well!