• Unix-Style Error Handling
• Posix-Style Error Handling
• DNS-Style Error Handling
• Summary of Error-Reporting Functions

Programmers should always check the error code returned by system-level functions. There are many subtle ways that things can go wrong, and it only makes sense to use the status information that the kernel provides us.

The idea is given some base system-level function foo, we define a wrapper function Foo with identical arguments. The wrapper calls the base function and checks for errors. If it detects and error, the wrapper prints an informative message and terminates the process. Otherwise, it behaves like the base function.

Programmers could use different styles for returning errors, there lists three styles: Unix-style, Posix-style, and DNS-style.

Unix-Style Error Handling

Functions such as fork and wait that were developed in early days of Unix overload the function return value with:

• useful return resutls
• error code: errno

This style has following form (take wait for example):

if ((pid = wait(NULL)) < 0) {
    fprintf(stderr, "wait error: %s\n", strerror(errno));
    exit(0);
}

Posix-Style Error Handling

Most of the newer Posix functions such as Pthreads

• use the return value to indicate success(0) or failure(1) only
• any useful results are returned in function arguments that are passed by reference

For example, pthread_create function indicates success or failure with its return value. And returns the ID of the newly created thread by reference in its first argument.

This style has following form (take pthread_create for example):

if ((retcode = pthread_create(&tid, NULL, thread, NULL)) != 0) {
    fprintf(stderr, "pthread_create error: %s\n", sterror(retcode))
    exit(0);
}

DNS-Style Error Handling

The gethostbyname and gethostbyaddr functions that retrieve DNS host entires have another approch for returning errors. These functions return a NULL pointer on failre and st the global h_errno variable.

This style has following form (take gethostbyname for example):

if ((p = gethostbyname(name)) == NULL){
    fprintf(stderr, "gethostbyname error: %s\n", hstrerror(h_errno));
    exit(0);
}

Summary of Error-Reporting Functions

Following error-reporint funtions corresponds to the above three styles of error handling:

void unix_error(char *msg){
    fprintf(stderr, "%s: %s\n", msg, strerror(errno));
}

void posix_error(int code, char *msg){
    fprintf(stderr, "%s: %s\n", msg, sterror(code));
}

void dns_errno(char *msg){
    fprintf(stderr, "%s: DNS error %d\n", msg, h_errno);
}