Error Handling¶

The `errno` Variable ¶

On error, system calls (and most C library functions) return -1 and set the global variable errno.

Error handling with system calls¶

ssize_t n = read(fd, buffer, sizeof(buffer));
if (n == -1)
    if (errno == EINTR) {
        /* interrupted system call, retry possible */
        ...
    }
    else {
        /* abort, reporting the error */
        ...
    }

`errno` Is A Global Variable ¶

Where’s the bug?

Bad Error Handling¶

ssize_t n = read(fd, buffer, sizeof(buffer));
if (n == -1) {
    fprintf(stderr, "Error %d\n", errno);
    if (errno == EINTR)
        /* ... */
}

Helper Functions ¶

Prototype	Description	Documentation
`void perror(const char *s)`	Message to `stderr`, beginning with `s`	man -s 3 perror
`char *strerror(int errnum)`	Modifiable pointer to error description	man -s 3 strerror
`char strerror_r(int errnum, char buf, size_t buflen)`	Cleanest alternative: `_r` ⟶ “reentrant” (thread safe)	man -s 3 strerror_r

Error output¶

if (n == -1)
    perror("read()");