errno in C

The errno is a global variable that is set by system calls and some library functions in the event of an error to indicate what went wrong. It is defined in the <errno.h> header file. The errno is set to zero at program startup, and any function that fails will set it to a nonzero value.

For example, after calling access() function for a file that doesn’t exist, the errno will be set to ENOENT (meaning “No such file or directory”) which is defined in the <errno.h> header file. You can check its value and get a human-readable message using perror() or strerror(errno).

if (access("nonexistent_file.txt", F_OK) == -1) {
    perror("Error");
    printf("errno: %d\n", errno);
}

Output:

Error: No such file or directory # (ENOENT)
errno: 2 # (ENOENT)