C Programming/stdio.h/perror

The POSIX error function, perror, is used in C and C++ to print an error message to stderr, based on the error state stored in errno. It prints str and an implementation-defined error message corresponding to the global variable errno.

History
The definition of perror was first released in Issue 1 of the System V Interface Definition.

Semantics
If the parameter prefix is non-NULL, perror will first print prefix followed by a colon and a space to standard error. Then, it will print the result of strerror to standard error, followed by a newline character. For instance the above example may print

open: Permission denied