std::perror (3) - Linux Manuals
Defined in header <cstdio>
void perror( const char *s );
Prints a textual description of the error code currently stored in the system variable errno to stderr.
The description is formed by concatenating the following components:
* the contents of the null-terminated byte string pointed to by s, followed by ": " (unless s is a null pointer or the character pointed to by s is the null character)
* implementation-defined error message string describing the error code stored in errno, followed by '\n'. The error message string is identical to the result of std::strerror(errno).
s - pointer to a null-terminated string with explanatory message
// Run this code
errno (macro variable)