PERROR(3) Library Functions Manual PERROR(3)
perror, sys_errlist, sys_nerr - system error messages
Perror produces a short error message on the standard error file
describing the last error encountered during a call to the system from
a C program. First the argument string s is printed, then a colon,
then the message and a new-line. Most usefully, the argument string is
the name of the program which incurred the error. The error number is
taken from the external variable errno (see intro(2)), which is set
when errors occur but not cleared when non-erroneous calls are made.
To simplify variant formatting of messages, the vector of message
strings sys_errlist is provided; errno can be used as an index in this
table to get the message string without the newline. Sys_nerr is the
number of messages provided for in the table; it should be checked
because new error codes may be added to the system before they are
added to the table.