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void perror(s) char *s;
#include <errno.h>
int sys_nerr; char *sys_errlist[]; int errno;
perror() produces a short error message on the standard error describing the last error encountered during a call to a system or library function. If s is not a NULL pointer and does not point to a null string, the string it points to is printed, followed by a colon, followed by a space, followed by the message and a NEWLINE. If s is a NULL pointer or points to a null string, just the message is printed, followed by a NEWLINE. To be of most use, the argument string should include the name of the program that 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.
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