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     The perror function prints an error message to stderr. The
     <string> argument is printed first, followed by a colon, the
     system error message for the last library call that produced the
     error, and a newline character. If <string> is a NULL pointer or a
     pointer to a null string, perror prints only the system error
     message.
 
     The actual error number is stored in the variable errno (defined
     in ERRNO.H). The system error messages are accessed through the
     variable sys_errlist, which is an array of messages ordered by
     error number. The perror function prints the appropriate error
     message by using the errno value as an index to sys_errlist. The
     value of the variable sys_nerr is defined as the maximum number of
     elements in the sys_errlist array.
 
     To produce accurate results, perror should be called immediately
     after a library routine returns with an error. Otherwise, the
     errno value may be overwritten by subsequent calls.
 
     Under DOS and OS/2, some of the errno values listed in ERRNO.H are
     not used. The perror function prints an empty string for any errno
     value not used under DOS or OS/2.
 
     Return Value
 
     None.
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