osf1 man page for errno

errno(5)							File Formats Manual							  errno(5)

NAME
       errno - Returns error condition value

SYNOPSIS
       #include <errno.h>

DESCRIPTION
       The  errno  external  variable  contains the most recent error condition set by a function. The symbolic values for errno are listed in the
       intro reference page and in the ERRORS section of the individual reference pages for each function.

       The exec() functions set errno to a value of 0 (zero) after successful completion. Typically, other functions only set errno to	a  nonzero
       value.

EXAMPLES
       The  following program uses the value of errno to determine why the requested file could not be opened.	If errno has one of the two tested
       values, the program prints an appropriate message; otherwise, the program uses the error() function to print out the  appropriate  message.
       This program does not have to set errno to a value of 0 (zero) because errno is tested only if the open() function has failed.

       #include  <errno.h>  #include <stdio.h> #include <string.h> #include <fcntl.h> #include <sys/stat.h> #include <sys/types.h> #define SLENGTH
       80

       main() {
	   char filespec[SLENGTH], *eol;
	   int opret;
	   while (TRUE) {
	       printf("Enter file to be checked: ");
	       fgets(filespec, SLENGTH, stdin);
	       if ((eol = strchr(filespec, '
')) != NULL) {
		   *eol = '';  /* Replace newline with null */
		   if (*filespec == '') return; /* exit program */
		   opret = open(filespec,O_RDONLY,0);
		   if (opret > 0)
		     printf("%s: File exists
",filespec);
		   else if (errno == ENOENT)
		     printf("%s: File does not exist
",filespec);
		   else if (errno == ENOTDIR)
		     printf("%s: Prefix in path is not a directory
",
		       filespec);
		   else
		     perror(filespec);

	       } else		/* Line entered too long */
		 printf("Line entered is too long
");
	   } }

NOTES
       To ensure that your programs are portable, you should not explicitly declare errno to be an extern int in your program.	You should rely on
       the type int declaration in the <errno.h> include file.	Full use.

SEE ALSO
       Functions: intro(2), perror(3), strerror(3)

																	  errno(5)