errno(5)errno(5)NAMEerrno - 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 refer‐
ence 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 pro‐
gram 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, '\n')) != NULL) {
*eol = '\0'; /* Replace newline with null */
if (*filespec == '\0') return; /* exit program */
opret = open(filespec,O_RDONLY,0);
if (opret > 0)
printf("%s: File exists\n",filespec);
else if (errno == ENOENT)
printf("%s: File does not exist\n",filespec);
else if (errno == ENOTDIR)
printf("%s: Prefix in path is not a directory\n",
filespec);
else
perror(filespec);
} else /* Line entered too long */
printf("Line entered is too long\n");
} }
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)
