errno

 

C

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Type support

Dynamic memory management

Error handling

Program utilities

Date and time utilities

Strings library

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Thread support (C11)

Atomic operations (C11)

 

Error handling

Error codes

Error codes

errno

Assertions

assert

 

Defined in header <errno.h>
#define errno /*implementation-defined*/

errno is a preprocessor macro that expands to a thread-local modifiable lvalue of type int. Several standard library functions indicate errors by writing positive integers to errno. Typically, the value of errno is set to one the error codes, listed in <errno.h> as macro constants that begin with the letter E, followed by uppercase letters or digits.

The value of errno is ​0​ at program startup, and although library functions are allowed to write positive integers to errno whether or not an error occurred, library functions never store ​0​ in errno.

Example

#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
 
int main (int argc, char **argv)
{
	int fd = -1;
	fd = open ("/dev/zer0", O_RDONLY);
 
	if (errno)
	{
		perror ("Ouuupsss");
		exit (EXIT_FAILURE);
	}
 
	close (fd);
 
	return EXIT_SUCCESS;
}

Output:

Ouuupsss: No such file or directory

See also

E2BIG, EACCES, ..., EXDEV	macros for standard POSIX-compatible error conditions (macro constant)
perror	displays a character string corresponding of the current error to stderr (function)
strerror	returns a text version of a given error code (function)