How to know what the 'errno' means in Linux?

Errno is a global variable in Linux and Unix systems that indicates the type of error that occurred during system calls and library function execution. When a system call or library function encounters an error, it sets the errno variable to a specific integer value that corresponds to the type of error. This mechanism allows developers to identify and handle different types of errors programmatically.

The errno variable is defined in the <errno.h> header file and is thread-local in modern implementations. Each error code has both a numeric value and a symbolic constant name written in uppercase letters.

Common Errno Values

Here are some frequently encountered errno symbols −

EPERM      - Operation not permitted
ENOENT     - No such file or directory  
ESRCH      - No such process
EINTR      - Interrupted system call
EIO        - Input/output error
ENXIO      - No such device or address

Installing the Errno Tool

While errno values are built into the system, you can install a command-line tool to easily look up error meanings −

For Ubuntu/Debian

sudo apt-get install errno

For CentOS/RHEL

yum install errno

Using the Errno Tool

Once installed, you can list all errno symbols and their descriptions −

errno -l

1 EPERM Operation not permitted
2 ENOENT No such file or directory
3 ESRCH No such process
4 EINTR Interrupted system call
5 EIO Input/output error
6 ENXIO No such device or address
7 E2BIG Argument list too long
8 ENOEXEC Exec format error
9 EBADF Bad file descriptor
10 ECHILD No child processes

The complete list contains over 130 different errno values. You can also query a specific errno number −

errno 2

ENOENT 2 No such file or directory

Alternative Methods

If the errno tool is not available, you can use other methods to interpret error codes −

perror "Error message"

Or check the system's error code documentation −

man 3 errno

Practical Usage

In C programs, you typically check errno after a system call fails −

#include <errno.h>
#include <stdio.h>
#include <string.h>

if (some_system_call() == -1) {
    printf("Error: %s
", strerror(errno));
}

Conclusion

Understanding errno values is essential for debugging Linux system errors and developing robust applications. The errno tool provides a convenient way to look up error meanings, while the underlying mechanism helps developers identify and handle specific error conditions in their code.