131 Linux Error Codes for C Programming Language using errno

Programmers should handle all kinds of errors to protect the program from failure.

In C programming language, there is no direct support for error handling. You have to detect the failure and handle the error. In C programming language, return values represents success or failure. Inside a C program, when a function fails, you should handle the errors accordingly, or at least record the errors in a log file.

When you are running some program on Linux environment, you might notice that it gives some error number. For example, “Error no is : 17”, which doesn’t really say much. You really need to know what error number 17 means.

This article shows all available error numbers along with it descriptions. This article might be a handy reference for you, when you encounter an error number and you would like to know what it means.

• In C programming language, there is an external variable called “errno”.
• From this errno variable you can use some error handling functions to find out the error description and handle it appropriately.
• You have to include errno.h header file to use external variable errno.
• perror function prints error description in standard error.
• The strerror function returns a string describing the error code passed in the argument errnum.

The following C code snippet tries to open a file through open system call. There are two flags in the open call. O_CREAT flag is to create a file, if the file does not exist. O_EXCL flag is used with O_CREAT, if the file is already exist open call will fail with the proper error number.

$ cat fileopen.c
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
main()
{
// Declaration of a file descriptor
int fd;

// Opening a file
fd = open("/root/sasikala/testing",O_CREAT|O_EXCL);
// If Open is failed
if ( fd < 0 ) {
        printf("Opening file : Failed\n");
        printf ("Error no is : %d\n", errno);
        printf("Error description is : %s\n",strerror(errno));
}
// If Open is success
else
        printf("Opening file : Success\n");

}

$ cc -o fileopen fileopen.c
$ ./fileopen

Opening file : Success

$ ./fileopen
Opening file : Failed
Error no is : 17
Error description is : File exists

At first execution, open got executed successfully, and it created the file since the file was not available. In next execution, it throws an error number 17, which is “File already exist”.

The following table shows list of error numbers and its descriptions in Linux operation system

When you see an error number thrown by a C program on a Linux environment, you might find the above table handy to identify what those error number means. Make sure to bookmark this article for future reference.