perror() Function - example program in C

By: Lakshmi

The perror() function is another of C's error-handling tools. When called, perror() displays a message on stderr describing the most recent error that occurred during a library function call or system call. The prototype, in STDIO.H, is

void perror(char *msg);

The argument msg points to an optional user-defined message. This message is printed first, followed by a colon and the implementation-defined message that describes the most recent error. If you call perror() when no error has occurred, the message displayed is no error.

A call to perror() does nothing to deal with the error condition. It's up to the program to take action, which might consist of prompting the user to do something such as terminate the program. The action the program takes can be determined by testing the value of errno and by the nature of the error. Note that a program need not include the header file ERRNO.H to use the external variable errno. That header file is required only if your program uses the symbolic error constants. Sample program below illustrates the use of perror() and errno for handling runtime errors.

Using perror() and errno to deal with runtime errors.

1: /* Demonstration of error handling with perror() and errno. */
2:
3: #include <stdio.h>
4: #include <stdlib.h>
5: #include <errno.h>
6:
7: main()
8: {
9:     FILE *fp;
10:     char filename[80];
11:
12:     printf("Enter filename: ");
13:     gets(filename);
14:
15:     if (( fp = fopen(filename, "r")) == NULL)
16:     {
17:         perror("You goofed!");
18:         printf("errno = %d.\n", errno);
19:         exit(1);
20:     }
21:     else
22:     {
23:         puts("File opened for reading.");
24:         fclose(fp);
25:     }
26:     return(0);
27: }
Enter file name: list19_4.c
File opened for reading.
Enter file name: notafile.xxx
You goofed!: No such file or directory
errno = 2.

ANALYSIS: This program prints one of two messages based on whether a file can be opened for reading. Line 15 tries to open a file. If the file opens, the else part of the if loop executes, printing the following message:

File opened for reading.

If there is an error when the file is opened, such as the file not existing, lines 17 through 19 of the if loop execute. Line 17 calls the perror() function with the string "You goofed!". This is followed by printing the error number. The result of entering a file that does not exist is

You goofed!: No such file or directory.
errno = 2

DO include the ERRNO.H header file if you're going to use the symbolic errors.
DON'T include the ERRNO.H header file if you aren't going to use the symbolic error constants.

DO check for possible errors in your programs. Never assume that everything is okay.