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As such C programming does not provide direct support for error handling but being a system programming language, it provides you access at lower level in the form of return values. Most of the C or even Unix function calls return -1 or NULL in case of any error and sets an error code errno is set which is global variable and indicates an error occurred during any function call. You can find various error codes defined in <error.h> header file.

So a C programmer can check the returned values and can take appropriate action depending on the return value. As a good practice, developer should set errno to 0 at the time of initialization of the program. A value of 0 indicates that there is no error in the program.

The errno, perror() and strerror()

The C programming language provides perror() and strerror() functions, which can be used to display the text message associated with errno.

  • The perror() function displays the string you pass to it, followed by a colon, a space, and then the textual representation of the current errno value.
  • The strerror() function, which returns a pointer to the textual representation of the current errno value.

Let's try to simulate an error condition and try to open a file which does not exist. Here I'm using both the functions to show the usage, but you can use one or more ways of printing your errors. Second important point to note is that you should use stderr file stream to output all the errors.

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

extern int errno ;

int main ()
   FILE * pf;
   int errnum;
   pf = fopen ("unexist.txt", "rb");
   if (pf == NULL)
      errnum = errno;
      fprintf(stderr, "Value of errno: %d\n", errno);
      perror("Error printed by perror");
      fprintf(stderr, "Error opening file: %s\n", strerror( errnum ));
      fclose (pf);
   return 0;

When the above code is compiled and executed, it produces the following result:

Value of errno: 2
Error printed by perror: No such file or directory
Error opening file: No such file or directory

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