Error Handling in C language

Error Handling-
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 ));
}
else
{
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