C Array Of Pointers C Standard Library Stdarg H C Exercise Example17 C Recursion C Function Tmpnam C Function Wctomb C Examples Lexicographical Order C Function Strncmp C Function Abs C Examples Alphabet C Examples Printf Int C Exercise Example26 C Exercise Example84 C Examples Write File C Function Div C Function Toupper C Exercise Example44 C Function Raise C Examples Quadratic Roots C Function Cos C Exercise Example75 C Examples Check Armstrong Number C Examples Output Array C Variables C Standard Library Ctype H C Examples Complex Number Add C Function Fgetpos C Exercise Example22 C Exercise Example69 C Examples Octal Decimal Convert C Exercise Example80 C Function Gmtime C Function Gets C Function Isgraph C Exercise Example58 C Type Casting C Exercise Example13 C Nested If C Function Puts C Command Line Arguments C Exercise Example98 C Examples Binary Decimal Convert C Exercise Example71 C If C Sort Algorithm C Examples Calculator Switch Case C Passing Arrays To Functions C Exercise Example40 C Function Atol C Function Localtime C Macro Edom C Function Putchar C Function Strncat C Function Scanf C Exercise Example6 C Function Signal C Examples Factors Number C Examples Fibonacci Series C Examples Add Matrix C Function Rewind C Function Strftime C Function Longjmp C Exercise Example94 C Function Clearerr C Exercise Example36 C Examples Five Fish Html C Exercise Example54 C Function Strcpy C Function Feof C Examples Divide Concatenation Array C Exercise Example65 C Examples Structure Store Information C Variable Arguments C Exercise Example90 C Examples Access Array Pointer C Examples Remainder Quotient C Function Mktime C Pointers C Examples Swapping Cyclic Order C Exercise Example79 C Exercise Example32 C Exercise Example2 C Function Frexp C Exercise Example48 C Function Strpbrk C Function Fwrite C Exercise Example61 C Examples C Fun Pointer Callback C Basic Syntax C Data Types C Function Tolower C Function Fflush C Exercise Example88 C Function Fseek C Function Sprintf C Function Perror C Examples Sum Array C Exercise Example50 C Macro Assert C Do While Loop C Goto Statement C Exercise Example7 C Examples Int Data Compare C Function Vsprintf C Function Qsort C Nested Switch C Examples String Copy C Exercise Example95 C Function Vprintf C Function Getc C Exercise Example37 C Function Pow C Function Putc C Function Call By Pointer C Function Sinh C Examples Source Code Output C Exercise Example55 C Function Fread C Examples Sum Prime Numbers C Exercise Example64 C Function Isprint C Examples Printf Float C Examples Vowel Consonant Frequency String C Exercise Example91 C Multi Dimensional Arrays C Examples Add Numbers C Macro Errno C Function Sscanf C Exercise Example33 C Examples Ascii Value Character C Exercise Example78 C Function Free C Exercise Example49 C Exercise Example3 C Break Statement C Function Getchar C Examples Matrix Transpose C Function Strtok C Function Setlocale C Function Log10 C Exercise Example60 C Macro Va_Start C Examples Printf Helloworld C Exercise Example89 C Pointer To Pointer C Function Wcstombs C Function Setbuf C Function Isalnum C Exercise Example51 C Standard Library Errno H C Strings C Function Sin C Examples Leap Year C Function Clock C Function Strspn C Return Pointer From Functions C Exercise Example16 C Function Memmove C Standard Library Time H C Enum C Exercise Example27 C Function Strtoul C Function Log C Function Strrchr C Exercise Example85 C Bit Fields C Exercise Example45 C Function Fabs C Function Atoi C Storage Classes C Function Memcpy C Examples Power Number C Function Ceil C Exercise Example74 C Exercise Example68 C Exercise Example23 C Function Fmod C Function Strerror C 100 Examples C Loops C Examples Table Style C Exercise Example81 C Exercise Example12 C Exercise Example59 C Typedef C Examples Palindrome Number C Exercise Example99 C Function Tanh C Function Getenv C Function Exit C Exercise Example70 C Function Fopen C Function Abort C Function Ldexp C Function Ispunct C Function Isdigit C Exercise Example41 C Intro C Environment Setup C Examples Reverse Sentence Recursion C Exercise Example5 C Function Ftell C Examples Frequency Character C Function Sqrt C Function System C Function Strstr C Examples Concatenate String C Structures C Exercise Example35 C Exercise Example97 C Standard Library Float H C Exercise Example57 C Examples Negative Positive Zero C Switch C Examples Dynamic Memory Allocation Largest C Examples Factorial C Function Rand C Examples String Length C Function Isalpha C Exercise Example66 C Standard Library String H C Examples Hcf Gcd C Examples Remove Characters String C Examples Copy Array C Function Realloc C Examples Average Arrays C Exercise Example31 C Exercise Example93 C Arrays C Function Memcmp C Function Ferror C Examples Lcm C Function Atan C Standard Library Stdlib H C Examples Time Structure C Exercise Example1 C Continue Statement C Exercise Example29 C Exercise Example62 C Examples Octal Binary Convert C Function Isxdigit C Function Cosh C Examples Multiplication Table C Exercise Example53 C Exercise Example18 C Pointer Arithmetic C Examples Reverse Number C Function Fgetc C Function Printf C Examples Product Numbers C Exercise Example14 C Function Bsearch C Function Asin C Exercise Example87 C Function Setvbuf C Nested Loops C Passing Pointers To Functions C Function Ldiv C Input Output C Functions C Exercise Example25 C Function Fprintf C Function Labs C Function Strxfrm C Function Iscntrl C Exercise Example47 C Examples For Even Odd C Scope Rules C Exercise Example76 C Function Mbtowc C Examples Array Largest Element C Function Fsetpos C Function Fputc C Function Memset C Function Atexit C Unions C Exercise Example83 C Examples Printf Double C Function Exp C Examples Sum Natural Numbers C Function Malloc C Exercise Example21 C Function Srand C Macro Va_End C Exercise Example10 C Exercise Example100 C Function Ctime C Exercise Example72 C Memory Management C Exercise Example39 C Function Mblen C While Loop C Preprocessors C Standard Library Limits H C Examples Prime Number C Function Localeconv C Function Strchr C Exercise Example43 C Exercise Example9 C Examples Even Odd C Exercise Example15 C Macro Offsetof C Macro Null C Function Difftime C Examples Pyramid Pattern C Exercise Example86 C Exercise Example24 C Program Structure C Examples Joseph Life Dead Game C Error Handling C Function Strtod C Function Calloc C Exercise Example46 C Function Strcat C Exercise Example77 C Function Freopen C Function Acos C Macro Va_Arg C Function Asctime C Constants C Exercise Example82 C Function Atan2 C Function Mbstowcs C Exercise Example20 C Function Strlen C Function Strcspn C Standard Library Stdio H C Quiz C Exercise Example11 C Function Isupper C Examples Largest Number Three C Function Rename C Function Strcmp C Exercise Example101 C Function Vfprintf C Exercise Example38 C Exercise Example73 C For Loop C Function Modf C Exercise Example8 C Function Call By Value C Function Time C Exercise Example42 C Return Arrays From Function C Operators C Function Strtol C Exercise Example4 C Exercise Example34 C Exercise Example96 C Header Files C Function Fscanf C Function Islower C Exercise Example56 C File Io C Standard Library Assert H C Examples Display Alphabets C Function Snprintf C Macro Erange C Examples Sizeof Operator C Exercise Example67 C Standard Library Setjmp H C Exercise Example30 C Examples Standard Deviation C Function Ungetc C Standard Library Math H C Function Isspace C Tutorial C If Else C Function Fgets C Exercise Example92 C Standard Library C Examples Smallest Array Element C Function Remove C Examples Vowel Consonant C Function Tmpfile C Examples Digits Count C Function Atof C Standard Library Stddef H C Examples Printf Char C Function Strcoll C Function Memchr C Function Strncpy C Standard Library Locale H C Function Floor C Macro Setjmp C Exercise Example63 C Exercise Example28 C Standard Library Signal H C Examples Swapping C Pointer To An Array C Exercise Example19 C Exercise Example52 C Decision C Function Fputs C Examples Read File C Function Fclose

#define NULL ((char *)0)

or

#define NULL 0L

or

#define NULL 0

#include <stddef.h>
#include <stdio.h>

int main ()
{
   FILE *fp;

   fp = fopen("file.txt", "r");
   if( fp != NULL ) 
   {
      printf("Successfully opened file file.txt\n");
      fclose(fp);
   }

   fp = fopen("nofile.txt", "r");
   if( fp == NULL ) 
   {
      printf("Cannot open file nofile.txt\n");
   }

   return(0);
}

Successfully opened file file.txt
Cannot open file nofile.txt