Error Handling

C does not provide any direct support for error handling.

errno: Global variable that stores program’s state.

errno value Error
1 Operation not permitted
2 No such file or directory
3 No such process
4 Interrupted system call
5 I/O error
6 No such device or address
7 Argument list too long
8 Exec format error
9 Bad file number
10 No child processes
11 Try again
12 Out of memory
13 Permission denied
Example: Using errno 
#include <stdio.h>
#include <errno.h>
#include <string.h>
int main () {
  FILE *fp;

  printf("Value of errno: %d\n ", errno);

  fp = fopen("IWillReturnError.txt", "r");

  printf("Value of errno: %d\n ", errno);

  printf("The error message is : %s\n", strerror(errno));
  perror("Message from perror");
  return 0;
}

Division by Zero

Division by zero leads to an error with no handling/status.

exit()

Used to indicate exit status.

Example: Using exit() 
if (ptr == NULL)
{
  puts("malloc failed");
  puts(strerror(errno));
  exit(EXIT_FAILURE);
}

Preprocessor Statements

The preprocessors will process directives inserted in the C source code.

Directive Description Example
#include Include another C file into the current file at the location of the #include statement prior to compiling the source code. #include <stdio.h>
#define Define a macro which can be used as a constant throughout the source code. #define AGE 50
#undef Clear a macro which was previously defined. #undef AGE
#if Conditional expresssion which can be used to include source code for compilation. #if AGE > 50
#ifdef Allows the inclusion of source code if the provided macro identifier has been defined. Equivalent to #if defined(identifier). #ifdef SOLARIS
#ifndef Allows the inclusion of source code if the provided macro identifier has not been defined. #ifndef WINDOWS
#elif Provides an alternate inclusion of source code when used with the #if, #ifdef, or #ifndef directives and the #elif condition evaluates to true. #elif YEARS_OLD > 10
#else Allows the inclusion of source code if the preceeding #if, #ifdef, or #ifndef directive expression evaluates to false. #else
#endif Signals the end of a #if, #ifdef or #ifndef condition . #endif
#warning Report a warning message and continue preprocessing. #warning Non-critical error found
#error Report error and stop preprocessing. #error Windows is an unsupported platform
Example: Using #define 
#include <stdio.h>
#define height 100
#define number 3.14
#define letter 'A'
#define letter_sequence "ABC"
#define backslash_char '\?'
void main()
{
  printf("value of height : %d \n", height );
  printf("value of number : %f \n", number );
  printf("value of letter : %c \n", letter );
  printf("value of letter_sequence : %s \n", letter_sequence);
  printf("value of backslash_char : %c \n", backslash_char);
}
Example: Using #ifdef, #else, and #endif 
#include <stdio.h>
#define COUNT 100
int main()
{
  #ifdef COUNT
  printf(”COUNT is defined. So, this line will be added in " \
  "this C file\n");
  #else
  printf(”COUNT is not defined\n");
  #endif
  return 0;
}
Example: Using #error 
#include <stdio.h>
#include <limits.h>
#define MILLISECONDS(age) (age * 365 * 24 * 60 * 60 * 1000)
int main() {
  int age;
  #if INT_MAX < MILLISECONDS(12)
  #error Integer size cannot hold our age in milliseconds
  #endif
  age = MILLISECONDS(12);
  printf("John is %d milliseconds old\n", age);
  return 0;
}

String Tokenizer in string.h

Format: char* strtok(char* s1, const char* s2);

Example: Using strtok() 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char line[] = "2x^0 - 4x^1 + 3x^3 + 54x^4";
int main() {
  int counter=0;
  printf("Polynomial is %s\n",line) ;
  // Reading terms of a polynomial, ignoring whitespace
  char *tok = strtok(line, "x^");
  while (tok) {
      printf("*%s*\n", tok);
      tok = strtok(NULL, "x^");
      counter++ ;
  }
  printf("Number of terms are %d\n", counter-1);
}

extern Variables

extern variables are like global variables, except they can be shared among multiple files.

Example: Using extern

f1.c:

global variable
#include <stdio.h>
int globalVar=3;
void fun();
int main() {
  fun();
  printf("Global var in f1 is %d\n", globalVar);
  return 1;
}

f2.c:

#include <stdio.h>
extern int globalVar;
void fun() {
  printf("Global var in f2 is %d\n",
  globalVar);
  globalVar++;
}

system() call in stdlib.h

Lets you run system calls in UNIX systems.

Example: Using system calls 
#include <stdio.h>
#include <stdlib.h>
system() call in stdlib.h
extern char **environ;
int main() {
  puts("Looping with index");
  for (int index = 0; environ[index] != NULL; ++index)
      puts(environ[index]);
  puts("===============");
  puts("Looping with pointer");
  for (char **p = environ; *p != NULL; ++p)
      puts(*p);
  puts("===============");
  puts("system(\"env\")");
  system("env"); //system command env at prompt
}