CError Handling

Error Handling in C

Error handling lets you detect and respond to problems in your program, like a file that can't be opened or memory that can't be allocated, so your program doesn't crash or behave unexpectedly.

Unlike some languages, C does not have built-in exception handling (liketry/catch). Instead, C uses return values, global error codes, and helper functions likeperror() andstrerror().

Using Return Values

In the previous chapter, you learned that functions likefopen() returnNULL when something goes wrong.

You can check forNULL using anif statement to detect and handle errors before your program crashes.

In the example below, we try to open a file that does not exist. Sincefopen() fails, it returnsNULL and we print an error message:

#include <stdio.h>int main() {  FILE *fptr = fopen("nothing.txt", "r");  if (fptr == NULL) {    printf("Error opening file.\\n");    return 1;  }  fclose(fptr);  return 0;}

Get More Details

If you want more details about what went wrong, you can use theperror() function.

It prints a custom error message followed by a description of the last error that occurred:

#include <stdio.h>int main() {  FILE *f = fopen("nothing.txt", "r");  if (f == NULL) {    perror("Error opening file");    return 1;  }  fclose(f);  return 0;}

Using strerror() and errno

errno is a global variable that stores the error code from the last failed operation. You can include<errno.h> to access it, andstrerror(errno) will convert the error code into a readable message:

#include <stdio.h>#include <errno.h>#include <string.h>int main() {  FILE *f = fopen("nothing.txt", "r");  if (f == NULL) {    printf("Error: %s\n", strerror(errno));    return 1;  }  fclose(f);  return 0;}

Common Error Codes

Error constants are defined in<errno.h>. You can compareerrno to them to detect specific issues:

#include <stdio.h>#include <errno.h>int main() {  FILE *f = fopen("nothing.txt", "r");  if (f == NULL) {    if (errno == ENOENT) {      printf("The file was not found.\n");    } else {      printf("Some other file error occurred.\n");    }    return 1;  }  fclose(f);  return 0;}

Using exit() to Stop the Program

If you want to stop the program immediately when an error occurs, you can useexit(). It lets you return a status code to the operating system.

Exit codes help signal whether the program finished successfully or with an error, like:

• 0 means success
• Non-zero values (like1 orEXIT_FAILURE) indicate errors

#include <stdio.h>#include <stdlib.h>int main() {  FILE *f = fopen("nothing.txt", "r");  if (f == NULL) {    printf("Failed to open file.\n");    exit(1);  }  fclose(f);  return 0;}

Common Exit Status Codes

#include <stdio.h>#include <stdlib.h>int main() {  FILE *f = fopen("nothing.txt", "r");  if (f == NULL) {    perror("Could not open nothing.txt");    exit(EXIT_FAILURE); // More readable than exit(1)  }  fclose(f);  return EXIT_SUCCESS;}

Summary

• Many C functions returnNULL when something goes wrong
• Useperror() to print a message about the error
• Usestrerror(errno) to get the error message as a string
• errno stores the error code from the last failed action
• You can compareerrno to values likeENOENT (file not found) orENOMEM (not enough memory)
• Useexit() to stop the program early if there's an error