Error Handling in C

C does not have built-in exception handling like C++ or Java. Instead, error handling is done through return values, errno, and error-reporting functions.

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Return Value Conventions

Functions signal errors by returning special values:

FILE *fp = fopen("file.txt", "r");
if (fp == NULL) {
    /* error: file could not be opened */
}

int *arr = (int *)malloc(100 * sizeof(int));
if (arr == NULL) {
    /* error: out of memory */
}

Common conventions:

• Return NULL pointer on failure (pointer-returning functions)
• Return -1 or negative value on failure (int-returning functions)
• Return 0 for success (e.g., main)

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errno — Error Number

The global variable errno (from <errno.h>) is set by system calls and library functions when an error occurs.

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

FILE *fp = fopen("missing.txt", "r");
if (fp == NULL) {
    printf("Error code: %d\n", errno);
    printf("Error: %s\n", strerror(errno));
}

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perror() — Print Error Message

FILE *fp = fopen("missing.txt", "r");
if (fp == NULL) {
    perror("fopen");
    /* Output: fopen: No such file or directory */
}

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strerror() — Get Error String

#include <string.h>
printf("%s\n", strerror(errno));

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Common errno Values

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assert() — Debug Assertions

#include <assert.h>

int divide(int a, int b) {
    assert(b != 0);   /* aborts program if b == 0 in debug mode */
    return a / b;
}

• Disabled in release builds by defining NDEBUG
• assert(expr) — if expr is false, prints message and calls abort()

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Error Handling Pattern

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

int processFile(const char *filename) {
    FILE *fp = fopen(filename, "r");
    if (!fp) {
        perror("fopen");
        return -1;   /* signal error */
    }
    
    char line[256];
    while (fgets(line, sizeof(line), fp)) {
        printf("%s", line);
    }
    
    if (ferror(fp)) {
        perror("fgets");
        fclose(fp);
        return -1;
    }
    
    fclose(fp);
    return 0;   /* success */
}

int main() {
    if (processFile("data.txt") != 0) {
        fprintf(stderr, "Failed to process file\n");
        return EXIT_FAILURE;
    }
    return EXIT_SUCCESS;
}