Error return values

Learn error return values from functions.

Error return values

Many C functions report failure by return value. fopen returns NULL, malloc NULL, fgetc EOF, printf a negative number, and main a non-zero value. Using the return value without checking is undefined behavior.

The good habit: make the call, test the result, handle the error. On error, either return upward or print with perror and continue. Read the library docs to know which value each function returns; each function picks its own value.

Error propagation patterns

When a function catches an error, you have two choices: propagate upward or handle it. To propagate, make the error the return value: int read_config(void) { ... return -1; }. The caller checks again.

Handling means accepting the error and switching to an alternative: FILE *f = fopen("config.txt", "r"); if (!f) f = fopen("default.txt", "r");. Which one to choose depends on context: propagate critical errors, handle recoverable ones. Use consistent return values to avoid mixing them.

Exercise Check the malloc result

Include <stdio.h> and <stdlib.h>. In main, allocate with int *p = malloc(sizeof(int));. If if (p == NULL), print printf("no memory\n"); and return 1;. Otherwise set *p = 42;, print printf("%d\n", *p);, and free with free(p);. End with return 0;.

Show example solution

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

int main(void) {
    int *p = malloc(sizeof(int));
    if (p == NULL) {
        printf("no memory\n");
        return 1;
    }
    *p = 42;
    printf("%d\n", *p);
    free(p);
    return 0;
}

Test yourself

Match function to its purpose

fopen

Opens a file
malloc

Allocates memory
fgetc

Reads a char
printf

Prints text
What does malloc return on failure?
- -1
- EOF
- NULL
- 0
Show answer

NULL — malloc returns NULL when it can't allocate memory.

Check the malloc result

int *p = malloc(sizeof(int));
if (p == ____) { return 1; }

Show answer

int *p = malloc(sizeof(int));
if (p == NULL) { return 1; }

Next lesson: Summary - Error handling

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