Error return values
- C course
- Intermediate
- Lesson 57/116
- 5 min
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.
Practice 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;.
Starter code
#include <stdio.h>
// Include stdlib.h
int main(void) {
// Allocate an int with malloc
// If NULL, printf and return 1
// Else *p = 42, printf, free
return 0;
}Sample 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;
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match function to its purpose
| Term | Meaning |
|---|---|
fopen |
Opens a file |
malloc |
Allocates memory |
fgetc |
Reads a char |
printf |
Prints text |
2 What does malloc return on failure?
- -1
- EOF
- NULL Correct answer
- 0
Explanation: malloc returns NULL when it can't allocate memory.
3 Check the malloc result
int *p = malloc(sizeof(int));
if (p == ____) { return 1; }int *p = malloc(sizeof(int));
if (p == NULL) { return 1; }Explanation: If malloc returns NULL, there's no memory.