Custom Errors (Categorizing with Error Codes)

This lesson covers expressing your own error categories in C using error codes, so you can distinguish between types of errors more specifically. It's for anyone searching "C error code categorization."

C has no exception classes, but defining error codes with enum makes it easier to distinguish between error types. Think of it like preparing a named constant for each category of error.

The example defines error types with typedef enum { ERR_NONE, ERR_INVALID_AGE } ErrorCode;, and the check_age() function returns ERR_INVALID_AGE when the age is negative. The caller can tell what went wrong by looking at the returned error code.

A common early mistake is not realizing an error code alone struggles to convey detailed information (like a message). If you need more detail, you'd need a technique like bundling error info into a struct instead.

In real projects, distinguishing between something like "an invalid input" error and "a network failure" error lets you write the appropriate response for each in code.

📖 Reference code

#include <stdio.h>

typedef enum {
    ERR_NONE,
    ERR_INVALID_AGE
} ErrorCode;

ErrorCode check_age(int age, int *result) {
    if (age < 0) {
        return ERR_INVALID_AGE;
    }
    *result = age;
    return ERR_NONE;
}

int main(void) {
    int result;
    ErrorCode err = check_age(-5, &result);
    if (err == ERR_INVALID_AGE) {
        printf("ValidationError: age must be 0 or greater\n");
    }
    return 0;
}

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).