Question

I would have preferred to add a comment to the answer to this question
but didn’t have enough points. Consider the following code:

enum _config_error
{
    E_SUCCESS = 0,
    E_INVALID_INPUT = -1,
    E_FILE_NOT_FOUND = -2, /* consider some way of returning the OS error too */
    ...
};

/* type to provide in your API */
typedef _config_error error_t;

/* use this to provide a perror style method to help consumers out */
struct _errordesc {
    int  code;
    char *message;
} errordesc[] = {
    { E_SUCCESS, "No error" },
    { E_INVALID_INPUT, "Invalid input" },
    { E_FILE_NOT_FOUND, "File not found" },
    ...
};

How does one lookup the error description from errordesc? I can see two problems with the version I come up with:

/* add E_COUNT = 3 to enum _config_error */
const char *errorstring(error_t errnum)
{
    unsigned int i;

    for (i = 0; i < E_COUNT; ++i) {
        if (errordesc[i].code == errnum) {
            return errordesc[i].message;
        }

    }

    return "Can't reach this point";
}

1. One does know the enum size and has to manually set E_COUNT to 3.
2. One cannot reach the return after the for loop, what to do there?
3. Is there a better solution?

Answer 1

1. You can calculate E_COUNT from sizeof(errordesc) / sizeof(struct _errordesc).
2. If you reach the end of the loop, simply return "Unknown error" or something similar.
3. Since your error codes seems to be consecutive (but negative) you could index directly into the array using -errnum.