Cannot distinguish between no-op insert and memory error

[scossu]

I preface that I'm quite new to C so this may be completely off. However, in:

c-algorithms/src/set.c

Lines 256 to 275 in 5b0555f

	if (set->equal_func(data, rover->data) != 0) {
	/* This data is already in the set */
	return 0;
	}
	rover = rover->next;
	}
	/* Not in the set. We must add a new entry. */
	/* Make a new entry for this data */
	newentry = (SetEntry *) malloc(sizeof(SetEntry));
	if (newentry == NULL) {
	return 0;
	}

the 0 return value is used both for an operation that does not change the state of a set (because the value already exists), which may or may not be an issue, and a memory error, which is more likely to be an issue, and of a different type.

Shall the former not use a kind of custom error code, and the latter an ENOMEM according to https://www.gnu.org/software/libc/manual/html_node/Error-Codes.html ?

Thanks.