replace string in C

str_replace.c

// fork from https://github.com/irl/la-cucina/blob/master/str_replace.c
char* str_replace(char* string, const char* substr, const char* replacement) {
	char* tok = NULL;
	char* newstr = NULL;
	char* oldstr = NULL;
	int   oldstr_len = 0;
	int   substr_len = 0;
	int   replacement_len = 0;

	newstr = strdup(string);
	substr_len = strlen(substr);
	replacement_len = strlen(replacement);

	if (substr == NULL || replacement == NULL) {
		return newstr;
	}

	while ((tok = strstr(newstr, substr))) {
		oldstr = newstr;
		oldstr_len = strlen(oldstr);
		newstr = (char*)malloc(sizeof(char) * (oldstr_len - substr_len + replacement_len + 1));

		if (newstr == NULL) {
			free(oldstr);
			return NULL;
		}

		memcpy(newstr, oldstr, tok - oldstr);
		memcpy(newstr + (tok - oldstr), replacement, replacement_len);
		memcpy(newstr + (tok - oldstr) + replacement_len, tok + substr_len, oldstr_len - substr_len - (tok - oldstr));
		memset(newstr + oldstr_len - substr_len + replacement_len, 0, 1);

		free(oldstr);
	}

	free(string);

	return newstr;
}

There is an infinite loop if you try to double a char:

str_replace("test_test", "_" "__");

expected output "test__test", result is infinite loop because you try to replace from the start of the newly created string at each step

The outcome is the same if you try to replace the subtring with the same string, meaning:

str_replace("test", "e", "e"); // infinite looop

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