Substring in C

substr.c

#include <stdio.h>
#include <string.h>

/* If you're feeling sassy, a macro one-liner w/out checks */
#define substr(dest, src, dest_size, startPos, strLen)	snprintf(dest, dest_size, "%.*s", strLen, src+startPos)

/* Python-like idiom of negative values to count from the end of the string. */
void substr (char *dest, const char *src, int startPos, size_t strLen) {
	/* Cannot do anything with NULL. */
	if (dest == NULL || src == NULL) return;

	size_t len = strlen (src);

	/* All negative positions to go from end, and cannot
	start before start of string, force to start. */
	if (startPos < 0) {
		startPos = len + startPos;
	}
	if (startPos < 0) {
		startPos = 0;
	}

	/* Force negative lengths to zero and cannot
	start after end of string, force to end. */
	if ((size_t)startPos > len) {
		startPos = len;
	}

	len = strlen (&src[startPos]);
	/* Adjust length if source string too short. */
	if (strLen > len) {
		strLen = len;
	}

	/* Copy string section */
	memcpy(dest, src+startPos, strLen);
	dest[strLen] = '\0';
}