Reverse a file

reverse.c

#include <stdio.h>
#include <unistd.h>

#define BLOCK_SIZE		512

#define min(a, b)		((a) < (b) ? (a) : (b))

inline void swap(char* buf, size_t a, size_t b) {
	char tmp = buf[a];
	buf[a] = buf[b];
	buf[b] = tmp;
}

static int reverse(FILE* in, FILE* out) {
	// not reentrant but faster than without 'static'
	static char buf[BLOCK_SIZE];

	// TODO: check return value
	fseek(in, 0, SEEK_END);

	long file_left = ftell(in);

	while(file_left > 0) {
		size_t cur_block = min(BLOCK_SIZE, file_left);
		size_t buf_filled = 0;

		// TODO: check return value
		fseek(in, -cur_block, SEEK_CUR);

		// make sure we read a full block
		do {
			ssize_t now_read = fread(buf + buf_filled, sizeof(char), cur_block - buf_filled, in);

			buf_filled += now_read;

			if(now_read <= 0) {
				return 1;
			}
		} while(buf_filled < cur_block);

		// TODO: check return value
		fseek(in, -cur_block, SEEK_CUR);

		file_left -= cur_block;

		// actual reversing
		for(int i = 0; i < cur_block / 2; ++i) {
			swap(buf, i, cur_block - 1 - i);
		}

		// make sure we write a full block
		do {
			ssize_t written = fwrite(buf + cur_block - buf_filled, sizeof(char), buf_filled, out);

			buf_filled -= written;

			if(written < 0) {
				return 1;
			}
		} while(buf_filled > 0);
	}

	return 0;
}

int main(int argc, char* args[]) {
	FILE *in, *out;

	if(argc > 1) {
		in = fopen(args[1], "r");

		if(in == NULL) {
			fprintf(stderr, "unable to open input file!\n");
			return 3;
		}
	} else {
		fprintf(stderr, "give me an input file!\n");
		return 2;
	}

	if(argc > 2) {
		out = fopen(args[2], "w");

		if(out == NULL) {
			fprintf(stderr, "unable to open output file!\n");
			return 3;
		}
	} else {
		out = stdout;
	}

	return reverse(in, out);
}