#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


int head(const int fd, int lines)
{
	static const size_t SIZE = 1024 * 4;
	char buffer[SIZE];
	while (0 < lines) {
		const ssize_t available = read(fd, buffer, SIZE);
		if (-1 == available)
			return 1;
		if (0 == available)
			return 0;
		size_t offset = 0;
		for (ssize_t n = 0; n < available; ++n) {
			if (buffer[n] == '\n') {
				lines--;
				if (0 <= lines) {
					const ssize_t result = write(1, buffer + offset, 1 + n - offset);
					if (-1 == result)
						return 1;
					offset = 1 + n;
				}
				if (0 >= lines)
					return 0;
			}
		}
		if (0 < lines) {
			const ssize_t result = write(1, buffer + offset, available - offset);
			if (-1 == result)
				return 1;
		}
	}
	return 0;
}


int main(int argc, char * argv[])
{
	int opt;
	int lines = 10;
	while (-1 != (opt=getopt(argc, argv, "n:"))) {
		switch (opt) {
		case 'n':
			errno = 0;
			lines = strtol(optarg, (char **) NULL, 10);
			if (lines < 0)
				errno = EINVAL;
			if (0 != errno) {
				perror(optarg);
				return 1;
			}
			break;
		default:
			dprintf(2, "Usage: %s [-n lines] [file...]\n", argv[0]);
			return 1;
		}
	}
	const int files_count = argc - optind;
	int i = optind;
	int overall = 0;
	do {
		const char * const path = argc == i ? "-" : argv[i];
		const unsigned int is_stdin = '-' == path[0] && 0 == path[1];
		const int fd = is_stdin ? 0 : open(path, O_RDONLY);
		if (1 < files_count)
			printf(optind == i ? "==> %s <==\n" : "\n==> %s <==\n", path);
		if (0 <= fd) {
			const int result = head(fd, lines) | is_stdin ? 0 : close(fd);
			if (0 != result)
				perror(path);
			overall |= result;
		}
		else {
			perror(path);
			overall = 1;
		}
	}
	while (++i < argc);
	return overall;
}