#include <sys/types.h>	/** open fstat					  **/
#include <sys/stat.h>	/** open fstat					  **/
#include <fcntl.h>	/** open					  **/
#include <unistd.h>	/** write close fstat				  **/
#include <stdio.h>	/** fprintf perror				  **/
#include <stdlib.h>	/** EXIT_FAILURE EXIT_SUCCESS			  **/

#define TESTFILE	"./tstfile"
#define LOOPMAX		0xF

void chklseek_then_read_byte(int fd, off_t pos); 
ssize_t chkwrite(int fd, const void *buf, size_t count);

int main(int argc, char **argv)
{
	int fd;
	unsigned char buf;
	
	if ((fd = open(TESTFILE, O_RDWR | O_CREAT | O_TRUNC,
					S_IRUSR | S_IWUSR)) == -1) {
		perror("main:: open failed");
		return EXIT_FAILURE;
	}
       	for (buf = 0; buf < LOOPMAX; buf++)
       		chkwrite(fd, &buf, 1);
       	if (close(fd) == -1) {
       		perror("main:: close failed");
		return EXIT_FAILURE;
	}
	return EXIT_SUCCESS;
}

ssize_t chkwrite(int fd, const void *buf, size_t count)
{
	ssize_t count_ret, delta;
	off_t init_pos, final_pos = -1;
	struct stat stat;

	if ((init_pos = lseek(fd, 0, SEEK_CUR)) == -1)
		perror("chkwrite:: lseek failed");
	if ((count_ret = write(fd, buf, count)) == -1) {
		perror("chkwrite:: write failed");
		return count_ret;
	}
       	if ((final_pos = lseek(fd, 0, SEEK_CUR)) == -1)
       		perror("chkwrite:: lseek failed");
	if (init_pos != -1 && final_pos != -1 
			&& (delta = final_pos - init_pos) != count) {
		fprintf(stderr, "chkwrite:: %s bytes written "
				"(according to lseek)\n"
				"\tsupposed to be     : %d byte(s)\n"
				"\tactual             : %d byte(s)\n",
				(delta > count) ? "extra" : "too few",
				count, delta);
		fprintf(stderr, "\tfile pos (pre-wr  ): %d\n", (int)init_pos);
		fprintf(stderr, "\tfile pos (post-wr ): %d\n", (int)final_pos);
		if (fstat(fd, &stat) == -1)
			perror("chkwrite:: fstat failed");
		else 
			fprintf(stderr, "\tfile size (fstat)  : %d\n", 
							(int)stat.st_size);
	}
	return count_ret;
}