read_write_tester.c

#include <stdio.h>

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

int main() {
  int ret;
  off_t size;
  ssize_t rw;
  char buf;

  ret = open("", O_RDWR);
  if (ret > 0) {
    if (ret < 3)
      printf("odd...\n");
    printf("open succeeded\n");
    size = lseek(ret, 0, SEEK_SET);
    if (size >= 0) {
      printf("size: %ld\n", size);
      rw = read(ret, &buf, 1);
      if (rw == 0) {
	buf = 0;
      } else if (rw < 0) {
	printf("read error %d\n", errno);
	goto close;
      } else {
	printf("read: 0x%x at offset %x\n", buf, size);
	size = lseek(ret, -1, SEEK_CUR);
      }
      rw = write(ret, &buf, 1);
      if (rw < 0)
	printf("write error %d\n", errno);
      else if (rw > 0)
	printf("write 0x%x at offset %x\n", buf, size);
      else
	printf("write returns 0\n");
    }
  close:
    ret = close(ret);
    if (ret == 0)
      printf("close succeeded\n");
  } else {
    printf("open error: %d\n", errno);
  }
}