Direct IO testing...

main.c

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

#define HDD_PATH "/dev/sdc"

#define BLOCK_SIZE 1

int main(int argc, char *argv[]) {
  printf("opening %s\n", HDD_PATH);

  //int fd = open(HDD_PATH, O_RDWR | O_DIRECT | O_SYNC);
  int fd = open(HDD_PATH, O_RDWR | O_SYNC);
  if (fd < 0) {
    printf("Failed to open the fd, exiting\n");
    return 1;
  }

  printf("FD %d opened for dev %s\n", fd, HDD_PATH);

  char* testStringIn = "Mary had a little lamb,\nlittle lamb,\nlittle lamb.\nMary had a little lamb\nwho's fleece was white as snow";
  size_t testStringLen = strlen(testStringIn);

  int blk_cnt = testStringLen / BLOCK_SIZE + 1;
  printf("String occupies %d blocks for a total size of %d bytes.\n", blk_cnt, blk_cnt * BLOCK_SIZE);


  char* inputBuffer = (char *)malloc(BLOCK_SIZE * blk_cnt);
  memset(inputBuffer, 0, BLOCK_SIZE * blk_cnt);
  memcpy(inputBuffer, testStringIn, testStringLen);


  printf("\n\nWriting test string of size %d to the drive:\n%s\n\n", (int)testStringLen, testStringIn);
  int success = pwrite(fd, inputBuffer, BLOCK_SIZE * blk_cnt, 0);
  if (success < 0) {
    printf("Write failed! %d\n", success);
  } else {

    char* outputBuffer = (char *)malloc(BLOCK_SIZE * blk_cnt);
    memset(outputBuffer, 0, BLOCK_SIZE * blk_cnt);
    size_t bytesRead = pread(fd, outputBuffer, BLOCK_SIZE * blk_cnt, 0);
    printf("\n\nRead %d bytes from the disk.\n", (int)bytesRead);
    printf("Read test string from disk:\n\n%s\n\n", outputBuffer);
    free(outputBuffer);
  }

  printf("closing fd for %s\n", HDD_PATH);
  close(fd);

  free(inputBuffer);

  return 0;
}

Output:

$ ./main
opening /dev/sdc
FD 3 opened for dev /dev/sdc
String occupies 104 blocks for a total size of 104 bytes.


Writing test string of size 103 to the drive:
Mary had a little lamb,
little lamb,
little lamb.
Mary had a little lamb
who's fleece was white as snow



Read 104 bytes from the disk.
Read test string from disk:

Mary had a little lamb,
little lamb,
little lamb.
Mary had a little lamb
who's fleece was white as snow

closing fd for /dev/sdc