Reading into a struct from a file.

mem.c

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

struct st {
  int num;
  char ch;
};

int main(void)
{
  /* requires C99 */
  unsigned char buf[sizeof(struct st)];
  struct st myst = { 37, 'n' };
  size_t bytes;
  FILE *f = fopen("mem.b", "wb");
  if (!f){
    perror("mem.b");
    return EXIT_FAILURE;
  }

  /* write bytes to file */
  if ((bytes = fwrite(&myst, sizeof(struct st), 1, f)) != 1){
    printf("error: fwrite wrote %lu bytes\n", bytes);
    return EXIT_FAILURE;
  }

  /* close the file */
  fclose(f);
  /* and reopen */
  if (!(f = fopen("mem.b", "rb"))){
    perror("mem.b");
    return EXIT_FAILURE;
  }

  /* sum debug */
  printf("before freading:\n");
  printf("  num: %d\n", myst.num);
  printf("   ch: %c (%d 0x%x)\n", myst.ch, myst.ch, myst.ch);

  /* zero out the struct */
  memset(&myst, 0, sizeof(struct st));

  /* sum debug */
  printf("after the zero-out:\n");
  printf("  num: %d\n", myst.num);
  printf("   ch: %c (%d 0x%x)\n", myst.ch, myst.ch, myst.ch);

  /* read bytes from file */
  if ((bytes = fread(buf, sizeof(struct st), 1, f)) != 1){
    printf("error: fread read %lu bytes\n", bytes);
    return EXIT_FAILURE;
  }

  memcpy(&myst, buf, sizeof(struct st));

  /* sum debug */
  printf("after freading:\n");
  printf("  num: %d\n", myst.num);
  printf("   ch: %c (%d 0x%x)\n", myst.ch, myst.ch, myst.ch);

  /* close the file */
  fclose(f);

  return 0;
}