fwrite in the middle of a file

[Forest Dragon]

Hello,

I need to write some records to a file and sometimes I need to insert a record in the middle and move the records before it. The problem is that when I use fseek to get to the right place to write a record, then if it's in the beginning / end of the file, fwrite works, but if it's in the middle, it doesn't overwrite the record that was there, neither does it cause an error.
What is the problem?

Thanks in advance.

[jim mcnamara]

Did you use: fopen("file.dat","rb+");
to open the file?

here is a big hunk of code that does a disk sort using a random access file (what you are trying to do) Look at the part that is red.

/* Disk sort for structures of type address. */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define NUM_ELEMENTS 4  /* This is an arbitrary number
                           that should be determined
                           dynamically for each list. */

struct address {
  char name[30];
  char street[40];
  char city[20];
  char state[3];
  char zip[11];
} ainfo;

struct address addrs[NUM_ELEMENTS] = {
  "A. Alexander", "101 1st St", "Olney", "Ga", "55555",
  "B. Bertrand", "22 2nd Ave", "Oakland", "Pa", "34232",
  "C. Carlisle", "33 3rd Blvd", "Ava", "Or", "92000",
  "D. Dodger", "4 Fourth Dr", "Fresno", "Mi", "45678"
};

void quick_disk(FILE *fp, int count);
void qs_disk(FILE *fp, int left, int right);
void swap_all_fields(FILE *fp, long i, long j);
char *get_zip(FILE *fp, long rec);

int main(void)
{
  FILE *fp;

  /* first, create a file to sort */
  if((fp=fopen("mlist", "wb"))==NULL) {
    printf("Cannot open file for write.\n");
    exit(1);
  }
  printf("Writing unsorted data to disk.\n");
  fwrite(addrs, sizeof(addrs), 1, fp);
  fclose(fp);

  /* now, sort the file */
  if((fp=fopen("mlist", "rb+"))==NULL) {
    printf("Cannot open file for read/write.\n");
    exit(1);
  }

  printf("Sorting disk file.\n");
  quick_disk(fp, NUM_ELEMENTS);
  fclose(fp);
  printf("List sorted.\n");

  return 0;
}

/* A Quicksort for files. */
void quick_disk(FILE *fp, int count)
{
  qs_disk(fp, 0, count-1);
}

void qs_disk(FILE *fp, int left, int right)
{
  long int i, j;
  char x[100];

  i = left; j = right;

  strcpy(x, get_zip(fp, (long)(i+j)/2)); /* get the middle zip */

  do {
    while((strcmp(get_zip(fp,i),x) < 0) && (i < right)) i++;
    while((strcmp(get_zip(fp,j),x) > 0) && (j > left)) j--;

    if(i <= j) {
      swap_all_fields(fp, i, j);
      i++; j--;
    }
  } while(i <= j);

  if(left < j) qs_disk(fp, left, (int) j);
  if(i < right) qs_disk(fp, (int) i, right);
}

void swap_all_fields(FILE *fp, long i, long j)
{
  char a[sizeof(ainfo)], b[sizeof(ainfo)];

  /* first read in record i and j */
  fseek(fp, sizeof(ainfo)*i, SEEK_SET);
  fread(a, sizeof(ainfo), 1, fp);

  fseek(fp, sizeof(ainfo)*j, SEEK_SET);
  fread(b, sizeof(ainfo), 1, fp);

  /* then write them back in opposite slots */
  fseek(fp, sizeof(ainfo)*j, SEEK_SET);
  fwrite(a, sizeof(ainfo), 1, fp);
  fseek(fp, sizeof(ainfo)*i, SEEK_SET);
  fwrite(b, sizeof(ainfo), 1, fp);
}

/* Return a pointer to the zip code */
char *get_zip(FILE *fp, long rec)
{
  struct address *p;

  p = &ainfo;

  fseek(fp, rec*sizeof(ainfo), SEEK_SET);
  fread(p, sizeof(ainfo), 1, fp);

  return ainfo.zip;
}