How to search within a binary file

Hello everybody,

I know how to search within a text file but I fail to do the same with a binary file. I can load the entire binary datafile using fread() but I am trying to load only one record (based on users' input like order no.) at a time.

Can anyone please tell me the command to use for loading a specific record (not the entire data file!) from a binary file. I did a board search but couldn't fine anything.

Thank you

Why is that problem?
For example, let's say you have records something like this:

Code:

typedef struct info
{
        char name[21];
        int order_num;
}info;

So you have simple structure and consisting of customer name and order number. Let's assume you have records like this stored in a binarny file. All you need to do is to open file in binary mode for reading, read sizeof info bytes (assuming records are written to the file that way) store it to a temp variable and check if that record matches certain criteria.

Here look at this code:

Code:

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

typedef struct info
{
        char name[21];
        int order_num;
}info;

int main(void)
{
    info cust[3];
    char buf[21];
    int i,on;
    info tmp;
    FILE * fp;
    /* first time uncommented*/
   /* for (i = 0; i < 3; ++i)
    {
        sprintf(buf, "Cust. no. %d",i+1);
        cust[i].order_num = i + 1;
        strcpy(cust[i].name, buf);
    }  
    fp = fopen("test.bin", "wb");
    
    for (i = 0; i < 3; i++)
    {
        fwrite(&cust[i], sizeof(info), 1, fp);
    }*/
    
    /* first time commented*/
    fp = fopen("test.bin","rb");
    
    on = 2; /*we search order no. 2*/
    
    while( (fread(&tmp, sizeof(info), 1, fp) == 1) && (tmp.order_num != on));
    if (tmp.order_num == on)
    {
        printf("%s",tmp.name);
    }
    
}

All error checking I'm leaving to you.
Just for demonstration purpose!
I hope this helps!

Also, if you want to jump to a specific record in Micko's example, look up the fseek() function.

Thanks Micko.

I've been trying similar methods without any success

Say, here's my structure

Code:

#define MAX_ITEM 10

typedef struct {

     int stk_num,items_in_stk;
     float price;
     char desc[26],ord_ref[9];
} record;

record List[MAX_ITEM];

FILE *datafile;

and here's the way I'm trying to read and load a specific record

Code:

   datafile = fopen("mydatafile.dat","rb");

    sn = 2; 
    while ((fread(&List, sizeof(List), 1, datafile) == 1) && (List->stk_num != sn)) ;

    if (List->stk_num == sn)
    {
        printf("%d",List->stk_num);
        printf("%d",List->item_in_stk);

    }

The problem I'm having is with the loop I guess. It seems to go through only once ! I've been trying different variations but still no luck

Edit : I also looked at fseek() but don't know how to use it in this case.

Ok...now I think its not the loop that's causing the problem. I mean even if I run the following statement

Code:

while ((fread(&List, sizeof(List), 1, datafile) == 1) && (List->stk_num != sn)) ;

more than once, the result is same.

Code:

if (List->stk_num == sn)

Here, List->stk_num is not changing and seems to point the first record only

Something is fishy with your code. Please write elements of array to file one by one and also read one by one.
Please explain to me what is this
fread(&List.....) what List means and what List->stk_num means?
When you start explaining to me start with what List is!!!
i know what is problem, but I'll try to navigate you to find out!

Thanks for the hint Micko. I think I can see the problem now. It sould be

Code:

while ((fread(&List, sizeof(record), 1, datafile) == 1) && (List->stk_num != on))  ;

Shouldn't it ?

Well, please explain me!
1. What List is???
2. How records are stored/written to a file?
3. What is List->stk_num?
4. Is List a pointer, if so, what kind of....?

I'm waiting for you to explain me how you want to read records and how you coded it...

The -> operator is not supported by C. Maybe this should be moved to the C++ forum.

Originally Posted by joshdick

The -> operator is not supported by C. Maybe this should be moved to the C++ forum.

What???

The -> operator is not supported by C. Maybe this should be moved to the C++ forum.

It is defined, and is very widely used. Indispensible, some would say.

When did that get added to C? I really thought that was just C++.

Originally Posted by joshdick

When did that get added to C? I really thought that was just C++.

Something like 1978.

Originally Posted by itsme86

Something like 1978.

My mistake. I misread a reference book.

@khpuce: In your latest 2 posts, containing sample code, List is defined as an array. If you're only wanting to load 1 record into memory, or rather, only ever have one record in memory, you don't want an array. Instead, you want 1 struct, much like Micko did in their example with the variable named tmp.

You basically read 1 structures worth of data into memory, check to see if its the one you want, if not, load the next lot of data over the top of it and test again. No array needed.