Example of working with binary files in C.

client.h

struct clientData {
    int acctNum; // account number
    char lastName[ 15 ]; // account last name
    char firstName[ 10 ]; // account first name
    double balance;  // account balance
};

create.c

/*
Create binary file with C to store customer data
Source: C How to Program by The Dietels
*/

#include <stdio.h>
#include "client.h"


int main( void ){
    int i;

    struct clientData blankClient = { 0, "", "", 0.0 };

    FILE *fPtr = fopen("client", "wb");

    if ( fPtr == NULL){
        printf("File could not be opened.\n");
    }
    else {

        // output 100 blank records to file
        for ( i = 0; i < 100; i++)
        {
            fwrite(&blankClient, sizeof( struct clientData ), 1, fPtr);
        }

        fclose(fPtr);
        
    }

    return 0;

}

read.c

/*
Read from binary file to get customer data
Source: C How to Program by The Dietels
*/

#include <stdio.h>
#include "client.h"


int main( void ){
    FILE *fPtr = fopen("client", "rb");

    struct clientData client = { 0, "", "", 0.0};

    if ( fPtr == NULL){
        printf("File could not be opened.\n");
    } else
    {
        printf("%-6s%-16s%-11s%10s\n", "Acct", "Last Name", "First Name", "Balance" );

        while ( !feof(fPtr) )
        {
            fread( &client, sizeof( struct clientData ), 1, fPtr );

            if ( client.acctNum != 0 ){
                printf( "%-6d%-16s%-11s%10.2f\n",
                    client.acctNum, client.lastName,
                    client.firstName, client.balance );
            }
        }
        fclose(fPtr);   
    }
    
    return 0;

}

write.c

/*
Write to binary file to create customer data
Source: C How to Program by The Dietels
*/



#include <stdio.h>
#include "client.h"


int main( void ){

    FILE *fPtr = fopen("client", "rb+");

    struct clientData client = { 0, "", "", 0.0 };

    if ( fPtr == NULL){
        printf("File could not be opened.\n");
    } else {

        // request for user's account number
        printf("Enter account number"
            " ( 1 to 100, 0 to end input )\n?" );
        scanf("%d", &client.acctNum);

        while ( client.acctNum != 0 )
        {
            printf("Enter lastname, firstname, balance\n?");

            fscanf( stdin, "%s%s%lf", client.lastName, client.firstName, &client.balance);

            fseek(fPtr, ( client.acctNum - 1 ) * sizeof( struct clientData), SEEK_SET);

            fwrite( &client, sizeof( struct clientData ), 1, fPtr );

            printf("Enter account number\n? " );
            scanf("%d", &client.acctNum);
        
        }
        
        fclose(fPtr);
    }

    return 0;

}