C fwrite

Summary: in this tutorial, you will learn how to use the C fwrite funtion to create a random access file.

Introduction to the C fwrite() function

The fwrite() function is defined in the stdio.h library. The fwrite() function allows you to write data to a binary file.

Here’s the syntax of the fwrite() function:

size_t fwrite ( const void * ptr, size_t size, size_t count, FILE * stream );
Code language: C++ (cpp)

The fwrite() function writes a block of data in the memory to a file. It has the following parameters:

• ptr is a pointer that points to the block of data in the memory to be written to the file. Typically, it’s a pointer to an array of data.
• size specifies the number of bytes of each element that the function will write to the file.
• count specifies the number of elements that the function will write to the file.
• stream is the file pointer to the file to which the function will write.

The fwrite() function returns the number of elements that the function successfully writes to the file.

A simple C fwrite() function example

The following example uses the fwrite() function to write numbers from 0 to 9 to a file:

#include <stdio.h>

int main()
{
    char *filename = "numbers.dat";

    // open the file for writing binary data
    FILE *fp = fopen(filename, "wb");
    if (fp == NULL)
    {
        printf("Error opening the file %s", filename);
        return -1;
    }
    // write numbers to the file
    for (int i = 0; i < 10; i++)
        fwrite(&i, sizeof(int), 1, fp);

    // close the file
    fclose(fp);

    return 0;
}
Code language: C++ (cpp)

How it works.

First, open the file numbers.dat using the fopen() function. The fopen() uses the wb mode for writing binary data to a file. If the file doesn’t exist, it’ll create a new file. However, if the file already exists, it’ll overwrite the contents of the file.

char *filename = "numbers.dat";

// open the file for writing binary data
FILE *fp = fopen(filename, "wb");
if (fp == NULL)
{
    printf("Error opening the file %s", filename);
    return -1;
}
Code language: C++ (cpp)

Second, iterate from 0 to 9 and write each number to the file using the fwrite() function:

for (int i = 0; i < 10; i++)
    fwrite(&i, sizeof(int), 1, fp);
Code language: C++ (cpp)

The following statement writes an integer to the file:

fwrite(&i, sizeof(int), 1, fp);
Code language: C++ (cpp)

Third, close the file using the fclose() function:

// close the file
fclose(fp);
Code language: C++ (cpp)

A practical C fwrite() function example

Suppose that you need to write inventory data to a file. The inventory includes a list of record, where each record contains the following information:

• Product name
• Quantity

First, define a structure that includes the product name and quantity:

typedef struct
{
    char name[40];
    unsigned qty;
} product;
Code language: C++ (cpp)

Second, define a function that allows users to enter a number of products:

bool input(product *product_list, const int size);
Code language: C++ (cpp)

The definition of the input() function is as follows:

bool input(product *product_list, const int size)
{
    int i = 0;
    do
    {
        printf("Name:");
        scanf("%s", &product_list[i].name);
        printf("Qty:");
        scanf("%d", &product_list[i].qty);
        i++;
    } while (i < size);
}
Code language: C++ (cpp)

Third, define a function that saves the records into the file specified by the filename:

bool save(product *product_list, const int size, const char *filename);
Code language: C++ (cpp)

The following shows the definition of the save() function:

bool save(product *product_list, const int size, const char *filename)
{
    // write the inventory to the file
    FILE *fp = fopen(filename, "wb");
    if (fp == NULL)
    {
        puts("Error opening the file");
        return false;
    }
    for (int i = 0; i < size; i++)
        fwrite(&product_list[i], sizeof(product), 1, fp);

    fclose(fp);

    return true;
}
Code language: C++ (cpp)

The save() function does three things: open the file, write each strcture in the product list into the file, and close the it.

Finally, define a function that displays the entered inventory:

void display(product *product_list, const int size)
;
Code language: C++ (cpp)

The logic of the display() function is as follows:

void display(product *product_list, const int size)
{
    // display the product list
    puts("Name\tQty");
    puts("-------------");
    for (int i = 0; i < size; i++)
        printf("%s \t %d\n", product_list[i].name, product_list[i].qty);
}
Code language: C++ (cpp)

Put it all together:

#include <stdio.h>
#include <stdbool.h>

typedef struct
{
    char name[40];
    unsigned qty;
} product;

bool input(product *product_list, const int size);
bool save(product *product_list, const int size, const char *filename);
void display(product *product_list, const int size);

int main()
{
    const int SIZE = 10;
    product product_list[SIZE];

    // input the product list
    input(product_list, SIZE);

    // save to the file
    save(product_list, SIZE, "inventory.dat");

    // show the product list to the screen
    display(product_list, SIZE);

    return 0;
}

void display(product *product_list, const int size)
{
    // display the product list
    puts("Name\tQty");
    puts("-------------");
    for (int i = 0; i < size; i++)
        printf("%s \t %d\n", product_list[i].name, product_list[i].qty);
}

// save product list to the file
bool save(product *product_list, const int size, const char *filename)
{
    // write the inventory to the file
    FILE *fp = fopen(filename, "w");
    if (fp == NULL)
    {
        puts("Error opening the file");
        return false;
    }
    for (int i = 0; i < size; i++)
        fwrite(&product_list[i], sizeof(product), 1, fp);

    fclose(fp);

    return true;
}

// input inventory data
bool input(product *product_list, const int size)
{
    int i = 0;
    do
    {
        printf("Name:");
        scanf("%s", &product_list[i].name);
        printf("Qty:");
        scanf("%d", &product_list[i].qty);
        i++;
    } while (i < size);
}
Code language: C++ (cpp)

Note that the SIZE constant defines the number of products that the users will enter per time.

Summary

• Use the C fwrite() function to write a block of data in the memory into a file.