Read/Write Structure From/to a File in C

fwrite() and fread() are standard C library functions used to write and read blocks of binary data. They are commonly used to store and retrieve structures directly from files without converting them to text.

• Structures are stored in binary format, preserving the exact memory layout and improving performance.
• The file should be opened in binary mode ("wb", "rb", or "ab") when using these functions.

Binary File Modes

• rb: Opens a file for binary reading.
• wb: Opens a file for binary writing (creates a new file or overwrites an existing one).
• ab: Opens a file for appending binary data at the end of the file.

#include <stdio.h>

struct Student {
    int id;
    char name[20];
    float marks;
};

int main() {
    struct Student s1 = {101, "Rahul", 89.5};
    struct Student s2;

    FILE *fp;

    // Write structure to file
    fp = fopen("student.dat", "wb");
    fwrite(&s1, sizeof(struct Student), 1, fp);
    fclose(fp);

    // Read structure from file
    fp = fopen("student.dat", "rb");
    fread(&s2, sizeof(struct Student), 1, fp);
    fclose(fp);

    printf("ID: %d\n", s2.id);
    printf("Name: %s\n", s2.name);
    printf("Marks: %.1f\n", s2.marks);

    return 0;
}

ID: 101
Name: Rahul
Marks: 89.5

Syntax

// Writing data

fwrite(ptr, size, count, file_pointer);

// Reading data

fread(ptr, size, count, file_pointer);

Parameters

• ptr: Pointer to the data to write/read.
• size: Size of each data element (usually sizeof(struct_name)).
• count: Number of elements to write/read.
• file_pointer: Pointer to the opened file.

Writing Structures to a File using fwrite

The first step for writing structures in a file is to open the file in "wb" or "ab" mode. Then, we can use the fwrite() function to easily write a structure to a file. The fwrite() function writes to the file stream in the form of a binary data block.

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

// a struct to be read and written
struct person {
    int id;
    char fname[20];
    char lname[20];
};

int main()
{
    FILE* outfile;

    // open file for writing
    outfile = fopen("person.bin", "wb");
    if (outfile == NULL) {
        fprintf(stderr, "\nError opened file\n");
        exit(1);
    }

    struct person input1 = { 1, "rohan", "sharma" };

    // write struct to file
    int flag = 0;
    flag = fwrite(&input1, sizeof(struct person), 1,
                  outfile);
    if (flag) {
        printf("Contents of the structure written "
               "successfully");
    }
    else
        printf("Error Writing to File!");

    // close file
    fclose(outfile);

    return 0;
}

Contents of the structure written successfully

Syntax

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)

Parameters

• ptr: pointer to the block of memory to be written.
• size: the size of each element to be written (in bytes).
• nmemb: number of elements.
• stream: FILE pointer to the output file stream.

Reading Structure from a File using fread

To read structure from a file we need to open the file in "rb" binary read mode and then we can simply read structures using fread() function. This function reads a block of memory from the given stream.

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

// struct person with 3 fields
struct person {
    int id;
    char fname[20];
    char lname[20];
};

int main()
{
    FILE* infile;

    // Open file for read + write
    infile = fopen("person1.dat", "wb+");
    if (infile == NULL) {
        fprintf(stderr, "\nError opening file\n");
        exit(1);
    }

    struct person people[] = {
        {1, "Rohan", "Sharma"},
        {2, "Amit", "Verma"},
        {3, "Neha", "Singh"}
    };

    int count = sizeof(people) / sizeof(people[0]);

    for (int i = 0; i < count; i++) {
        fwrite(&people[i], sizeof(struct person), 1, infile);
    }

    struct person read_struct;

    // Move file pointer to start
    rewind(infile);

    printf("Reading records:\n");

    while (fread(&read_struct, sizeof(struct person), 1, infile) == 1) {
        printf("ID: %d, Name: %s %s\n",
               read_struct.id,
               read_struct.fname,
               read_struct.lname);
    }

    fclose(infile);
    return 0;
}

Reading records:
ID: 1, Name: Rohan Sharma
ID: 2, Name: Amit Verma
ID: 3, Name: Neha Singh

Syntax

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)

Parameters

• ptr: pointer to the block of memory to read.
• size: the size of each element to read (in bytes).
• nmemb: number of elements.
• stream: FILE pointer to the input file stream.

Return Value: Number of objects read.