Read/Write structure to a file using C

In C programming, structures can be written to and read from files using the fwrite() and fread() functions. This allows you to store complex data types persistently and retrieve them later.

fwrite() Syntax

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

Parameters:

• ptr − A pointer to the data to be written
• size − Size in bytes of each element
• nmemb − Number of elements to write
• stream − Pointer to the FILE object

fread() Syntax

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

Parameters:

• ptr − Pointer to memory block to store data
• size − Size in bytes of each element
• nmemb − Number of elements to read
• stream − Pointer to the FILE object

Example

Here's a complete example showing how to write and read student records to/from a file −

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

struct Student {
    int roll_no;
    char name[20];
};

int main() {
    FILE *file;
    
    // Writing structures to file
    file = fopen("students.txt", "wb");
    if (file == NULL) {
        fprintf(stderr, "Error opening file for writing<br>");
        return 1;
    }
    
    struct Student student1 = {1, "Ram"};
    struct Student student2 = {2, "Shyam"};
    
    fwrite(&student1, sizeof(struct Student), 1, file);
    fwrite(&student2, sizeof(struct Student), 1, file);
    
    printf("Student records written to file successfully!<br>");
    fclose(file);
    
    // Reading structures from file
    file = fopen("students.txt", "rb");
    if (file == NULL) {
        fprintf(stderr, "Error opening file for reading<br>");
        return 1;
    }
    
    struct Student student;
    printf("\nReading student records from file:<br>");
    
    while (fread(&student, sizeof(struct Student), 1, file)) {
        printf("Roll No: %d, Name: %s<br>", student.roll_no, student.name);
    }
    
    fclose(file);
    return 0;
}

Student records written to file successfully!

Reading student records from file:
Roll No: 1, Name: Ram
Roll No: 2, Name: Shyam

Key Points

• Use "wb" and "rb" modes for binary file operations with structures
• Always check if file opening was successful before proceeding
• The sizeof(struct Student) ensures correct byte count for each record
• Remember to close files after operations

Conclusion

Using fwrite() and fread() with structures provides an efficient way to store and retrieve complex data in C. Always use binary mode and proper error checking for reliable file operations.