File I/O

Duration: 50 min

File I/O allows you to read from and write to files. C provides functions for opening, closing, reading, and writing files. Understanding file operations is essential for working with persistent data.

Opening and Closing Files

#include <stdio.h>

int main() {
    // Open file for writing
    FILE *file = fopen("data.txt", "w");
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    // Use the file
    fprintf(file, "Hello, File!\n");
    
    // Close the file
    fclose(file);
    
    return 0;
}

File Modes

#include <stdio.h>

int main() {
    // "r"  - Read (file must exist)
    // "w"  - Write (creates or truncates file)
    // "a"  - Append (creates or appends to file)
    // "r+" - Read and write
    // "w+" - Write and read (truncates)
    // "a+" - Append and read
    
    FILE *file = fopen("example.txt", "w");
    if (file != NULL) {
        fprintf(file, "Line 1\n");
        fprintf(file, "Line 2\n");
        fclose(file);
    }
    
    return 0;
}

Writing to Files

#include <stdio.h>

int main() {
    FILE *file = fopen("output.txt", "w");
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    // fprintf: formatted write
    fprintf(file, "Name: %s\n", "Alice");
    fprintf(file, "Age: %d\n", 25);
    fprintf(file, "Score: %.2f\n", 95.5);
    
    // fputs: write string
    fputs("End of file\n", file);
    
    fclose(file);
    
    return 0;
}

Reading from Files

#include <stdio.h>

int main() {
    FILE *file = fopen("input.txt", "r");
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    char line[100];
    
    // fgets: read line
    while (fgets(line, sizeof(line), file) != NULL) {
        printf("%s", line);
    }
    
    fclose(file);
    
    return 0;
}

Reading Formatted Data

#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "r");
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    char name[50];
    int age;
    float score;
    
    // fscanf: formatted read
    while (fscanf(file, "%s %d %f", name, &age, &score) == 3) {
        printf("Name: %s, Age: %d, Score: %.2f\n", name, age, score);
    }
    
    fclose(file);
    
    return 0;
}

Binary File Operations

#include <stdio.h>

typedef struct {
    int id;
    char name[50];
    float salary;
} Employee;

int main() {
    // Write binary data
    FILE *file = fopen("employees.bin", "wb");
    
    Employee emp1 = {1, "Alice", 50000.0};
    Employee emp2 = {2, "Bob", 60000.0};
    
    // fwrite: write binary data
    fwrite(&emp1, sizeof(Employee), 1, file);
    fwrite(&emp2, sizeof(Employee), 1, file);
    
    fclose(file);
    
    // Read binary data
    file = fopen("employees.bin", "rb");
    
    Employee emp;
    // fread: read binary data
    while (fread(&emp, sizeof(Employee), 1, file) == 1) {
        printf("ID: %d, Name: %s, Salary: %.2f\n", 
               emp.id, emp.name, emp.salary);
    }
    
    fclose(file);
    
    return 0;
}

File Positioning

#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "r");
    
    if (file == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    
    // ftell: get current position
    printf("Current position: %ld\n", ftell(file));
    
    // fseek: move to position
    fseek(file, 0, SEEK_END);  // Go to end
    long size = ftell(file);
    printf("File size: %ld bytes\n", size);
    
    fseek(file, 0, SEEK_SET);  // Go to beginning
    
    // SEEK_SET - beginning
    // SEEK_CUR - current position
    // SEEK_END - end
    
    fclose(file);
    
    return 0;
}

Practical Example: Student Records

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

typedef struct {
    int id;
    char name[50];
    float gpa;
} Student;

int main() {
    FILE *file = fopen("students.txt", "w");
    
    // Write student records
    Student students[] = {
        {101, "Alice", 3.8},
        {102, "Bob", 3.5},
        {103, "Carol", 3.9}
    };
    
    for (int i = 0; i < 3; i++) {
        fprintf(file, "%d %s %.2f\n", 
                students[i].id, 
                students[i].name, 
                students[i].gpa);
    }
    
    fclose(file);
    
    // Read and display
    file = fopen("students.txt", "r");
    
    Student s;
    while (fscanf(file, "%d %s %f", &s.id, s.name, &s.gpa) == 3) {
        printf("ID: %d, Name: %s, GPA: %.2f\n", s.id, s.name, s.gpa);
    }
    
    fclose(file);
    
    return 0;
}

Quiz 1: File Modes

Quiz 2: fopen Return Value

Quiz 3: fprintf

Quiz 4: fread and fwrite

Quiz 5: fseek