File Handling in C

File handling allows C programs to read from and write to files on disk, enabling permanent data storage.

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File Pointer

All file operations use a FILE pointer:

FILE *fp;

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Opening and Closing Files

FILE *fp = fopen("filename.txt", "mode");

if (fp == NULL) {
    printf("Error opening file!\n");
    return 1;
}

fclose(fp);

File opening modes:

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Writing to a File

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "w");
    if (fp == NULL) return 1;
    
    fprintf(fp, "Hello, File!\n");
    fputs("Another line\n", fp);
    fputc('A', fp);
    
    fclose(fp);
    return 0;
}

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Reading from a File

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "r");
    if (fp == NULL) return 1;
    
    char line[100];
    while (fgets(line, sizeof(line), fp) != NULL) {
        printf("%s", line);
    }
    
    fclose(fp);
    return 0;
}

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File I/O Functions

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Binary File Example

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

/* Write binary */
FILE *fp = fopen("students.dat", "wb");
struct Student s = {1, "Alice", 90.5};
fwrite(&s, sizeof(s), 1, fp);
fclose(fp);

/* Read binary */
fp = fopen("students.dat", "rb");
struct Student t;
fread(&t, sizeof(t), 1, fp);
printf("%d %s %.1f\n", t.id, t.name, t.marks);
fclose(fp);