Reading & Writing Files in C

Why File Handling?

• Store data permanently
• Process large datasets
• Record logs like apps & websites
• Load configuration or settings
• Exchange data between programs

Important File Functions

• fopen() – open a file
• fclose() – close file
• fprintf() – write formatted text
• fscanf() – read formatted text
• fputs() – write string
• fgets() – read string
• fread() – read block of data
• fwrite() – write block of data

File Opening Modes

• "r" – read only
• "w" – write only (overwrites)
• "a" – append to end
• "r+" – read + write
• "w+" – read + write (overwrite)
• "a+" – read + append
• "rb/wb/ab" – binary modes

Reading Files

You can read files using:

• fgetc() – read one character
• fgets() – read line
• fscanf() – read formatted input
• fread() – read binary block

Writing Files

• fputc() – write single character
• fputs() – write string
• fprintf() – formatted write
• fwrite() – write binary block

Error Handling

• Check file pointer == NULL
• Use perror() or strerror() to display error

All 10 File Read/Write Examples

#include <stdio.h>

/* Example 1 – Write text to file */
int main() {
    FILE *f = fopen("data.txt", "w");
    fprintf(f, "Hello C File Handling!");
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 2 – Append text */
int main() {
    FILE *f = fopen("data.txt", "a");
    fputs(" New line added.\n", f);
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 3 – Read entire file character by character */
int main() {
    FILE *f = fopen("data.txt", "r");
    int ch;
    while((ch = fgetc(f)) != EOF){
        putchar(ch);
    }
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 4 – Read line using fgets() */
int main() {
    FILE *f = fopen("data.txt", "r");
    char line[100];
    fgets(line, sizeof(line), f);
    printf("%s", line);
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 5 – Write multiple values using fprintf() */
int main() {
    FILE *f = fopen("marks.txt", "w");
    fprintf(f, "Math: %d\nScience: %d", 90, 85);
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 6 – Read formatted data with fscanf() */
int main() {
    FILE *f = fopen("marks.txt", "r");
    int m, s;
    fscanf(f, "Math: %d\nScience: %d", &m, &s);
    printf("%d %d", m, s);
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 7 – Write binary data */
int main() {
    int nums[3] = {1,2,3};
    FILE *f = fopen("bin.dat", "wb");
    fwrite(nums, sizeof(int), 3, f);
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 8 – Read binary data */
int main() {
    int nums[3];
    FILE *f = fopen("bin.dat", "rb");
    fread(nums, sizeof(int), 3, f);
    printf("%d %d %d", nums[0], nums[1], nums[2]);
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 9 – Count number of characters */
int main() {
    FILE *f = fopen("data.txt", "r");
    int count = 0, ch;
    while((ch = fgetc(f)) != EOF) count++;
    printf("Characters: %d", count);
    fclose(f);
    return 0;
}

#include <stdio.h>

/* Example 10 – Copy content from one file to another */
int main() {
    FILE *src = fopen("input.txt", "r");
    FILE *dst = fopen("output.txt", "w");

    int ch;
    while((ch = fgetc(src)) != EOF){
        fputc(ch, dst);
    }

    fclose(src);
    fclose(dst);
    return 0;
}