File Handling in C

File handling allows programs to store data permanently on disk and read it back when needed. C provides powerful file handling through the standard library (stdio.h).

1. Opening a File in C

Use fopen() function to open a file. Returns FILE* pointer or NULL on failure.

              
C

            FILE *fp = fopen("data.txt", "r");
if(fp == NULL) {
    printf("Error opening file!\n");
    return 1;
}

Mode	Description	File must exist?	Creates new file?	Position
r	Read	Yes	No	Beginning
w	Write	No	Yes (overwrites)	Beginning
a	Append	No	Yes (if not exist)	End
r+	Read + Write	Yes	No	Beginning
w+	Read + Write	No	Yes (overwrites)	Beginning
a+	Read + Append	No	Yes	End (for write)

2. Reading from a File

              
            
C

          Common reading methods
        
// Character by character
int ch;
while((ch = fgetc(fp)) != EOF) putchar(ch);

// Line by line
char line[256];
while(fgets(line, sizeof(line), fp)) printf("%s", line);

// Formatted
int age; float salary;
fscanf(fp, "%d %f", &age, &salary);

3. Writing to a File

              
C

            fprintf(fp, "Name: %s Age: %d\n", "Rahul", 25);
fputs("Hello World\n", fp);
fputc('A', fp);

4. Mode 'a' - Append Mode

Opens file for **appending** data at the end. If file doesn't exist → creates it. Never overwrites existing content.

              
C

            FILE *fp = fopen("log.txt", "a");
if(fp) {
    fprintf(fp, "[%s] User logged in\n", __TIME__);
}

5. Mode 'a+' - Read + Append

Allows both **reading** and **appending**. Writing always goes to the **end** of file. Useful for logs where you sometimes need to read old data.

Note: fseek() can move read position, but write always appends.

6. Reading and Writing Binary Files

Use modes "rb", "wb", "ab", etc. and functions fread(), fwrite()

              
            
C

          Writing structure to binary file
        
struct Student {
    char name[50];
    int roll;
    float marks;
};

struct Student s = {"Aman", 101, 92.5};

FILE *fp = fopen("students.dat", "wb");
fwrite(&s, sizeof(struct Student), 1, fp);
fclose(fp);

// Reading back
struct Student s2;
fp = fopen("students.dat", "rb");
fread(&s2, sizeof(struct Student), 1, fp);

7. File Positioning Functions

ftell(fp) → returns current position (long)
fseek(fp, offset, whence) → moves pointer
rewind(fp) → moves to beginning
whence values: SEEK_SET (beginning), SEEK_CUR (current), SEEK_END (end)

              
C

            fseek(fp, 0, SEEK_END);          // go to end
long size = ftell(fp);                // file size
fseek(fp, 0, SEEK_SET);               // back to start
fseek(fp, -10, SEEK_END);             // last 10 bytes

8. Closing a File

Always close files using fclose() to free resources and flush buffers.

              
C

            if(fclose(fp) == 0)
    printf("File closed successfully\n");
else
    printf("Error closing file!\n");

9. Command Line Arguments

Access arguments passed while running program using main(int argc, char *argv[])

              
            
C

          Copy file content (like simple cp command)
        
#include <stdio.h>

int main(int argc, char *argv[]) {
    if(argc != 3) {
        printf("Usage: %s source.txt destination.txt\n", argv[0]);
        return 1;
    }

    FILE *src = fopen(argv[1], "rb");
    FILE *dest = fopen(argv[2], "wb");

    if(!src || !dest) {
        printf("Error opening files!\n");
        return 1;
    }

    char buffer[1024];
    size_t bytes;
    while((bytes = fread(buffer, 1, sizeof(buffer), src)) > 0)
        fwrite(buffer, 1, bytes, dest);

    fclose(src);
    fclose(dest);
    printf("File copied successfully!\n");
    return 0;
}