File Handling in C, A Complete Hands-on Hacking

This hacks explains file handling in C using stdio from the ground up. It assumes you know basic C syntax but have not implemented file I/O hands-on yet.

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1. What Is a File in C?

A file is data stored on disk (HDD/SSD) and managed by the Operating System. In C, we never access the disk directly.

Instead, the flow is:

C Program
→ C Standard Library (stdio)
→ System Calls (open, read, write)
→ Kernel
→ Disk

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2. What Is FILE *?

FILE *fp;

• FILE is a structure defined inside <stdio.h>
• fp points to a memory structure, NOT the file on disk
• This structure contains:
  • File descriptor (fd)
  • Buffer
  • Current position
  • Mode (read/write)
  • EOF and error flags

Conceptually:

fp → FILE structure → file descriptor → kernel → disk file

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3. Opening and Closing a File

fopen()

FILE *fp = fopen("data.txt", "r");

• Opens a file
• Creates an internal buffer
• Returns NULL on failure

fclose()

fclose(fp);

• Flushes buffered data
• Releases OS resources
• Must always be called

Error Check

if (fp == NULL) {
    perror("fopen failed");
}

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4. File Open Modes (IMPORTANT)

Basic Modes

Mode	Meaning
"r"	Read (file must exist)
"w"	Write (truncate or create)
"a"	Append (write at end)

Extended Modes (+)

Mode	Meaning
"r+"	Read + write (must exist)
"w+"	Read + write (truncate/create)
"a+"	Read + append

Binary Modes

Mode	Meaning
"rb"	Read binary
"wb"	Write binary
"rb+"	Read/write binary

Use binary mode for:

• Images
• Structs
• Network data

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5. Writing Text to a File

Using fprintf

FILE *fp = fopen("demo.txt", "w");
fprintf(fp, "Name: %s\nAge: %d\n", "Nishad", 25);
fclose(fp);

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Using fputs

FILE *fp = fopen("demo.txt", "a");
fputs("This is appended text\n", fp);
fclose(fp);

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6. Reading Text from a File

Using fgets (RECOMMENDED)

FILE *fp = fopen("demo.txt", "r");
char buf[100];

while (fgets(buf, sizeof(buf), fp)) {
    printf("%s", buf);
}

fclose(fp);

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Using fscanf (RISKY)

char name[20];
int age;

fscanf(fp, "Name: %s\nAge: %d", name, &age);

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7. Binary File I/O (fread / fwrite)

Writing a Struct

struct Person {
    int id;
    float height;
};

struct Person p = {1, 5.8};

FILE *fp = fopen("data.bin", "wb");
fwrite(&p, sizeof(p), 1, fp);
fclose(fp);

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Reading a Struct

struct Person p;

FILE *fp = fopen("data.bin", "rb");
fread(&p, sizeof(p), 1, fp);
printf("%d %.2f\n", p.id, p.height);
fclose(fp);

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8. End-of-File Handling (feof)

⛔ WRONG

while (!feof(fp)) {
    fgets(buf, 100, fp);
}

✅ CORRECT

while (fgets(buf, sizeof(buf), fp) != NULL) {
    printf("%s", buf);
}

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9. File Cursor Control (fseek, ftell)

fseek(fp, 0, SEEK_SET);
fseek(fp, 10, SEEK_CUR);
fseek(fp, 0, SEEK_END);

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10. Buffering (CRITICAL CONCEPT)

stdio uses buffered I/O.

fprintf(fp, "Hello");
fclose(fp);

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11. Complete Combined Example

#include <stdio.h>

int main() {
    FILE *fp = fopen("demo.txt", "w+");
    if (!fp) return 1;

    fprintf(fp, "Line 1\n");
    fputs("Line 2\n", fp);

    fseek(fp, 0, SEEK_SET);

    char buf[50];
    while (fgets(buf, sizeof(buf), fp)) {
        printf("%s", buf);
    }

    fclose(fp);
    return 0;
}

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12. Common Mistakes to Avoid

• Forgetting fclose
• Using feof incorrectly
• Writing structs blindly
• Assuming write is immediate
• Using fscanf for general input

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13. Summary

✔ FILE * is a pointer to a memory structure
✔ stdio is buffered
✔ Modes control existence, truncation, and position
✔ fgets is safer than fscanf
✔ fread/fwrite are fast but risky
✔ Always close files