C File I/O Tutorial with Examples: fopen, fclose, fread, fwrite

Table of Contents

Introduction to File Handling in C
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In C programming, there are two main ways to persist data:

Files (stored on disk)
Databases (structured data storage)

This tutorial focuses on file I/O (Input/Output) using the C Standard Library.

Text Files vs. Binary Files in C
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Text files: store characters in readable form (ASCII, Unicode). Example: .txt files.
Binary files: store raw data not directly readable by humans. Example: executables, images, audio files.

Opening and Closing Files in C (fopen, fclose)
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fopen() – Open a File
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FILE *fopen(const char *path, const char *mode);

path: file name or path (e.g., "./file.txt")
mode: read, write, or append
Returns: file stream pointer on success, or NULL if failed

Common File Modes in C
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Mode	Meaning	Behavior
r	Read only	File must exist, read from start
w	Write only	Creates or overwrites file
a	Append only	Creates if not exists, writes at end
r+	Read/Write	File must exist, read & write from start
w+	Write/Read	Creates or overwrites file
a+	Append/Read	Creates if not exists, writes at end, reads from start

fclose() – Close a File
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int fclose(FILE *fp);

Closes an open file stream
Returns 0 on success, EOF on failure

Basic Steps for File I/O in C
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Declare a file pointer
Open the file with fopen()
Perform operations (read/write)
Close the file with fclose()

#include <stdio.h>

int main() {
    FILE *fp = NULL;              
    fp = fopen("./1.txt", "r");   // open file for reading

    if (fp == NULL) {
        printf("Failed to open file!\n");
        return 1;
    }

    // perform file operations here...

    fclose(fp);                   
    fp = NULL;                    
    return 0;
}

Formatted File I/O in C (fprintf, fscanf)
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Writing to a File with fprintf
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int fprintf(FILE *stream, const char *format, ...);

Example:

#include <stdio.h>
int main() {
    FILE *fp = fopen("./test.txt", "w+");  

    if (fp == NULL) {
        printf("File open error!\n");
        return 1;
    }

    fprintf(fp, "%s %d", "Score", 100);   // write "Score 100" to file

    fclose(fp);
    return 0;
}

Reading from a File with fscanf
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int fscanf(FILE *stream, const char *format, ...);

Example:

#include <stdio.h>
int main() {
    FILE *fp = fopen("./test.txt", "r");  
    if (fp == NULL) {
        printf("File open error!\n");
        return 1;
    }

    char str[100];
    int num;
    fscanf(fp, "%s %d", str, &num);   
    printf("Read: %s %d\n", str, num);

    fclose(fp);
    return 0;
}

Binary File I/O in C (fwrite, fread)
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Writing Binary Data with fwrite
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size_t fwrite(const void *ptr, size_t size, size_t count, FILE *stream);

Example:

#include <stdio.h>
int main() {
    FILE *fp = fopen("./bin.dat", "w+");  
    if (fp == NULL) return 1;

    int a[5] = {10, 20, 30, 40, 50};
    fwrite(a, sizeof(int), 5, fp);   // write array to file

    fclose(fp);
    return 0;
}

Reading Binary Data with fread
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size_t fread(void *ptr, size_t size, size_t count, FILE *stream);

Example:

#include <stdio.h>
int main() {
    FILE *fp = fopen("./bin.dat", "r");  
    if (fp == NULL) return 1;

    int buffer[5];
    fread(buffer, sizeof(int), 5, fp);   // read data into buffer

    for (int i = 0; i < 5; i++) {
        printf("%d ", buffer[i]);
    }

    fclose(fp);
    return 0;
}

Output:

10 20 30 40 50

File Positioning in C (fseek, ftell)
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Sometimes you need to move around inside a file instead of reading sequentially.

fseek() – Set File Position
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int fseek(FILE *stream, long offset, int whence);

SEEK_SET: from beginning
SEEK_CUR: from current position
SEEK_END: from end

Examples:

fseek(fp, 8, SEEK_SET);   // move 8 bytes from start
fseek(fp, -12, SEEK_END); // move 12 bytes back from end

ftell() – Get File Position
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long ftell(FILE *stream);

Example:

#include <stdio.h>
int main() {
    FILE *fp = fopen("./bin.dat", "r");
    if (fp == NULL) return 1;

    fseek(fp, 8, SEEK_SET);      
    long pos = ftell(fp);        
    printf("Current position: %ld\n", pos);

    fclose(fp);
    return 0;
}