File Handling in C Programming

File handling lets you store data permanently - beyond the lifetime of your program. Instead of typing data each time the program runs, you can read from and write to files on disk.

C handles files through a special type called FILE*, defined in <stdio.h>. You don’t create FILE variables directly - you get a pointer to one by calling fopen().

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "w");  // Open for writing

    if (fp == NULL) {
        printf("Could not open file\n");
        return 1;
    }

    fprintf(fp, "Hello, File!\n");
    fclose(fp);  // Always close when done

    return 0;
}

Open a File in C

fopen() opens a file and returns a FILE* pointer. If it fails, it returns NULL.

FILE *fopen(const char *filename, const char *mode);

#include <stdio.h>

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

    if (fp == NULL) {
        printf("File not found or cannot be opened\n");
        return 1;
    }

    printf("File opened successfully\n");
    fclose(fp);
    return 0;
}

Always check if fopen returns NULL - the file might not exist, lack permissions, or the disk could be full.

File Opening Modes

Mode	Meaning	If file exists	If file doesn’t exist
"r"	Read (text)	Read from start	Returns NULL
"w"	Write (text)	Overwrite contents	Creates new file
"a"	Append (text)	Write at end	Creates new file
"r+"	Read + Write (text)	Read/write from start	Returns NULL
"w+"	Read + Write (text)	Overwrite contents	Creates new file
"a+"	Read + Append (text)	Read from start, write at end	Creates new file
"rb"	Read (binary)	Read from start	Returns NULL
"wb"	Write (binary)	Overwrite contents	Creates new file
"ab"	Append (binary)	Write at end	Creates new file

Create a File

Use "w" or "w+" mode - the file is created if it doesn’t exist.

#include <stdio.h>

int main() {
    FILE *fp = fopen("newfile.txt", "w");

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

    printf("File created successfully\n");
    fclose(fp);
    return 0;
}

Write to a File

C provides several functions to write data:

fprintf - formatted output

#include <stdio.h>

int main() {
    FILE *fp = fopen("students.txt", "w");
    if (fp == NULL) return 1;

    fprintf(fp, "Name: %s, Age: %d, Grade: %.1f\n", "Alice", 20, 92.5);
    fprintf(fp, "Name: %s, Age: %d, Grade: %.1f\n", "Bob", 22, 85.0);

    fclose(fp);
    return 0;
}

fputc - write a single character

#include <stdio.h>

int main() {
    FILE *fp = fopen("letters.txt", "w");
    if (fp == NULL) return 1;

    for (char ch = 'A'; ch <= 'Z'; ch++) {
        fputc(ch, fp);
    }

    fclose(fp);
    return 0;
}

fputs - write a string

#include <stdio.h>

int main() {
    FILE *fp = fopen("quotes.txt", "w");
    if (fp == NULL) return 1;

    fputs("C is a powerful language.\n", fp);
    fputs("File handling is essential.\n", fp);

    fclose(fp);
    return 0;
}

Read from a File

fscanf - formatted input

#include <stdio.h>

int main() {
    FILE *fp = fopen("students.txt", "r");
    if (fp == NULL) return 1;

    char name[50];
    int age;
    float grade;

    while (fscanf(fp, "Name: %s, Age: %d, Grade: %f\n", name, &age, &grade) == 3) {
        printf("%s is %d years old (%.1f%%)\n", name, age, grade);
    }

    fclose(fp);
    return 0;
}

fgetc - read a single character

#include <stdio.h>

int main() {
    FILE *fp = fopen("letters.txt", "r");
    if (fp == NULL) return 1;

    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);
    }
    printf("\n");

    fclose(fp);
    return 0;
}

fgets - read a line

#include <stdio.h>

int main() {
    FILE *fp = fopen("quotes.txt", "r");
    if (fp == NULL) return 1;

    char line[100];
    while (fgets(line, sizeof(line), fp) != NULL) {
        printf("Read: %s", line);
    }

    fclose(fp);
    return 0;
}

Close a File

Always close files when you’re done. fclose() flushes any buffered data and releases system resources.

int fclose(FILE *fp);

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "w");
    if (fp == NULL) return 1;

    fprintf(fp, "This data needs to be saved\n");

    if (fclose(fp) != 0) {
        printf("Error closing file\n");
        return 1;
    }

    printf("File closed successfully\n");
    return 0;
}

If you don’t call fclose, data may be lost if the program crashes before the buffer is flushed.

Move File Pointer

Every open file has an internal file position indicator that tracks where the next read or write will occur. You can move it with fseek, ftell, and rewind.

ftell - get current position

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "w");
    if (fp == NULL) return 1;

    fprintf(fp, "Hello World");

    long pos = ftell(fp);
    printf("Current position: %ld bytes from start\n", pos);  // 11

    fclose(fp);
    return 0;
}

fseek - move to a specific position

int fseek(FILE *fp, long offset, int whence);

whence	Meaning
SEEK_SET	From the beginning of the file
SEEK_CUR	From the current position
SEEK_END	From the end of the file

#include <stdio.h>

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

    fprintf(fp, "0123456789");

    // Read character at position 4
    fseek(fp, 4, SEEK_SET);
    printf("Char at 4: %c\n", fgetc(fp));  // '4'

    // Go back 2 from current position
    fseek(fp, -2, SEEK_CUR);
    printf("Char at -2 from current: %c\n", fgetc(fp));  // '3'

    // Read last character
    fseek(fp, -1, SEEK_END);
    printf("Last char: %c\n", fgetc(fp));  // '9'

    fclose(fp);
    return 0;
}

rewind - go back to the start

#include <stdio.h>

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

    fprintf(fp, "Data at the end");
    rewind(fp);  // Move back to the beginning

    char buf[50];
    fgets(buf, sizeof(buf), fp);
    printf("Read after rewind: %s\n", buf);

    fclose(fp);
    return 0;
}

Delete a File

Use remove() to delete a file. Returns 0 on success.

#include <stdio.h>

int main() {
    if (remove("temp.txt") == 0) {
        printf("File deleted successfully\n");
    } else {
        printf("Failed to delete file\n");
    }

    return 0;
}

Rename a File

Use rename() to rename a file. Returns 0 on success.

#include <stdio.h>

int main() {
    if (rename("oldname.txt", "newname.txt") == 0) {
        printf("File renamed successfully\n");
    } else {
        printf("Failed to rename file\n");
    }

    return 0;
}

Read and Write in a Binary File

Text files store data as human-readable characters. Binary files store data in the same format it’s stored in memory - more compact and exact.

Writing to a Binary File

Use fwrite() to write blocks of data to a binary file.

#include <stdio.h>

typedef struct {
    int id;
    char name[50];
    float salary;
} Employee;

int main() {
    Employee emp = {101, "Alice", 75000.50};

    FILE *fp = fopen("employee.bin", "wb");
    if (fp == NULL) return 1;

    fwrite(&emp, sizeof(Employee), 1, fp);

    fclose(fp);
    printf("Employee written to binary file\n");

    return 0;
}

Reading from a Binary File

Use fread() to read blocks of data from a binary file.

#include <stdio.h>

typedef struct {
    int id;
    char name[50];
    float salary;
} Employee;

int main() {
    Employee emp;

    FILE *fp = fopen("employee.bin", "rb");
    if (fp == NULL) return 1;

    fread(&emp, sizeof(Employee), 1, fp);

    printf("ID: %d\n", emp.id);
    printf("Name: %s\n", emp.name);
    printf("Salary: %.2f\n", emp.salary);

    fclose(fp);
    return 0;
}

Binary files are ideal for structured data like arrays of structs - they’re faster to read/write and take less space than text equivalents.

More Functions for C File Operations

Function	Description
feof(fp)	Returns non-zero if end-of-file is reached
ferror(fp)	Returns non-zero if an error occurred
perror(msg)	Prints a descriptive error message to stderr
fflush(fp)	Forces buffered data to be written to disk
tmpfile()	Creates a temporary file that auto-deletes on close

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "r");
    if (fp == NULL) {
        perror("Failed to open file");  // Prints: Failed to open file: No such file or directory
        return 1;
    }

    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);
    }

    if (feof(fp)) {
        printf("\n--- End of file reached ---\n");
    }

    if (ferror(fp)) {
        printf("An error occurred while reading\n");
    }

    fclose(fp);
    return 0;
}