Basics of File Handling in C

File handling in C is the process in which we create, open, read, write, and close operations on a file. C language provides different functions such as fopen(), fwrite(), fread(), fseek(), fprintf(), etc. to perform input, output, and many different C file operations in our program.

C File Operations

C language provides the following different operations that we can perform on a file from our C program:

1. Creating a new file.
2. Opening an existing file.
3. Reading from file.
4. Writing to a file.
5. Moving to a specific location in a file.
6. Closing a file.

Open a File in C

For opening a file in C, the fopen() function is used with the filename or file path along with the required access modes.

Syntax:

FILE* fopen(*file_name, *access_mode);

Parameters

• file_name: name of the file when present in the same directory as the source file. Otherwise, full path.
• access_mode: Specifies for what operation the file is being opened.

Return Value

• If the file is opened successfully, returns a file pointer to it.
• If the file is not opened, then returns NULL.

File Opening Modes

File opening modes or access modes specify the allowed operations on the file to be opened. They are passed as an argument to the fopen() function. Some of the commonly used file access modes are listed below:

As given above, if you want to perform operations on a binary file, then you have to append 'b' at the last. For example, instead of “w”, you have to use “wb”, instead of “a+” you have to use “a+b”.

Example:

#include <stdio.h>
#include <stdlib.h>

int main() {
    
    // File pointer to store the 
    // value returned by fopen
    FILE* fptr;

    // Opening the file in read mode
    fptr = fopen("filename.txt", "r");

    // checking if the file is 
    // opened successfully
    if (fptr == NULL) {
        printf("The file is not opened.");
    }
    return 0;
}

The file is not opened.

The file is not opened because it does not exist in the source directory. But the fopen() function is also capable of creating a file if it does not exist.

Note: If is essential to check for NULL values that might be returned by the fopen() function to avoid any errors.

Create a File

The fopen() function can not only open a file but also can create a file if it does not exist already. For that, we have to use the modes that allow the creation of a file if not found such as w, w+, wb, wb+, a, a+, ab, and ab+.

Example:

#include <stdio.h>
#include <stdlib.h>

int main() {
    
    // File pointer
    FILE* fptr;

    // Creating file using fopen()
    // with access mode "w"
    fptr = fopen("file.txt", "w");

    // checking if the file is created
    if (fptr == NULL) 
        printf("The file is not opened.");
    else 
        printf("The file is created Successfully.");
    return 0;
}

The file is created Successfully.

Write to a File

The file write operations can be performed by the functions fprintf() and fputs(). C programming also provides some other functions that can be used to write data to a file such as:

Example:

#include <stdio.h>
#include <stdlib.h>

int main() {
    
    // File pointer
    FILE* fptr;

    // Get the data to be written in file
    char data[50] = "GeeksforGeeks-A Computer "
                    "Science Portal for Geeks";

    // Creating file using fopen()
    // with access mode "w"
    fptr = fopen("file.txt", "w");

    // Checking if the file is created
    if (fptr == NULL) 
        printf("The file is not opened.");
    else{
        printf("The file is now opened.\n");
        fputs(data, fptr);
        fputs("\n", fptr);

        // Closing the file using fclose()
        fclose(fptr);
        printf("Data successfully written in file "
               "file.txt\n");
        printf("The file is now closed.");
    }
    return 0;
}

The file is now opened.
Data successfully written in file file.txt
The file is now closed.

Reading From a File

The file read operation in C can be performed using functions fscanf() or fgets(). Both the functions performed the same operations as that of scanf() and gets but with an additional parameter, the file pointer. There are also other functions we can use to read from a file. Such functions are listed below:

Example:

#include <stdio.h>
#include <string.h>

int main() {
    FILE* fptr;

    // Declare the character array 
    // for the data to be read from file
    char data[50];
    fptr = fopen("file.txt", "r");

    if (fptr == NULL) {
        printf("file.txt file failed to open.");
    }
    else {

        printf("The file is now opened.\n");

        // Read the data from the file
        // using fgets() method
        while (fgets(data, 50, fptr)
               != NULL) {

            // Print the data
            printf("%s", data);
        }

        // Closing the file using fclose()
        fclose(fptr);
    }
    return 0;
}

Output

The file is now opened.
GeeksforGeeks-A Computer Science Portal for Geeks

The getc() and some other file reading functions return EOF (End Of File) when they reach the end of the file while reading. EOF indicates the end of the file, and its value is implementation-defined. Reading more after EOF results in undefined error so, it is always recommended to check for EOF while reading a file.

Note: One thing to note here is that after reading a particular part of the file, the file pointer will be automatically moved to the end of the last read character.

Closing a File

The fclose() function is used to close the file. After successful file operations, you must always close a file to remove it from the memory.

Syntax:

fclose(file_pointer);

Move File Pointer

File pointer generally points to the position according to the mode or last read/write operation. We can manually move this pointer to any position in the file using fseek() function.

Syntax:

fseek(fptr, offset, pos);

where, pos is the position from where offset is counted and offset is the number of positions to shift from pos (it can be negative or positive).

Example:

While writing to a file opened in rw+ mode, the file pointer moves to the end of the file. In case where we want to replace a word, then first we have to move the file pointer to the position where that word starts.

#include <stdio.h>
#include <stdlib.h>

int main() {
    
    // File pointer
    FILE* fptr;

    // Get the data to be written in file
    char data[50] = "GeeksforGeeks-A Computer "
                    "Science Portal for Geeks";

    // Creating file using fopen()
    // with access mode "w"
    fptr = fopen("file.txt", "w");

    // Checking if the file is created
    if (fptr == NULL) 
        printf("The file is not opened.");
    else{
        printf("The file is now opened.\n");
        fputs(data, fptr);
        fputs("\n", fptr);
        fseek(fptr, -6, SEEK_END);
        
        fputs("GeeksforGeeks", fptr);

        // Closing the file using fclose()
        fclose(fptr);
        printf("Data successfully written in file "
               "file.txt\n");
        printf("The file is now closed.");
    }
    return 0;
}

The file is now opened.
Data successfully written in file file.txt
The file is now closed.

Now, imagine you want to read this file after writing. We can use fseek() here too, but these is one more function specifically for this purpose which is rewind().

Read and Write in a Binary File

Till now, we have only discussed text file operations. The operations on a binary file are similar to text file operations with little difference.

Opening a Binary File

To open a file in binary mode, we use the rb, rb+, ab, ab+, wb, and wb+ access mode in the fopen() function. We also use the .bin file extension in the binary filename.

Example:

fptr = fopen("filename.bin", "rb");

Write to a Binary File

We use fwrite() function to write data to a binary file. The data is written to the binary file in the form of bits (0's and 1's).

Syntax of fwrite()

fwrite(ptr, size, nmemb, file_pointer);

Parameters:

• ptr: pointer to the block of memory to be written.
• size: size of each element to be written (in bytes).
• nmemb: number of elements.
• file_pointer: FILE pointer to the output file stream.

Return Value:

• Number of objects written.

Example: Program to write to a Binary file using fwrite()

#include <stdio.h>
#include <stdlib.h>

struct threeNum {
    int n1, n2, n3;
};

int main() {
    int n =1 ;
    
    // Structure variable declared.
    struct threeNum num;
    FILE* fptr;
    fptr = fopen("binaryFile.bin", "wb");
    int flag = 0;
    num.n1 = n;
    num.n2 = 5 * n;
    num.n3 = 5 * n + 1;
    
    // Write the Structure data
    // to binary file.
    flag = fwrite(&num, sizeof(struct threeNum), 1,
                  fptr);

    // Checking if the data is written.
    if (!flag)
        printf("Write Operation Failure");
    else
        printf("Write Operation Successful");
    fclose(fptr);
    return 0;
}

Write Operation Successful

Reading from Binary File

The fread() function can be used to read data from a binary file in C. The data is read from the file in the same form as it is stored i.e. binary form.

Syntax:

fread(ptr, size, nmemb, file_pointer);

Parameters:

• ptr: pointer to the block of memory to read.
• size: the size of each element to read (in bytes).
• nmemb: number of elements.
• file_pointer: FILE pointer to the input file stream.

Return Value:

• Number of objects read.

Example: Program to Read from a binary file using fread()

#include <stdio.h>
#include <stdlib.h>

// Structure that store
// binary file data
struct threeNum {
    int n1, n2, n3;
};

int main() {
    int n;
    struct threeNum num;
    FILE* fptr;
    fptr = fopen("binaryFile.bin", "rb");
    
    // Read the data from binary 
    // file and print that data
    fread(&num, sizeof(struct threeNum), 1, fptr);
    printf("n1: %d\tn2: %d\tn3: %d\n", num.n1, num.n2,
           num.n3);
    fclose(fptr);
    return 0;
}

Output

n1: 1   n2: 5   n3: 6

More Functions for C File Operations

The following table lists some more functions that can be used to perform file operations or assist in performing them.