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.

Why do we need File Handling in C?

So far the operations using the C program are done on a prompt/terminal which is not stored anywhere. The output is deleted when the program is closed. But in the software industry, most programs are written to store the information fetched from the program. The use of file handling is exactly what the situation calls for.

In order to understand why file handling is important, let us look at a few features of using files:

• Reusability: The data stored in the file can be accessed, updated, and deleted anywhere and anytime providing high reusability.
• Portability: Without losing any data, files can be transferred to another in the computer system. The risk of flawed coding is minimized with this feature.
• Efficient: A large amount of input may be required for some programs. File handling allows you to easily access a part of a file using few instructions which saves a lot of time and reduces the chance of errors.
• Storage Capacity: Files allow you to store a large amount of data without having to worry about storing everything simultaneously in a program.

If you’re interested in working with files and integrating them into larger data structures, the C Programming Course Online with Data Structures provides detailed lessons on file management in C.

Types of Files in C

A file can be classified into two types based on the way the file stores the data. They are as follows:

• Text Files
• Binary Files

1. Text Files

A text file contains data in the form of ASCII characters and is generally used to store a stream of characters.

• Each line in a text file ends with a new line character (‘\n’).
• It can be read or written by any text editor.
• They are generally stored with .txt file extension.
• Text files can also be used to store the source code.

2. Binary Files

A binary file contains data in binary form (i.e. 0’s and 1’s) instead of ASCII characters. They contain data that is stored in a similar manner to how it is stored in the main memory.

• The binary files can be created only from within a program and their contents can only be read by a program.
• More secure as they are not easily readable.
• They are generally stored with .bin file extension.

C File Operations

C file operations refer to the different possible operations that we can perform on a file in C such as:

1. Creating a new file – fopen() with attributes as “a” or “a+” or “w” or “w+”
2. Opening an existing file – fopen()
3. Reading from file – fscanf() or fgets()
4. Writing to a file – fprintf() or fputs()
5. Moving to a specific location in a file – fseek(), rewind()
6. Closing a file – fclose()

The highlighted text mentions the C function used to perform the file operations.

Functions for C File Operations

File Pointer in C

A file pointer is a reference to a particular position in the opened file. It is used in file handling to perform all file operations such as read, write, close, etc. We use the FILE macro to declare the file pointer variable. The FILE macro is defined inside <stdio.h> header file.

Syntax of File Pointer

FILE* pointer_name;

File Pointer is used in almost all the file operations in C.

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 of fopen()

FILE* fopen(const char *file_name, const char *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 in C

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 of Opening a File

// C Program to illustrate file opening
#include <stdio.h>
#include <stdlib.h>

int main()
{
    // file pointer variable 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. The program will "
               "now exit.");
        exit(0);
    }

    return 0;
}

The file is not opened. The program will now exit.

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. It is shown below

Create a File in C

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+.

FILE *fptr;
fptr = fopen("filename.txt", "w");

Example of Opening a File

// C Program to create a file
#include <stdio.h>
#include <stdlib.h>

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

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

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

The file is created Successfully.

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:

So, it depends on you if you want to read the file line by line or character by character.

Example:

FILE * fptr; 
fptr = fopen(“fileName.txt”, “r”);
fscanf(fptr, "%s %s %s %d", str1, str2, str3, &year);
char c = fgetc(fptr);

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.

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.

Write to a File

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

Example:

FILE *fptr ; 
fptr = fopen(“fileName.txt”, “w”);
fprintf(fptr, "%s %s %s %d", "We", "are", "in", 2012);
fputc("a", fptr);

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 of fclose()

fclose(file_pointer);

where the file_pointer is the pointer to the opened file.

Example:

FILE *fptr ;
fptr= fopen(“fileName.txt”, “w”);
---------- Some file Operations -------
fclose(fptr);

Examples of File Handing in C

Example 1: Program to Create a File, Write in it, And Close the File

// C program to Open a File,
// Write in it, And Close the File
#include <stdio.h>
#include <string.h>

int main()
{

    // Declare the file pointer
    FILE* filePointer;

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

    // Open the existing file GfgTest.c using fopen()
    // in write mode using "w" attribute
    filePointer = fopen("GfgTest.c", "w");

    // Check if this filePointer is null
    // which maybe if the file does not exist
    if (filePointer == NULL) {
        printf("GfgTest.c file failed to open.");
    }
    else {

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

        // Write the dataToBeWritten into the file
        if (strlen(dataToBeWritten) > 0) {

            // writing in the file using fputs()
            fputs(dataToBeWritten, filePointer);
            fputs("\n", filePointer);
        }

        // Closing the file using fclose()
        fclose(filePointer);

        printf("Data successfully written in file "
               "GfgTest.c\n");
        printf("The file is now closed.");
    }
  
    return 0;
}

The file is now opened.
Data successfully written in file GfgTest.c
The file is now closed.

This program will create a file named GfgTest.c in the same directory as the source file which will contain the following text: “GeeksforGeeks-A Computer Science Portal for Geeks”.

Example 2: Program to Open a File, Read from it, And Close the File

// C program to Open a File,
// Read from it, And Close the File
#include <stdio.h>
#include <string.h>

int main()
{

    // Declare the file pointer
    FILE* filePointer;

    // Declare the variable for the data to be read from
    // file
    char dataToBeRead[50];

    // Open the existing file GfgTest.c using fopen()
    // in read mode using "r" attribute
    filePointer = fopen("GfgTest.c", "r");

    // Check if this filePointer is null
    // which maybe if the file does not exist
    if (filePointer == NULL) {
        printf("GfgTest.c file failed to open.");
    }
    else {

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

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

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

        // Closing the file using fclose()
        fclose(filePointer);

        printf(
            "Data successfully read from file GfgTest.c\n");
        printf("The file is now closed.");
    }
    return 0;
}

Output

The file is now opened.
GeeksforGeeks-A Computer Science Portal for Geeks
Data successfully read from file GfgTest.c
The file is now closed.

This program reads the text from the file named GfgTest.c which we created in the previous example and prints it in the console.

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 from of bits (0’s and 1’s).

Syntax of fwrite()

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *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()

// C 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;
    // Structure variable declared here.
    struct threeNum num;
    FILE* fptr;
    if ((fptr = fopen("C:\\program.bin", "wb")) == NULL) {
        printf("Error! opening file");
        // If file pointer will return NULL
        // Program will exit.
        exit(1);
    }
    int flag = 0;
    // else it will return a pointer to the file.
    for (n = 1; n < 5; ++n) {
        num.n1 = n;
        num.n2 = 5 * n;
        num.n3 = 5 * n + 1;
        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 of fread()

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *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 written.

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

// C Program to Read from a binary file using fread()
#include <stdio.h>
#include <stdlib.h>
struct threeNum {
    int n1, n2, n3;
};
int main()
{
    int n;
    struct threeNum num;
    FILE* fptr;
    if ((fptr = fopen("C:\\program.bin", "rb")) == NULL) {
        printf("Error! opening file");
        // If file pointer will return NULL
        // Program will exit.
        exit(1);
    }
    // else it will return a pointer to the file.
    for (n = 1; n < 5; ++n) {
        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
n1: 2   n2: 10  n3: 11
n1: 3   n2: 15  n3: 16
n1: 4   n2: 20  n3: 21

fseek() in C

If we have multiple records inside a file and need to access a particular record that is at a specific position, so we need to loop through all the records before it to get the record. Doing this will waste a lot of memory and operational time. To reduce memory consumption and operational time we can use fseek() which provides an easier way to get to the required data. fseek() function in C seeks the cursor to the given record in the file.

Syntax for fseek()

int fseek(FILE *ptr, long int offset, int pos);

Example of fseek()

// C Program  to demonstrate the use of fseek() in C
#include <stdio.h>

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

    // Moving pointer to end
    fseek(fp, 0, SEEK_END);

    // Printing position of pointer
    printf("%ld", ftell(fp));

    return 0;
}

Output

81

rewind() in C

The rewind() function is used to bring the file pointer to the beginning of the file. It can be used in place of fseek() when you want the file pointer at the start.

Syntax of rewind()

rewind (file_pointer);

Example

// C program to illustrate the use of rewind
#include <stdio.h>

int main()
{
    FILE* fptr;
    fptr = fopen("file.txt", "w+");
    fprintf(fptr, "Geeks for Geeks\n");

    // using rewind()
    rewind(fptr);

    // reading from file
    char buf[50];
    fscanf(fptr, "%[^\n]s", buf);

    printf("%s", buf);

    return 0;
}

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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.

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Article Tags :

• C Language
• C-File Handling