File Input and Output

File input and output (I/O) operations are essential in C programming for reading data from files and writing data to files. This capability allows programs to interact with external files, enabling data persistence and communication between programs. In this chapter, we will explore the concepts of file I/O, from basic operations to advanced techniques, using clear examples and detailed explanations.

Opening a File

To begin operations on a file, it must be opened using the fopen() function. This function requires the filename and the mode of operation as parameters. Modes include read ("r"), write ("w"), and append ("a").

				
					FILE *file_pointer;
file_pointer = fopen("filename.txt", "r");

Closing a File

After completing file operations, it’s essential to close the file using fclose() to release system resources associated with it.

				
					fclose(file_pointer);

Reading from a File

Reading data from a file is achieved using functions like fscanf() or fgets() to read individual elements or entire lines, respectively.

				
					fscanf(file_pointer, "%d", &number);
fgets(buffer, sizeof(buffer), file_pointer);

Writing to a File

Writing data to a file involves using functions like fprintf() or fputs() to write formatted output or strings to the file.

				
					fprintf(file_pointer, "The value is: %d\n", value);
fputs("Hello, world!\n", file_pointer);

Error Handling

It’s essential to handle errors that may occur during file operations. Checking if the file pointer is NULL after opening a file is a common practice.

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

Binary File I/O

In addition to text files, C supports reading and writing binary files. For binary file I/O, functions like fwrite() and fread() are used to handle binary data.

				
					fwrite(data, sizeof(data[0]), num_elements, file_pointer);
fread(data, sizeof(data[0]), num_elements, file_pointer);

Seeking within a File

The fseek() function allows positioning the file pointer at a specific location within the file, enabling random access to data.

				
					fseek(file_pointer, offset, SEEK_SET); // SEEK_SET, SEEK_CUR, SEEK_END

Explanation:

  • offset: Number of bytes to move the file pointer.
  • SEEK_SET, SEEK_CUR, SEEK_END: Constants indicating the starting point for seeking.

 

  • SEEK_SET sets the file pointer relative to the beginning of the file.
  • SEEK_CUR sets the file pointer relative to its current position.
  • SEEK_END sets the file pointer relative to the end of the file.

Reading from a File ("r" mode)

This example demonstrates reading data from a text file named “input.txt”.

				
					#include <stdio.h>

int main() {
    FILE *file_pointer;
    char buffer[100];

    file_pointer = fopen("input.txt", "r");
    if (file_pointer == NULL) {
        printf("File open error!");
        return 1;
    }

    fgets(buffer, sizeof(buffer), file_pointer);
    printf("Data read from file: %s", buffer);

    fclose(file_pointer);
    return 0;
}

Explanation:

  • The program opens a file named “input.txt” in read mode ("r").
  • It checks if the file pointer is NULL to handle any errors during file opening.
  • Using fgets(), it reads a line from the file and stores it in the buffer array.
  • Finally, it prints the data read from the file.
  • The file is closed using fclose() to release system resources.

Output: If “input.txt” contains the text “Hello, world!”:

Writing to a File ("w" mode)

This example demonstrates writing data to a text file named “output.txt”.

				
					#include <stdio.h>

int main() {
    FILE *file_pointer;
    int value = 100;

    file_pointer = fopen("output.txt", "w");
    if (file_pointer == NULL) {
        printf("File open error!");
        return 1;
    }

    fprintf(file_pointer, "The value is: %d\n", value);

    fclose(file_pointer);
    return 0;
}

Explanation:

  • The program opens a file named “output.txt” in write mode ("w").
  • It checks if the file pointer is NULL to handle any errors during file opening.
  • Using fprintf(), it writes formatted data to the file.
  • Finally, it closes the file using fclose().

Output: A file named “output.txt” is created with the content “The value is: 100”.

Appending to a File ("a" mode)

This example demonstrates appending data to a text file named “data.txt”.

				
					#include <stdio.h>

int main() {
    FILE *file_pointer;

    file_pointer = fopen("data.txt", "a");
    if (file_pointer == NULL) {
        printf("File open error!");
        return 1;
    }

    fputs("New data appended!\n", file_pointer);

    fclose(file_pointer);
    return 0;
}

Explanation:

  • The program opens a file named “data.txt” in append mode ("a").
  • It checks if the file pointer is NULL to handle any errors during file opening.
  • Using fputs(), it appends a string to the end of the file.
  • Finally, it closes the file using fclose().

Output: If “data.txt” already contains some data, the new string “New data appended!” will be added to the end of the file.

Binary File I/O

This example demonstrates reading and writing binary data to a binary file named “binary_data.bin”.

				
					#include <stdio.h>

int main() {
    FILE *file_pointer;
    int data_write[] = {10, 20, 30, 40, 50};
    int data_read[5];

    // Writing to binary file
    file_pointer = fopen("binary_data.bin", "wb");
    if (file_pointer == NULL) {
        printf("File open error!");
        return 1;
    }

    fwrite(data_write, sizeof(data_write[0]), sizeof(data_write)/sizeof(data_write[0]), file_pointer);
    fclose(file_pointer);

    // Reading from binary file
    file_pointer = fopen("binary_data.bin", "rb");
    if (file_pointer == NULL) {
        printf("File open error!");
        return 1;
    }

    fread(data_read, sizeof(data_read[0]), sizeof(data_read)/sizeof(data_read[0]), file_pointer);
    fclose(file_pointer);

    // Displaying data read from file
    printf("Data read from file: ");
    for (int i = 0; i < sizeof(data_read)/sizeof(data_read[0]); i++) {
        printf("%d ", data_read[i]);
    }
    printf("\n");

    return 0;
}

Explanation:

  • The program first writes an array of integers data_write to a binary file named “binary_data.bin” in write mode ("wb").
  • Then, it reads the data back into another array data_read from the same binary file in read mode ("rb").
  • The data read from the file is then displayed.
				
					// output //
Data read from file: 10 20 30 40 50

File Operations

Opening a File (fopen()):

  • Description: Opens a file for further operations such as reading, writing, or appending.
  • Function Prototype: FILE *fopen(const char *filename, const char *mode);
  • Parameters:
    • filename: Name of the file to be opened.
    • mode: Mode of file access (“r”, “w”, “a”, “rb”, “wb”, “ab”, etc.).
  • Return Value: A file pointer to the opened file, or NULL if an error occurs.

Closing a File (fclose()):

  • Description: Closes a previously opened file, releasing associated resources.
  • Function Prototype: int fclose(FILE *stream);
  • Parameters:
    • stream: File pointer to the file to be closed.
  • Return Value: 0 if successful, EOF (end-of-file) if an error occurs.

Reading from a File (fscanf(), fgets(), fread(), etc.):

  • Description: Reads data from a file.
  • Functions:
    • int fscanf(FILE *stream, const char *format, ...);
    • char *fgets(char *str, int n, FILE *stream);
    • size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
  • Parameters:
    • stream: File pointer to the file from which data is read.
    • format (for fscanf()): Format string specifying the data format to read.
    • str (for fgets()): Buffer to store the read data.
    • n (for fgets()): Maximum number of characters to read.
    • ptr (for fread()): Pointer to the buffer where data is to be stored.
    • size (for fread()): Size in bytes of each element to be read.
    • nmemb (for fread()): Number of elements to be read.
  • Return Value: Number of items read, or EOF if an error occurs.

Writing to a File (fprintf(), fputs(), fwrite(), etc.):

  • Description: Writes data to a file.
  • Functions:
    • int fprintf(FILE *stream, const char *format, ...);
    • int fputs(const char *str, FILE *stream);
    • size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
  • Parameters:
    • stream: File pointer to the file where data is to be written.
    • format (for fprintf()): Format string specifying the format of the data to write.
    • str (for fputs()): String to be written.
    • ptr (for fwrite()): Pointer to the data to be written.
    • size (for fwrite()): Size in bytes of each element to be written.
    • nmemb (for fwrite()): Number of elements to be written.
  • Return Value: Number of items written, or an error indicator.

Error Checking (feof(), ferror(), etc.):

  • Description: Checks for errors or end-of-file conditions during file operations.
  • Functions:
    • int feof(FILE *stream);
    • int ferror(FILE *stream);
  • Parameters:
    • stream: File pointer to the file being checked.
  • Return Value:
    • feof(): Non-zero if end-of-file indicator is set, 0 otherwise.
    • ferror(): Non-zero if error indicator is set, 0 otherwise.

Seeking within a File (fseek()):

  • Description: Sets the file position indicator to a specified position within the file.
  • Function Prototype: int fseek(FILE *stream, long int offset, int whence);
  • Parameters:
    • stream: File pointer to the file where seeking is performed.
    • offset: Number of bytes to offset from the reference point specified by whence.
    • whence: Reference point for seeking (SEEK_SET, SEEK_CUR, SEEK_END).
  • Return Value: 0 on success, non-zero otherwise.

File input and output operations are fundamental in C programming, enabling interaction with external files. Understanding file I/O allows developers to create applications capable of efficient data handling. This chapter equips readers with comprehensive knowledge and practical examples to master file I/O operations effectively, empowering them to tackle diverse file-related tasks with confidence. Happy coding!❤️

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