**Course Title:** Mastering C: From Fundamentals to Advanced Programming **Section Title:** File I/O in C **Topic:** Develop a C program that reads from and writes to files.(Lab topic) **Objective:** By the end of this lab, you will be able to develop a C program that reads from and writes to files effectively. **Introduction:** In this lab, we will create a C program that reads data from a file and writes data to another file. This will help you understand the practical implementation of file I/O operations in C. **Understanding File Modes:** Before we start reading and writing to files, it's essential to understand the different file modes available in C. * `r`: Opens the file for reading. If the file does not exist, the program will terminate. * `w`: Opens the file for writing. If the file does not exist, a new file will be created. If the file already exists, its contents will be deleted. * `a`: Opens the file for appending. If the file does not exist, a new file will be created. If the file already exists, new data will be appended to the end of the file. * `r+`: Opens the file for both reading and writing. If the file does not exist, the program will terminate. * `w+`: Opens the file for both reading and writing. If the file does not exist, a new file will be created. If the file already exists, its contents will be deleted. * `a+`: Opens the file for both reading and appending. If the file does not exist, a new file will be created. If the file already exists, new data will be appended to the end of the file. **Reading from a File:** To read from a file, we can use the `fopen` function to open the file in read mode (`r`) and then use the `fscanf` function to read data from the file. ```c #include <stdio.h> #include <stdlib.h> int main() { FILE *filePointer; char filename[] = "example.txt"; char data[100]; // Open the file in read mode filePointer = fopen(filename, "r"); // Check if the file is opened successfully if (filePointer == NULL) { printf("Could not open file %s", filename); exit(1); } // Read data from the file fscanf(filePointer, "%s", data); printf("Data read from file: %s\n", data); // Close the file fclose(filePointer); return 0; } ``` **Writing to a File:** To write to a file, we can use the `fopen` function to open the file in write mode (`w`) and then use the `fprintf` function to write data to the file. ```c #include <stdio.h> #include <stdlib.h> int main() { FILE *filePointer; char filename[] = "example.txt"; char data[] = "Hello, World!"; // Open the file in write mode filePointer = fopen(filename, "w"); // Check if the file is opened successfully if (filePointer == NULL) { printf("Could not open file %s", filename); exit(1); } // Write data to the file fprintf(filePointer, "%s", data); printf("Data written to file: %s\n", data); // Close the file fclose(filePointer); return 0; } ``` **Lab Exercise:** Create a C program that reads a string from the user and writes it to a file named "output.txt". Then, read the data from the file and display it on the console. **Practical Tips:** * Always check if the file is opened successfully before performing any file operations. * Use the `fclose` function to close the file after reading or writing to it. * Use the `fscanf` and `fprintf` functions to read and write data to files. **External Resources:** For more information on file I/O in C, you can refer to the following resources: * [File Input/Output](https://www.gnu.org/software/libc/manual/html_node/Input_002fOutput-Overview.html) by the GNU C Library * [File I/O in C](https://www.tutorialspoint.com/cprogramming/c_file_io.htm) by Tutorials Point **Leave a Comment:** If you have any questions or need help with the lab exercise, feel free to leave a comment below. **What's Next:** In the next topic, we will cover [Understanding preprocessor directives: #include, #define, #ifdef.](./Preprocessor-Directives-and-Macros.md)