C File Handling Lecture Notes PDF

MCS5101- Programming and Problem solving Lecture 9 –File Handling in C Professor Noel Fernando What is a File? A File is a collection of data stored in the secondary storage. The data was entered into the programs through the keyboard or other alternative method.. So, File...

MCS5101- Programming and Problem solving Lecture 9 –File Handling in C Professor Noel Fernando What is a File? A File is a collection of data stored in the secondary storage. The data was entered into the programs through the keyboard or other alternative method.. So, Files are used for storing information that can be processed by the programs. A file is a collection of related data that a computers treats as a single unit. When a computer reads a file, it copies the file from the storage device to memory; when it writes to a file, it transfers data from memory to the storage device. In C language, we use a structure pointer of FILE type to declare a file. FILE is defined in stdio.h library. Why file handling is important and a few features of using files: Reusability: File handling allows us to preserve the information/data generated after we run the program. Saves Time: Some programs might require a large amount of input from their users. In such cases, file handling allows you to easily access a part of a code using individual commands. Commendable storage capacity: When storing data in files, you can leave behind the worry of storing all the info in bulk in any program. Portability: The contents available in any file can be transferred to another one without any data loss in the computer system. This saves a lot of effort and minimises the risk of flawed coding. Types of Files in a C Program There are two types of files in C Summary Text files Text File: Binary files Contains text in ASCII characters generally alphabets and numeric. Human readable Text files Takes more space compared to binary files. Text files are the normal.txt files. You can easily create text files using any simple text editors such as Notepad. When you open those files, you'll see all the contents within the file as plain text. You can easily edit or delete the contents. They take minimum effort to maintain, are easily readable, and provide the least security and takes bigger storage space. Types of Files in a C Program Binary files Binary File: Binary files are mostly Contains text in binary form the.bin files in your computer. Not Human readable Faster access compared to text Instead of storing data in plain files. text, they store it in the binary form (0's and 1's). They can hold a higher amount of data, are not readable easily, and provides better security than text files. File Operations File handling in C enables us to create, update, read, and delete the files stored on the local file system through our C program. The following operations can be performed on a file. Creation of the new file Opening an existing file Reading from the file Writing to the file Deleting the file C File Operations C file operations refer to the different possible operations that we can perform on a file in C such as: Creating a new file – fopen() with attributes as “a” or “a+” or “w” or “w+” Opening an existing file – fopen() Reading from file – fscanf() or fgets() Writing to a file – fprintf() or fputs() Moving to a specific location in a file – fseek(), rewind() Closing a file – fclose() Working with files When working with files, you need to declare a pointer of type file. This declaration is needed for communication between the file and the program. FILE *fptr Opening a file - for creation and edit Opening a file is performed using the fopen() function defined in the stdio.h header file. Opening a File The file mode tells C how the program will use the file. The filename indicates the system name and location for the file. We assign the return value of fopen() to our pointer variable: Relative path Absolute path 9. Opening a File Consider the following command FILE *filePointer = fopen("filename", "mode"); FILE *filePointer is a pointer to a FILE object, which holds information about the file being accessed. "filename" is the name of the file you want to open. This can include the file path if it's located in a different directory. "mode" specifies how the file should be opened. The most common modes are: "r" - Open a file for reading. If the file doesn't exist, fopen() returns NULL "w" - Open a file for writing. If the file doesn't exist, it will be created. If it does exist, its contents are erased. And more …. Closing a File When we finish with a mode, we need to close the file before ending the program or beginning another mode with that same file. The file (both text and binary) should be closed after reading/writing. Closing a file is performed using the fclose() function. fclose(fptr); Working with files The syntax for opening a file in standard I/O is: ptr = fopen("fileopen","mode"); For example, 1) fopen("E:\\cprogram\\newprogram.txt","w"); 2) fopen("E:\\cprogram\\oldprogram.bin","rb"); 1) If newprogram.txt file is not existed in the location, The first function creates a new file named “newprogram.txt” and opens it for writing as per the mode 'w’. The writing mode allows you to create and edit (overwrite) the contents of the file. Working with files (2) fopen("E:\\cprogram\\oldprogram.bin","rb"); The second binary file “oldprogram.bin"," exists in the location. The second function opens the existing file for reading in binary mode 'rb’. The reading mode only allows you to read the file, you cannot write into the file. Functions for file handling There are many functions in the C library to open, read, write, search and close the file. A list of file functions are given below: Functions for file handling No. Function Description 1 fopen() opens new or existing file 2 fprintf() write data into the file 3 fscanf() reads data from the file 4 fputc() writes a character into the file 5 fgetc() reads a character from file 6 fclose() closes the file 7 fseek() sets the file pointer to given position 8 fputw() writes an integer to file 9 fgetw() reads an integer from file 10 ftell() returns current position 11 rewind() sets the file pointer to the beginning of the file File opening modes in C Mode Description r opens a text file in read mode w opens a text file in write mode a opens a text file in append mode r+ opens a text file in read and write mode w+ opens a text file in read and write mode a+ opens a text file in read and write mode rb opens a binary file in read mode wb opens a binary file in write mode ab opens a binary file in append mode rb+ opens a binary file in read and write mode wb+ opens a binary file in read and write mode ab+ opens a binary file in read and write mode Print to a File Syntax: fprintf(, , ); It's just like printf() but output is sent to a file, rather than standard output. It's used when we wish to values in a file. The first argument is a pointer to file. 17 Example 18 fscanf() fscanf() Reads data from the file stream and stores them according to the parameter format into the locations pointed by the additional arguments. Syntax: fscanf(, , ); scanf() reads from stdin; fscanf() reads from a user specified stream 19 getc & putc getc() or fgetc() reads a single character from a given file and increment the file pointer position. putc() or fputc() writes a single character to the output file, pointed to by the file pointer. 20 END of File There are few ways to check the end of the file condition. Using function feof() using macro EOF. 21 fwrite() This function is used to write an entire block of data structure to a given binary file. Declaration: size_t fwrite(const *ptr, size_t size, size_t nmemb, FILE *stream) Note: The size_t data type in C is an unsigned 1. ptr -- This is the pointer to the array of elements to be written. integer type used to 2. size -- This is the size in bytes of each element to be written. represent the size of objects in bytes. 3. nmemb -- This is the number of elements to be written to the file. It is defined in the 4. stream -- This is the pointer to a FILE object that specifies an output stream. stddef.h header and is commonly used to represent the size of arrays, memory blocks, and strings. 22 fwrite() This function returns the total number of elements successfully written is returned as a size_t object, which is an integral data type. If this number differs from the nmemb parameter, it will show an error. This function store data in the file in binary form, so you may not be able to read the file. However, you can read the information stored in the file using fread() 23 Reading and writing to a text file For reading and writing to a text file, we use the functions fprintf() and fscanf(). They are just the file versions of printf() and Scanf(). The only difference is that fprintf() and fscanf() expects a pointer to the structure FILE. Write a C program to takes a number from the user and stores it in the text file. Write a C program to takes a number from the user and stores it in the text file. #include #include int main() { int num; FILE *fptr; // use appropriate location if you are using MacOS or Linux fptr = fopen("C:\\program.txt","w"); Write a C program to takes a number from the user and stores it in the text file. if(fptr == NULL) { printf("Error!"); Note : After you compile and run exit(1); this program, you can see a text } file ’program.txt’ created in C printf("Enter num: "); drive of your computer. scanf("%d",&num); When you open the file, you can fprintf(fptr,"%d",num); see the integer you entered. fclose(fptr); return 0; Write a C program to read the number from the text file. Write a C program to read the number from the text file. #include #include int main() { int num; FILE *fptr; if ((fptr = fopen("C:\\program.txt","r")) == NULL){ printf("Error! opening file"); // Program exits if the file pointer returns NULL. exit(1); } Write a C program to read the number from the text file. fscanf(fptr,"%d", &num); printf("Value of n=%d", num); fclose(fptr); return 0; } Note: This program reads the integer present in the ‘program.txt’ file and prints it onto the screen. Other functions like fgetchar(), fputc() etc. can be used in a similar way. fread() This function is used to read an entire block of data structure from a given binary file. Declaration: size_t fread(const *ptr, size_t size, size_t nmemb, FILE *stream) 1. ptr -- This is the pointer to a block of memory with a minimum size of size*nmemb bytes.. 2. size -- This is the size in bytes of each element to be read. 3. nmemb -- This is the number of elements, each one with a size of size bytes. 4. stream -- This is the pointer to a FILE object that specifies an input stream. 31 Question : Write a C program to read Student Name, age and Gpa from a disk file and print on the screen. Example: fread() 33 fseek() This function is used for setting the file position pointer at the specified byte. Declaration: int fseek(FILE *stream, int offset, int origin) 1. fp -- This is the pointer to a FILE object that identifies the stream. 2. offset -- This is the number of bytes to offset from origin. 3. origin -- This is the position from where offset is added. It is specified by one of the following constants: SEEK_SET Seeks from beginning of file SEEK_CUR Seeks from current position SEEK_END Seeks from end of file 34 fseek() This function returns zero if successful, else it returns nonzero value. Output : This is C programming Language 35 Write down the output of the following program #include void main(){ FILE *fp; fp = fopen("myfile.txt","w+"); fputs("This is javatpoint", fp); fseek( fp, 12, SEEK_SET ); fputs(“Lesson Number 1", fp); fclose(fp); } Write down the output of the following program #include void main(){ FILE *fp; fp = fopen("myfile.txt","w+"); OUTPUT: fputs("This is javatpoint", fp); This is Java Lesson Number 1 fseek( fp, 12, SEEK_SET ); fputs(“Lesson Number 1", fp); fclose(fp); } Example: Read and print int from a txt file 38 Writing to a binary file To write into a binary file, you need to use the fwrite() function. The functions take four arguments: address of data to be written in the disk size of data to be written in the disk number of such type of data pointer to the file where you want to write. fwrite(addressData, sizeData, numbersData, pointerToFile); Writing to a binary file #include #include struct threeNum { int n1, n2, n3; }; int main() { int n; struct threeNum num; FILE *fptr; Writing to a binary file if((fptr = fopen("C:\\program.bin","wb")) == NULL) { printf("Error! opening file"); Write the output of the // Program exits if the file pointer returns NULL. program. exit(1); } for(n = 1; n < 5; ++n) { num.n1 = n; num.n2 = 5*n; num.n3 = 5*n + 1; fwrite(&num, sizeof(struct threeNum), 1, fptr); } fclose(fptr); return 0; } Writing to a binary file if((fptr = fopen("C:\\program.bin","wb")) == NULL) { We declare a structure threeNum with printf("Error! opening file"); // Program exits if the file pointer returns NULL. three numbers - n1, n2 and n3, exit(1); and define it in the main function as } num. for(n = 1; n < 5; ++n) { The first parameter takes the address num.n1 = n; of num and the second parameter num.n2 = 5*n; takes the size of the structure num.n3 = 5*n + 1; threeNum fwrite(&num, sizeof(struct threeNum), 1, fptr); } fclose(fptr); we're only inserting one instance of return 0; OUTPUT: num the third parameter is 1 And, the } last parameter *fptr points to the file we're storing the data. Question: Write a C program to read names, ages and GPAs of 10 students from the keyboard and write on a disk file. Question: Write a C program to read names, ages and GPAs of 10 students from the keyboard and write on a disk file. Example: Copy a file 45 Write down the output of the program : int main() { FILE* fptr; fptr = fopen("file.txt", "w+"); fprintf(fptr, “Sri Lanka Vs India \n"); // using rewind() rewind(fptr); // reading from file char buf; fscanf(fptr, "%[^\n]s", buf); printf("%s", buf); return 0; } Write down the output of the program : int main() { FILE* fptr; fptr = fopen("file.txt", "w+"); fprintf(fptr, “SriLanka Vs India \n"); // using rewind() rewind(fptr); // reading from file char buf; Sri Lanka Vs India fscanf(fptr, "%[^\n]s", buf); printf("%s", buf); return 0; } Question : Write down the output of the following program #include main() { FILE *fp; fp = fopen("/tmp/test.txt", "w+"); fprintf(fp, "This is testing for fprintf...\n"); fputs("This is testing for fputs...\n", fp); fclose(fp); } Write down the output of the following program #include Output ( in a file : main() ("/tmp/test.txt“) { This is testing for fprintf... FILE *fp; fp = fopen("/tmp/test.txt", "w+"); This is testing for fputs... fprintf(fp, "This is testing for fprintf...\n"); fputs("This is testing for fputs...\n", fp); fclose(fp); } Write down the output of the following program if one read the contents of the previous file (("/tmp/test.txt“) #include main() { FILE *fp; char buff; fp = fopen("/tmp/test.txt", "r"); fscanf(fp, "%s", buff); printf("1 : %s\n", buff ); fgets(buff, 255, (FILE*)fp); printf("2: %s\n", buff ); fgets(buff, 255, (FILE*)fp); printf("3: %s\n", buff ); fclose(fp); } Output of the above program #include The string is only scanned till a whitespace is encountered main() { FILE *fp; char buff; Output : fp = fopen("/tmp/test.txt", "r"); 1 : This fscanf(fp, "%s", buff); 2: is testing for fprintf... printf("1 : %s\n", buff ); 3: This is testing for fputs... fgets(buff, 255, (FILE*)fp); printf("2: %s\n", buff ); fgets(buff, 255, (FILE*)fp); printf("3: %s\n", buff ); fclose(fp); The newline character ( \n ) is called an escape sequence, and it forces the cursor to change its position to the beginning of } the next line on the screen.