Bài giảng Lập trình C - Session 12: File Handling

File Handling Session 12 Elementary Programming with C/Session 12/ * of 28 Objectives Explain streams and files Discuss text streams and binary streams Explain the various file functions Explain file pointer Discuss current active pointer Explain command-line arguments Elementary Programming with C/Session 12/ * of 28 File Input/Output All I/O operations in C are carried out using functions from the standard library This approach makes the C file system very powerful and flexible I/O in C is unique because data may be transferred in its internal binary representation or in a human-readable text format Elementary Programming with C/Session 12/ * of 28 Streams The C file system works with a wide variety of devices including printers, disk drives, tape drives and terminals Though all these devices are very different from each other, the buffered file system transforms each device into a logical device called a stream Since all streams act similarly, it is easy to handle the different devices There are two types of streams - the text and binary streams Elementary Programming with C/Session 12/ * of 28 Text Streams A text stream is a sequence of characters that can be organized into lines terminated by a new line character In a text stream, certain character translations may occur as required by the environment Therefore, there may not be a one-to-one relationship between the characters that are written (or read) and those in the external device Also, because of possible translations, the number of characters written (or read) may not be the same as those in the external device Elementary Programming with C/Session 12/ * of 28 Binary Streams A binary stream is a sequence of bytes with a one-to-one correspondence to those in the external device, that is, there are no character translations The number of bytes written (or read) is the same as the number on the external device Binary streams are a flat sequence of bytes, which do not have any flags to indicate the end of file or end of record The end of file is determined by the size of the file Elementary Programming with C/Session 12/ * of 28 Files A file can refer to anything from a disk file to a terminal or a printer A file is associated with a stream by performing an open operation and disassociated by a close operation When a program terminates normally, all files are automatically closed When a program crashes, the files remain open Elementary Programming with C/Session 12/ * of 28 Basic File Functions Elementary Programming with C/Session 12/ * of 28 File Pointer A file pointer is essential for reading or writing files It is a pointer to a structure that contains the file name, current position of the file, whether the file is being read or written, and whether any errors or the end of the file have occurred The definitions obtained from stdio.h include a structure declaration called FILE The only declaration needed for a file pointer is: 	FILE *fp Elementary Programming with C/Session 12/ * of 28 Opening a Text File The fopen() function opens a stream for use and links a file with that stream The fopen() function returns a file pointer associated with the file The prototype for the fopen() function is: 	FILE *fopen(const char *filename, const char *mode); Elementary Programming with C/Session 12/ * of 28 Closing a Text File It is important to close a file once it has been used This frees system resources and reduces the risk of overshooting the limit of files that can be open Closing a stream flushes out any associated buffer, an important operation that prevents loss of data when writing to a disk The fclose() function closes a stream that was opened by a call to fopen() The prototype for fclose() is: 	int fclose(FILE *fp); The fcloseall() function closes all open streams Elementary Programming with C/Session 12/ * of 28 Writing a Character – Text File Streams can be written to either character by character or as strings The fputc() function is used for writing characters to a file previously opened by fopen() The prototype is: 	int fputc(int ch, FILE *fp); Elementary Programming with C/Session 12/ * of 28 Reading a Character – Text File The fgetc() function is used for reading characters from a file opened in read mode, using fopen() The prototype is: 	int fgetc(FILE *fp); The fgetc() function returns the next character from the current position in the input stream, and increments the file position indicator Elementary Programming with C/Session 12/ * of 28 String l/O The functions fputs() and fgets() write and read character strings to and from a disk file The fputs() function writes the entire string to the specified stream The fgets() function reads a string from the specified stream until either a new line character is read or length-1 characters have been read The prototypes are: 	int fputs(const char *str, FILE *fp); 	char *fgets( char *str, int length, FILE *fp); Elementary Programming with C/Session 12/ * of 28 Opening a File-Binary The fopen() function opens a stream for use and links a file with that stream The fopen() function returns a file pointer associated with the file The prototype for the fopen() function is: 	FILE *fopen(const char *filename, const char *mode); Elementary Programming with C/Session 12/ * of 28 Closing a File Binary The fclose() function closes a stream that was opened by a call to fopen() The prototype for fclose() is: 	int fclose(FILE *fp); Elementary Programming with C/Session 12/ * of 28 The fread() and fwrite() functions The functions fread() and fwrite() are referred to as unformatted read or write functions They are used to read and write an entire block of data to and from a file The most useful application involves reading and writing user-defined data types, especially structures The prototypes for the functions are: 	size_t fread(void *buffer, size_t num_bytes, size_t count, FILE *fp); 	size_t fwrite(const void *buffer, size_t num_bytes, size_t count, FILE *fp); Elementary Programming with C/Session 12/ * of 28 Using feof() The function feof() returns true if the end of the file has been reached, otherwise it returns false (0) This function is used while reading binary data The prototype is: 	int feof (FILE *fp); Elementary Programming with C/Session 12/ * of 28 The rewind() function The rewind() function resets the file position indicator to the beginning of the file It takes the file pointer as its argument Syntax: 	rewind(fp ); Elementary Programming with C/Session 12/ * of 28 The ferror() function The ferror() function determines whether a file operation has produced an error As each operation sets the error condition, ferror() should be called immediately after each operation; otherwise, an error may be lost Its prototype is: 	int ferror(FILE *fp); Elementary Programming with C/Session 12/ * of 28 Erasing Files The remove() function erases a specified file Its prototype is: 	int remove(char *filename); Elementary Programming with C/Session 12/ * of 28 Flushing streams The fflush() function flushes out the buffer depending upon the file type A file opened for read will have its input buffer cleared, while a file opened for write will have its output buffer written to the files Its prototype is: 	int fflush(FILE *fp); The fflush() function, with a null, flushes all files opened for output Elementary Programming with C/Session 12/ * of 28 The Standard Streams Whenever a C program starts execution under DOS, five special streams are opened automatically by the operating system The standard input (stdin) The standard output (stdout) The standard error (stderr) The standard printer (stdprn) The standard auxiliary (stdaux) Elementary Programming with C/Session 12/ * of 28 Current Active Pointer A pointer is maintained in the FILE structure to keep track of the position where I/O operations take place Whenever a character is read from or written to the stream, the current active pointer (known as curp) is advanced The current location of the current active pointer can be found with the help of the ftell() function The prototype is: 	long int ftell(FILE *fp); Elementary Programming with C/Session 12/ * of 28 Setting Current Position-1 The fseek() function repositions the curp by the specified number bytes from the start, the current position or the end of the stream depending upon the position specified in the fseek() function The prototype is: 	int fseek (FILE *fp, long int offset, int origin); Elementary Programming with C/Session 12/ * of 28 Setting Current Position-2 The origin indicates the starting position of the search and has values as follows: Elementary Programming with C/Session 12/ * of 28 fprintf() and fscanf()-1 The buffered I/O system includes fprintf() and fscanf() functions that are similar to printf() and scanf() except that they operate with files The prototypes of are: 	int fprintf(FILE * fp, const char *control_string,..); 	int fscanf(FILE *fp, const char *control_string,...); Elementary Programming with C/Session 12/ * of 28 fprintf() and fscanf()-2 The fprintf() and fscanf() though the easiest, are not always the most efficient Extra overhead is incurred with each call, since the data is written in formatted ASCII data instead of binary format So, if speed or file size is a concern, fread() and fwrite() are a better choice