Text and Binary File Processing

Text and Binary File Processing 程式設計潘仁義 CCU COMM

User Program C Library OS File System File C File I/O Overview(1/3)

C File I/O Overview(3/3) - stdio.h 與檔案相關之函式-

12.1 Input and Output Files, Review • text file • a named collection of characters saved in secondary storage • input (output) stream • continuous stream of character codes representing textual input (or output) data • (FILE *) stdin • system file pointer for keyboard’s input stream • (FILE *) stdout, stderr • system file pointers for screen’s output stream

TABLE 12.2Placeholders for printf Format Strings

TABLE 12.1 Meanings of Common Escape Sequences 語言

TABLE 12.4 Comparison of I/O with Standard Files and I/O with User-Defined File Pointers

Figure 12.1 Program to Make a Backup Copy of a Text File

Figure 12.1 Program to Make a Backup Copy of a Text File (cont’d) 可以換成feof( inp)嗎? 不行,還是有點不一樣

Figure 12.2 Input and Output Streams for File Backup Program

File Open Mode

12.2 Binary Files • Formatted Text files • contain variable length records • must be accessed sequentially, processing all records from the start of file to access a particular record • Binary Files (random access file) • a file containing binary numbers that are the computer’s internal representation of each file component • contain fixed length records • can be accessed directly, directly accessing the record that is required • Binary files are appropriate for online transaction processing systems, • e.g. airline reservation, order processing, banking systems, • sizeof • operator that finds the number of bytes used for storage of a data type

The Data Hierarchy • Bit - smallest data item • Value of 0 or 1 • Byte – 8 bits • Used to store a character • Decimal digits, letters, and special symbols • Field - group of characters conveying meaning • Example: your name • Record– group of related fields • Represented astruct or a class • Example: In a payroll system, a record for a particular employee that contained his/her identification number, name, address, etc. • File– group of related records • Example: payroll file • Database – group of related files

In a Random Access File … • Data • Data unformatted (stored as "raw bytes") in random access files • All data of the same type (ints, for example) use the same memory • All records of the same type have a fixed length • Data not human readable

Random Access • Access individual records without searching through other records • Instant access to records in a file • Data can be inserted without destroying other data • Data previously stored can be updated or deleted without overwriting. • Implemented using fixed length records • Sequential files do not have fixed length records

Random Access a File -- fread () • fread --Transfer bytes from a file to a location in memory • Function fread requires four arguments • ret = fread(buffer, size, num, myptr); • the number of objects read • buffer: Address of first memory cell to fill • size: Size of one value • num: Maximum number of elements to copy from the file into memory • myptr: File pointer to a binary file opened in mode “rb” using function fopen How to distinguish error and EOF? feof ( ), ferror( )

Random Access a File – fwrite() • fwrite - Transfer bytes from a location in memory to a file • fwrite( &number, sizeof( int ), 1, myPtr ); • &number - Location to transfer bytes from • sizeof( int ) - Number of bytes to transfer • 1 - For arrays, number of elements to transfer • In this case, "one element" of an array is being transferred • myPtr - File to transfer to or from

Random Access a File – fwrite() (II) • Writing structs fwrite( &myObject, sizeof (struct myStruct), 1, myPtr ); • sizeof - Returns size in bytes of object in parentheses • To write several array elements • Pointer to array as first argument • Number of elements to write as third argument

Figure 12.3 Creating a Binary File of Integers

目前存取位置 位移後 file offset origin: SEEK_SET SEEK_CUR SEEK_END Access Data Randomly in a Random Access File • fseek • Sets file position pointer to a specific position • fseek( myPtr, offset, symbolic_constant); • myPtr- pointer to file • offset - file position pointer (0 is first location) • symbolic_constant - specifies where in file we are reading from • SEEK_SET - seek starts at beginning of file • SEEK_CUR - seek starts at current location in file • SEEK_END - seek starts at end of file • ftell • Return the current position in a stream • ftell( myptr) • myPtr- pointer to file

12.3 SEARCHING A DATABASE • database • a vast electronic file of information that can be quickly searched using subject headings or keywords • 本節請自修, 寫作業時可參考

12.4 COMMON PROGRAMMING ERRORS(1/2) • Remember to declare a file pointer variable (type FILE *) for each file you want to process • fscanf, fprintf, getc and putc must be used for text I/O only • fread andfwrite are applied exclusively to binary files • getc, fscanf and fprintftake the file pointer as their first argument • getc, putc, fread and fwrite take the file pointer as their last argument

12.4 COMMON PROGRAMMING ERRORS (2/2) • Opening a file for output by calling fopen with a second argument of “w” or “wb” typically results in a loss of any existing file whose name matches the first argument • Binary files cannot be created, viewed, or modified using an editor or word processor program • Depending on OS, a read cannot follow a write, a write cannot follow a read, except using a fseek(fp, 0, SEEK_CUR) between them

Ch.12 之番外篇 • 懶惰的C語言 • 讀取失敗了, 再來看原因是什麼 (別的語言不一定要等到讀失敗) • getc( )的EOF, fread的傳回0 • 用feof( ), ferror( )可分辨是檔案結束或是讀取錯誤 • 想知道檔案的大小嗎? • fseek( ) + ftell( ) • stat(), fstat( ), struct stat {... st_size;…}; • +模式的陷阱 • 交互讀寫可能會有奇怪的現象 • man fopen • Reads and writes may be intermixed on read/write streams in any order, and do not require an intermediate seek as in previous versions of stdio. • This is not portable to other systems, however; ANSI C requires that a file positioning function intervene between output and input, unless an input operation encounters end-of-file.

Text and Binary File Processing