Introduction to Computer and Program Design. Lesson 6. File I/O. James C.C. Cheng Department of Computer Science National Chiao Tung University

Transcription

1 Introduction to Computer and Program Design Lesson 6 File I/O James C.C. Cheng Department of Computer Science National Chiao Tung University

2 File System in OS Microsoft Windows Filename DriveID : /DirctoryName/MainFileName.ExtensionName EX: C:/test.txt D:/MyDoc/Reports/ ppt Storage Devices Directories or Folders Files Files Directories or Folders Files 2

3 File System in OS Microsoft Windows Filename DriveID A and B are used for floppy disk drivers C is the master boot device DirctoryName MainFileName ExtensionName = type name Path Absolute path = full path, Relative path a path relative to the working directory EX: if the the working directory is D:/ICP/2011 ABC.h D:/ICP/2011/ABC.h /0515/test.dat D:/ICP/2011/0515/test.dat../xyz.txt D:/ICP/xyz.txt (.. represents the parent diretory)../../t_t.b D:/T_T.b 3

4 File System in OS Microsoft Windows FAT32 (File Allocation Table 32-bit) Max. file size: 4GB Max. length of MainFileName: 255 (in Windows), 8+3(DOS) Protection: Read-Only, Hidden and System exfat (Extended File Allocation Table) Max. file size: 64ZebiB = 2 70 bytes Max. length of MainFileName: 255 Protection: Read-Only, Hidden and System NTFS (New Technology File System) Max. file size: 16 TB Max. length of MainFileName: 255 UTF-16 code units Protection: Read-Only, Hidden, System and Windows Access Control List(ACL) 4

5 File I/O in C I/O redirection Redirect the standard I/O to file I/O < input redirection > output redirection EX: MyProg.exe < input.txt > ouput.txt #include <stdio.h> void main(){ int x = 0, y = 0; do{ scanf("%d %d", &x, &y); printf("x = %d, y = %d, x * y = %d\n", x, y, x*y); }while( x>=0); } 5

6 File I/O in C I/O redirection int fprintf ( FILE *stream, const char *format [, argument ]...); stream is a pointer to point a I/O device It returns the number of bytes written. It returns -1 if function failed. #include <stdio.h> void main(){ int x = 10, y = 20; int r = printf("%d, %d\n", x, y); // 10, 20\n printf ("%d\n", r); // 7 r = fprintf( stdout, "%d, %d\n", x, y); // 10, 20\n, it can be redirected to file printf ("%d\n", r); // 7 r = fprintf( stderr, "%d, %d\n", x, y); // 10, 20\n, it cannot be redirected to file printf ("%d\n", r); // 7 } r = fprintf( stdin, "%d, %d\n", x, y); // printf ("%d\n", r); // -1 6

7 File I/O in C Stream I/O 1. Open file FILE * file; file = fopen( "C:/MyDoc/data.txt", "r" ); /* filename format: DriveID : /DirctoryName/MainFileName.ExtensionName Each slash repreasents a directory */ // Do not use back-slash character in the filename, why?: file = fopen( "C:\MyDoc\data.txt", "r" ); 2. Input (read) 3. Output (write) 4. Close file int x[10]; fread( &x, sizeof( int ), 10, file ); char *s = " Hello world"; fwrite( s, 1, strlen(s), file ); fclose(file); 7

8 File I/O in C Stream I/O Open file FILE *fopen( const char *filename, const char *mode ); mode can be: "r Opens for reading. The file must be existed. "w Opens an empty file for writing. If the given file exists, its contents are destroyed. "a Opens for appending; creates the file first if it doesn t exist. Writing always at the end of the file (appending) "r+ = r with writing. (The file must exist.) "w+ = w with reading "a+ = a with reading. 8

9 File I/O in C Stream I/O Open file FILE *fopen( const char *filename, const char *mode ); mode with translation type : mode + t : Open in text mode. It inserts \r (0x0D) before \n (0x0A) in write-out data. It discard \r (0x0D) before \n (0x0A) in read-in data. EX: "rt", "wt", "at", "r+t", "w+t", "a+t" mode + b : Open in binary mode. EX: "rb", "wb", "ab", "r+b", "w+b", "a+b t + mode and b + mode are wrong usages and fopen returns NULL EX: bw, ta, The default translation type depends on the OS. You should explicitly specify the translation type at any time. 9

10 File I/O in C Stream I/O Input (read) / Output (write) size_t fread( void *buffer, size_t size, size_t count, FILE *stream ); size_t fwrite( const void *buffer, size_t size, size_t count, FILE *stream ); buffer : Storage location for data size: Item size in bytes count: Maximum number of items to be read stream: Pointer to FILE structure 10

11 File I/O in C Stream I/O Other Stream I/O functions: long ftell( FILE *stream ); Gets the current position of a file pointer. int fseek( FILE *stream, long offset, int origin ); Moves the file pointer to a specified location. Origin = SEEK_CUR current position + offset SEEK_END files size(in byte) + offset SEEK_SET offset int feof( FILE *stream ); Returns true if the cursor is at the end of file int fprintf( FILE *stream, const char *format [, argument ]...); int fscanf( FILE *stream, const char *format [, argument ]...);

12 List Files In Directory dirent.h #include <stdio.h> #include <dirent.h> int main(){ DIR *dirp; dirent *entry; if(dirp = opendir("c:/")){ while(entry = readdir (dirp)) printf("%s\n", entry->d_name); closedir (dirp); } getchar(); return 0; } dirent.h is known to be included in the following compilers: Turbo C++ (DOS) GCC (Cross-platform) MinGW (Microsoft Windows) Borland C++ Builder (Microsoft Windows) Microsoft Visual C++ does not include dirent.h Download the API package:

13 File I/O in C++ #include <fstream> 1. Open file fstream file("c:/mydoc/test.txt ", ios::in ios::out ios::trunc ); 2. Input (read) int A[10]; file.read((char *)A, 10 * sizeof(int)); 3. Output (write) int A[10]; file.write((const char *)A, 10 * sizeof(int)); int x = 0; file << x << " + 1 = " << x + 1 << std::endl; 4. Close file file.close(); 13

14 File I/O in C++ Open file Use the constructor of fstream Constructor: the function that creates an object. fstream( const char* szname, int nmode, int nprot = filebuf::openprot ); nmode can be: ios::app, appending to the end of file. ios::ate, to seek to the end of file. ios::binary, binary mode ios::in, read mode. ios::out, write mode. ios::trunc, to delete contents of an existing file. nprot: sharing mode, just use the default value. 14

15 File I/O in C++ Input (read) / Output (write) istream& fstream::read(char* buffer, streamsize size ); ostream& fstream::write( const *char buffer, streamsize size ); buffer : Storage location for data size: Item size in bytes. (streamsize = int)

16 File I/O in C++ Stream I/O Other Stream I/O functions: streampos fstream::tellg(); Get position of get pointer streampos fstream::tellp(); Get position of put pointer istream& fstream::seekg(streamoff off, ios_base::seekdir dir ); Sets the position of the get pointer dir = ios_base::beg beginning of the stream buffer ios_base::cur current position in the stream buffer ios_base::end end of the stream buffer ostream& fstream::seekp(streamoff off, ios_base::seekdir dir ); Sets the position of the put pointer bool fstream::eof ( ) const; Returns true if the cursor is at the end of file