Accessing Files in C. Professor Hugh C. Lauer CS-2303, System Programming Concepts

Transcription

1 Accessing Files in C Professor Hugh C. Lauer CS-2303, System Programming Concepts (Slides include materials from The C Programming Language, 2 nd edition, by Kernighan and Ritchie, Absolute C++, by Walter Savitch, The C++ Programming Language, Special Edition, by Bjarne Stroustrup, and from C: How to Program, 5 th and 6 th editions, by Deitel and Deitel) 1

2 Two Kinds of File Access Stream Raw File is treated as a sequence of bytes Access is sequential i.e., in byte order Cannot replace data in the middle of a file File is a sequence of blocks Any block can be read and/or written independently 2

3 Definition File A (potentially) large amount of information or data that lives a (potentially) very long time Often much larger than the memory of the computer Often much longer than any computation Sometimes longer than life of machine itself (Usually) organized as a linear array of bytes or blocks Internal structure is imposed by application (Occasionally) blocks may be variable length (Often) requiring concurrent access by multiple threads or processes Even by processes on different machines! 3

4 Implementations of Files Usually on disks (or devices that mimic disks) Magnetic hard drive or floppy Optical CD, DVD Flash drives electronic memory, organized as disks Requirement Preserve data contents during power-off or disasters Directory / Folder Special kind of file that contains links pointing to other files Associates names with files 4

5 Opening and Closing Files FILE *fopen(char *name, char *mode) Makes a file available for use mode may be "r ", "w", "a", etc. If mode == "w" or "a", file is created if it does not already exist. mode == "w" overwrite file from beginning mode == a" add to end of file int fclose(file *fp); Disconnects file from program Flushes output buffers, cleans up internal data structures 9

6 Stream File Access Declared in <stdio.h> fgetc(), fgets(), fputc(), fputs(), fscanf(), fprintf(), fopen(), fclose(), Familar tools fread(), fwrite(), fseek(), ftell(), rewind(), fgetpos(), fsetpos() Not so familiar All take FILE * argument to identify the file. Note: if you seek to a position in a file and start writing, file is truncated at that point 10

7 Standard Streams FILE is typedef declared in <stdio.h> Whatever shell or window system connects to stream named stdin E.g., keyboard E.g., file redirection: command < filename scanf( ) is same as fscanf(stdin, ) FILE *stdin; stdin, stdout, stderr are extern variables declared in <stdio.h> FILE *stdout; Whatever shell or window system connects to stream named stdout E.g., window or screen E.g., file redirection: command > filename printf( ) is same as fprintf(stdout, ) Opened by OS before program starts FILE *stderr; Usually the window or screen; may be redirected to a file may be redirected to file using command 2> filename Redirections may be combined 11

8 Raw File Access See Kernighan & Ritchie, Chapter 8 Raw file access Without simplifying stream functions e.g., scanf, fscanf, printf, fprintf, fgetc, etc. read and write raw disk blocks Seek to a file position lseek, fseek sets file pointer to specified location Subsequent read, write, etc., start there ftell returns file pointer 12

9 Raw File Access (continued) See Chapter 8 for more details Streams are a layer of abstraction on top of raw access See K&R for example implementations 13

10 Questions? 17