1. Accessing Files in C
Professor Hugh C. Lauer CS-2303, System Programming Concepts
(Slides include materials from The C Programming Language, 2nd 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, 5th and 6th editions, by Deitel and Deitel)
CS-2303, A-Term 2012
Files in C

2. Two Kinds of File Access
Stream
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
Raw
File is a sequence of blocks
Any block can be read and/or written independently
CS-2303, A-Term 2012
Files in C

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!
CS-2303, A-Term 2012
Files in C 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
CS-2303, A-Term 2012
Files in C 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
CS-2303, A-Term 2012
Files in C

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
Note:– if you seek to a position in a file and start writing, file is truncated at that point
All take FILE * argument to identify the file.
CS-2303, A-Term 2012
Files in C

7. Standard Streams FILE *stdin; FILE *stdout; FILE *stderr;
stdin, stdout, stderr are extern variables declared in <stdio.h>
FILE is typedef declared in <stdio.h>
FILE *stdin;
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 *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, …)
FILE *stderr;
Usually the window or screen; may be redirected to a file
may be redirected to file using command 2> filename
Opened by OS before program starts
Redirections may be combined
CS-2303, A-Term 2012
Files in C

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
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Files in C 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 §8.1–8.6 for example implementations
CS-2303, A-Term 2012
Files in C

10. Questions?
CS-2303, A-Term 2012
Files in C