physical

logical

- may be very different

 

- usually thought of as being in one piece

- can be anything (ASCII, pictures, records, music)

- sub-structure imposed by software that is uses

 

- organization of actual bytes on storage medium may be quite different from logical organization

- It's important to remember the distinction between the physical files and devices and the logical structure of the same thing.

To access: (every language has some way to accomplish these:)

• need to establish connection between logical and physical file
• need to 'announce' our intentions (read/write/new/append/etc.)
• sometimes, need to position the file pointer
• do the read & write operations
• close the file

So, what are the fundamental operations done on files?

Basic File Operations in C: (see text for C++ routines)

FILE structure defined in stdio ; keeps current position, end of file location, location in memory of associated buffer, etc.

 

opening files :

FILE *fopen(const char *file_id, const char *mode)

file_id : pathname of file

mode : describes how file will be used

"r" = read; "w" = write; "a" = append (create file if it doesn't exist); "+" after mode means read and write but must execute fseek before switching from read to write or vice-versa.

Returns pointer to file structure that holds file description

 

closing files :

int fclose(FILE *file);

file : pointer to file structure

Returns 0 if successful; EOF if not

seeking :

int fseek(FILE *file, long offset, int origin)

file : pointer to file descriptor

offset : longint = # of bytes to move from origin, can be negative

origin : SEEK_SET (0) = from beginning of file

SEEK_CUR (1) = from current position

SEEK_END (2) = from end of file

WARNING: C doesn't stop you from positioning the pointer beyond the end or beginning of the file.

Returns 0 if OK, non-zero if not.

special characters: ^z; CR; LF

^z : typical end-of-file marker in DOS

UNIX just keeps track of file size and current position

CR/LF : end-of-line pair

UNIX uses NL (ASCII 10)

 

reading:

size_t fread (const void *location, size_t bytes, size_t items, FILE *file)

location : where in memory to put stuff read

bytes : # of bytes in each item

items : # of items to read

file : pointer to file descriptor

Returns # items or partial items actually read.

WARNING: make sure you have enough space at location; C doesn't care.

 

writing:

size_t fwrite (const void *location, size_t bytes, size_t items, FILE *file)

location : where in memory to get stuff to write

bytes : # of bytes per item

items : # of items to write

file : pointer to file descriptor

Returns # items or partial items actually written.

• UNIX directory structure:
  • physical VS logical files
    • again how we see directory vs. how UNIX sees it
    • to UNIX each file is merely a sequence of bytes
  • STDIN, STDOUT, STDERR as files
  • I/O redirection and pipes

- basically sets up switches between STDIN, STDOUT, and regular files