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11

C File Processing

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I read part of it all the way through.

Samuel Goldwyn

Hats off!

The flag is passing by.

Henry Holcomb Bennett

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Consciousness … does not appear to itself chopped up in bits. … A “river” or a “stream” are the metaphors by which it is most naturally described.

William James

I can only assume that a “Do Not File” document is filed in a “Do Not File” file.

Senator Frank Church

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OBJECTIVES

In this chapter you will learn:

To create, read, write and update files.
Sequential access file processing.
Random-access file processing.

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11.1 Introduction

11.2 Data Hierarchy

11.3 Files and Streams

11.4 Creating a Sequential-Access File

11.5 Reading Data from a Sequential-Access File

11.6 Random-Access Files

11.7 Creating a Random-Access File

11.8 Writing Data Randomly to a Random-Access File

11.9 Reading Data from a Random-Access File

11.10 Case Study: Transaction-Processing Program

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11.1 Introduction

Data files

Can be created, updated, and processed by C programs
Are used for permanent storage of large amounts of data

Storage of data in variables and arrays is only temporary

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11.2 Data Hierarchy

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 by a struct or a class
Example: In a payroll system, a record for a particular employee that contained his/her identification number, name, address, etc.

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11.2 Data Hierarchy

Data Hierarchy (continued):

File – group of related records

Example: payroll file

Database – group of related files

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Fig. 11.1 | Data hierarchy.

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11.2 Data Hierarchy

Data files

Record key

Identifies a record to facilitate the retrieval of specific records from a file

Sequential file

Records typically sorted by key

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11.3 Files and Streams

C views each file as a sequence of bytes

File ends with the end-of-file marker

Or, file ends at a specified byte

Stream created when a file is opened

Provide communication channel between files and programs
Opening a file returns a pointer to a FILE structure

Example file pointers:
stdin - standard input (keyboard)
stdout - standard output (screen)
stderr - standard error (screen)

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11.3 Files and Streams

FILE structure

File descriptor

Index into operating system array called the open file table

File Control Block (FCB)

Found in every array element, system uses it to administer the file

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Fig. 11.2 | C’s view of a file of n bytes.

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11.3 Files and Streams

Read/Write functions in standard library

fgetc

Reads one character from a file
Takes a FILE pointer as an argument
fgetc( stdin ) equivalent to getchar()

fputc

Writes one character to a file
Takes a FILE pointer and a character to write as an argument
fputc( 'a', stdout ) equivalent to putchar( 'a' )

fgets

Reads a line from a file

fputs

Writes a line to a file

fscanf / fprintf

File processing equivalents of scanf and printf

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11.4 Creating a Sequential-Access File

C imposes no file structure

No notion of records in a file
Programmer must provide file structure

Creating a File

FILE *cfPtr;

Creates a FILE pointer called cfPtr

cfPtr = fopen(“clients.dat", “w”);

Function fopen returns a FILE pointer to file specified
Takes two arguments – file to open and file open mode
If open fails, NULL returned

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11.4 Creating a Sequential-Access File

fprintf

Used to print to a file
Like printf, except first argument is a FILE pointer (pointer to the file you want to print in)

feof( FILE pointer )

Returns true if end-of-file indicator (no more data to process) is set for the specified file

fclose( FILE pointer )

Closes specified file
Performed automatically when program ends
Good practice to close files explicitly

Details

Programs may process no files, one file, or many files
Each file must have a unique name and should have its own pointer

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Outline

fig11_03.c

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FILE pointer definition creates new file pointer

fopen function opens a file; w argument means the file is opened for writing

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Outline

fig11_03.c

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feof returns true when end of file is reached

fprintf writes a string to a file

fclose closes a file

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Common Programming Error 11.1

Opening an existing file for writing ("w") when, in fact, the user wants to preserve the file, discards the contents of the file without warning.

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Common Programming Error 11.2

Forgetting to open a file before attempting�to reference it in a program is a logic error.

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Common Programming Error 11.3

Using the wrong file pointer to refer to a file is a logic error.

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Error-Prevention Tip 11.1

Be sure that calls to file processing functions�in a program contain the correct file pointers.

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Fig. 11.4 | End-of-file key combinations for various popular�operating systems.

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Good Programming Practice 11.1

Explicitly close each file as soon as it is known that the program will not reference the file again.

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Performance Tip 11.1

Closing a file can free resources for which other users or programs may be waiting.

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Common Programming Error 11.4

Opening a nonexistent file for reading is an error.

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Common Programming Error 11.5

Opening a file for reading or writing without having been granted the appropriate access rights to the file (this is operating-system dependent) is an error.

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Common Programming Error 11.6

Opening a file for writing when no disk space is available is an error.

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Fig. 11.5 | Relationship between FILE pointers, FILE structures and FCBs.

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Common Programming Error 11.7

Opening a file with the incorrect file mode is�a logic error. For example, opening a file in write mode ("w") when it should be opened in update mode ("r+") causes the contents of the file to be discarded.

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Fig. 11.6 | File opening modes.

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Error-Prevention Tip 11.2

Open a file only for reading (and not update) if the contents of the file should not be modified. This prevents unintentional modification of the file’s contents. This is another example of the principle of least privilege.

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11.5 Reading Data from a Sequential-Access File

Reading a sequential access file

Create a FILE pointer, link it to the file to read

cfPtr = fopen( “clients.dat", "r" );

Use fscanf to read from the file

Like scanf, except first argument is a FILE pointer

fscanf( cfPtr, "%d%s%f", &accounnt, name, &balance );

Data read from beginning to end
File position pointer

Indicates number of next byte to be read / written
Not really a pointer, but an integer value (specifies byte location)
Also called byte offset

rewind( cfPtr )

Repositions file position pointer to beginning of file (byte 0)

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Outline

fig11_07.c

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fopen function opens a file; r argument means the file is opened for reading

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Outline

fig11_07.c

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fscanf function reads a string from a file

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Outline

fig11_08.c

(1 of 4 )

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Outline

fig11_08.c

(2 of 4 )

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Outline

fig11_08.c

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Outline

fig11_08.c

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rewind function moves the file pointer back to the beginning of the file

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Outline

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11.5 Reading Data from a Sequential-Access File

Sequential access file

Cannot be modified without the risk of destroying other data
Fields can vary in size

Different representation in files and screen than internal representation
1, 34, -890 are all ints, but have different sizes on disk

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11.6 Random-Access Files

Random access files

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

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Fig. 11.10 | C’s view of a random-access file.

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11.7 Creating a Random-Access File

Data in random access files

Unformatted (stored as "raw bytes")

All data of the same type (ints, for example) uses the same amount of memory
All records of the same type have a fixed length
Data not human readable

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11.7 Creating a Random-Access File

Unformatted I/O functions

fwrite

Transfer bytes from a location in memory to a file

fread

Transfer bytes from a file to a location in memory

Example:

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

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11.7 Creating a Random-Access File

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

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Outline

fig11_11.c

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Outline

fig11_11.c

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fopen function opens a file; wb argument means the file is opened for writing in binary mode

fwrite transfers bytes into a random-access file

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11.8 Writing Data Randomly to a Random-Access File

fseek

Sets file position pointer to a specific position
fseek( pointer, offset, symbolic_constant );

pointer – 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

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Outline

fig11_12.c

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Outline

fig11_12.c

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fseek searches for a specific location in the random-access file

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Outline

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Fig. 11.14 | File position pointer indicating an offset of 5 bytes from�the beginning of the file.

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11.9 Reading Data from a Random-Access File

fread

Reads a specified number of bytes from a file into memory

fread( &client, sizeof (struct clientData), 1, myPtr );

Can read several fixed-size array elements

Provide pointer to array
Indicate number of elements to read

To read multiple elements, specify in third argument

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Outline

fig11_16.c

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Outline

fig11_16.c

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