C File Processing

Objectives

• To be able to create, write and update files.

• To become familiar with sequential access file processing.

• To become familiar with random-access file processing.

Date Hierarchy

Tom Green

Judy Smith

Stan Miller

Randy Walter

File

Randy Walter Record

Randy Field

01010010 Byte (ASCII Character R)

1 Bit

C’s View of a File of n-bytes

0 1 2 3 4 5 n-1 eof

Each file will be accompanied with three streams when being executed.They are: stadin, stdout and stderr.

Standard library then provides many functions for reading data from files and writing data to files. Such as: fgets read one char from a file. ( fgetc(stdin) then get from the Standard input=getchar ( ) )

Similarly, fscanf and fprintf will handle file input and output.

Creating a Sequential File

• C impose no structure on a file.

• Thus, notions like a record of a file do not exist as part of the C language.

• The programmer must provide any file structure to meet the requirements of each particular application.

• Example: week10 cfile.c

File Open Modes

• Mode Description• r open a file for reading w open a file for writing a Append r+ open a file for update (r & w) w+ Create a file for update a+ Append; open or create a file for

update; writing is done at the end of the file. ** w and w+ will discard the existing file.

Open an Existing File

• Example: /week10/copen.c

• fscanf( cfPtr, “%d%s%lf”, &account, name, &balance);

More Example

• Credit inquiry program.

• week10 credit.c

• rewind ( cfPtr)

Random-Access Files

• Normally, they have fixed –length records for individual records.

• May be accessed directly (and thus quickly) without searching through other records.

• Main applications:• Airline reservation system, banking system,

point-of-sale system and other kinds of transaction processing systems that require rapid access to specific data.

Functions Used in Such Files

• 1. fwrite – transfers a specified number of bytes beginning at a specified location in memory to a file.

• fwrite ( &number, sizeof( int ), 1, fPtr )

• Will write 4-bytes from variable number to the file represented by fPtr.

• 2. fread --

Example

• Create a credit processing system capable of storing up to 100 fixed-length records. Each record should consists of an account number that will be used as the record key, a last name, a first name and a balance.

• The first program—create the random file.

• c-RandF.c

Program Remark

• The statement of fwrite( &blankClient, sizeof(struct clientsData), 1,

cfPtr );Causes the structure blankClient of sizeof( struct

clientsData ) to be written to the file pointed by cfPtr.

*** sizeof is a compile-time unary operator that returns an unsigned integer.

*** sizeof is not a function and it will not generate the execution-time overhead of a function call.