Abstractly, a file is a collection of bytes stored on a secondary storage device, which is generally a disk of some kind. The collection of bytes may be interpreted, for example, as characters, words, lines, paragraphs and pages from a textual document; fields and records belonging to a database; or pixels from a graphical image. The meaning attached to a particular file is determined entirely by the data structures and operations used by a program to process the file. It is conceivable (and it sometimes happens) that a graphics file will be read and displayed by a program designed to process textual data. The result is that no meaningful output occurs (probably) and this is to be expected. A file is simply a machine decipherable storage media where programs and data are stored for machine usage.
Essentially there are two kinds of files that programmers deal with text files and binary files.
ASCII Text files :
A text file can be a stream of characters that a computer can process sequentially. It is not only processed sequentially but only in forward direction. For this reason a text file is usually opened for only one kind of operation (reading, writing, or appending) at any given time.
Similarly, since text files only process characters, they can only read or write data one character at a time. (In C Programming Language, Functions are provided that deal with lines of text, but these still essentially process data one character at a time.) A text stream in C is a special kind of file. Depending on the requirements of the operating system, newline characters may be converted to or from carriage-return/linefeed combinations depending on whether data is being written to, or read from, the file. Other character conversions may also occur to satisfy the storage requirements of the operating system. These translations occur transparently and they occur because the programmer has signalled the intention to process a text file.
Binary files :
A binary file is no different to a text file. It is a collection of bytes. In C Programming Language a byte and a character are equivalent. Hence a binary file is also referred to as a character stream, but there are two essential differences.
1. No special processing of the data occurs and each byte of data is transferred to or from the disk unprocessed.
2. C Programming Language places no constructs on the file, and it may be read from, or written to, in any manner chosen by the programmer.
Binary files can be either processed sequentially or, depending on the needs of the application, they can be processed using random access techniques. In C Programming Language, processing a file using random access techniques involves moving the current file position to an appropriate place in the file before reading or writing data. This indicates a second characteristic of binary files.
They a generally processed using read and write operations simultaneously.
For example, a database file will be created and processed as a binary file. A record update operation will involve locating the appropriate record, reading the record into memory, modifying it in some way, and finally writing the record back to disk at its appropriate location in the file. These kinds of operations are common to many binary files, but are rarely found in applications that process text files.
C supports a number of functions that have the ability to perform basic file operations, which include:
1. Naming a file
2. Opening a file
3. Reading from a file
4. Writing data into a file
5. Closing a file
Real life situations involve large volume of data and in such cases, the console oriented I/O operations pose two major problems. It becomes cumbersome and time consuming to handle large volumes of data through terminals. The entire data is lost when either the program is terminated or computer is turned off therefore it is necessary to have more flexible approach where data can be stored on the disks and read whenever necessary, without destroying the data. This method employs the concept of files to store data.
File operation functions in C:
Function Name Operation
fopen() Creates a new file for use
Opens a new existing file for use
fclose() Closes a file which has been opened for use
getc() Reads a character from a file
putc() Writes a character to a file
fprintf() Writes a set of data values to a file
fscanf() Reads a set of data values from a file
getw() Reads a integer from a file
putw() Writes an integer to the file
fseek() Sets the position to a desired point in the file
ftell() Gives the current position in the file
rewind() Sets the position to the begining of the file
1. Defining and opening a file :
If we want to store data in a file into the secondary memory, we must specify certain things about the file to the operating system. They include the fielname, data structure, purpose.
The general format of the function used for opening a file is
FILE *fp;
fp=fopen(“filename”,”mode”);
The first statement declares the variable fp as a pointer to the data type FILE. As stated earlier, File is a structure that is defined in the I/O Library. The second statement opens the file named filename and assigns an identifier to the FILE type pointer fp. This pointer, which contains all the information about the file, is subsequently used as a communication link between the system and the program.
The second statement also specifies the purpose of opening the file. The mode does this job.
R open the file for read only.
W open the file for writing only.
A open the file for appending data to it.
Consider the following statements:
FILE *p1, *p2;
p1=fopen(“data”,”r”);
p2=fopen(“results”,”w”);
In these statements the p1 and p2 are created and assigned to open the files data and results respectively the file data is opened for reading and result is opened for writing. In case the results file already exists, its contents are deleted and the files are opened as a new file. If data file does not exist error will occur.
2. Closing a file :
The input output library supports the function to close a file; it is in the following format.
fclose(file_pointer);
A file must be closed as soon as all operations on it have been completed. This would close the file associated with the file pointer.
Observe the following program.
….
FILE *p1 *p2;
p1=fopen (“Input”,”w”);
p2=fopen (“Output”,”r”);
….
…
fclose(p1);
fclose(p2)
The above program opens two files and closes them after all operations on them are completed, once a file is closed its file pointer can be reversed on other file.
The getc and putc functions are analogous to getchar and putchar functions and handle one character at a time. The putc function writes the character contained in character variable c to the file associated with the pointer fp1. ex putc(c,fp1); similarly getc function is used to read a character from a file that has been open in read mode. c=getc(fp2).
The getw and putw functions :
These are integer-oriented functions. They are similar to get c and putc functions and are used to read and write integer values. These functions would be usefull when we deal with only integer data. The general forms of getw and putw are:
putw(integer,fp);
getw(fp);
The fprintf & fscanf functions :
The fprintf and fscanf functions are identical to printf and scanf functions except that they work on files. The first argument of theses functions is a file pointer which specifies the file to be used. The general form of fprintf is
fprintf(fp,”control string”, list);
Where fp id a file pointer associated with a file that has been opened for writing. The control string is file output specifications list may include variable, constant and string.
fprintf(f1,%s%d%f”,name,age,7.
Here name is an array variable of type char and age is an int variable
The general format of fscanf is
fscanf(fp,”controlstring”,
This statement would cause the reading of items in the control string.
Random access to files :
Sometimes it is required to access only a particular part of the and not the complete file. This can be accomplished by using the following function:
1 > fseek
fseek function :
The general format of fseek function is a s follows:
fseek(file pointer,offset, position);
This function is used to move the file position to a desired location within the file. Fileptr
is a pointer to the file concerned. Offset is a number or variable of type long, and position in an integer number. Offset specifies the number of positions (bytes) to be moved from the location specified bt the position. The position can take the 3 values.
Value Meaning
0 Beginning of the file
1 Current position
2 End of the file.
C.
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Preeti Bagad [BE(CS)]
SW Engineer Cum Blogger
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