FILE I/O
The last chapter
explained the standard input and output devices handled by C
programming language. This chapter covers how C programmers can create,
open, close text or binary files for their data storage.
A file represents a sequence of bytes, regardless of it being a text file or a
binary
file. C programming language provides access on high-level functions as well as
low-level (OS level) calls to handle file on your storage devices. This chapter
will
take you through the important calls for file management.
Opening
Files
You can use the fopen( ) function to create a new file or to open an
existing file.
This call will initialize an object of the type FILE, which contains all
the
information necessary to control the stream. The prototype of this function
call is
as follows:
FILE *fopen( const char * filename, const char * mode );
Here, filename is a string literal, which you will use to name your
file, and
access mode can have one of the following values:
Mode Description
r Opens an existing text file for reading purpose.
w Opens a text file for writing. If it does not exist, then a new file is
created. Here your program will start writing content from the
beginning of the file.
a Opens a text file for writing in appending mode. If it does not exist,
then a new file is created. Here your program will start appending
content in the existing file content.
r+ Opens a text file for both reading and writing.
w+ Opens a text file for both reading and writing. It first truncates the file
to zero length if it exists, otherwise creates a file if it does not exist.
22. FILE I/O
a+ Opens a text file for both reading and writing. It creates the file if it
does not exist. The reading will start from the beginning but writing
can only be appended.
If you are going to handle binary files, then you will use the following access
modes instead of the above-mentioned ones:
|
"rb", "wb", "ab",
"rb+", "r+b", "wb+", "w+b",
"ab+", "a+b"
|
Closing a File
To close a file, use the fclose( ) function. The prototype of this function
is:
The fclose() function
returns zero on success, or EOF if there is an error in
closing the file. This function actually flushes any data still pending in the
buffer
to the file, closes the file, and releases any memory used for the file. The
EOF is
a constant defined in the header file stdio.h.
There are various functions provided by C standard library to read and write a
file, character by character, or in the form of a fixed length string.
Writing
a File
Following is the simplest function
to write individual characters to a stream:
|
int fputc( int c, FILE *fp );
|
The function fputc() writes the character value of the argument c to the
output
stream referenced by fp. It returns the written character written on success
otherwise EOF if there is an error. You can use the following functions
to write a
null-terminated string to a stream:
|
int fputs( const char *s, FILE *fp );
|
The function fputs()
writes the string s to the output stream referenced by fp. It
returns a non-negative value on success, otherwise EOF is returned in
case of
any error. You can use int fprintf(FILE *fp,const char *format, ...) function
as well to write a string into a file. Try the following example.
Make sure you have /tmp directory available. If it is not, then before
proceeding, you must create this directory on your machine.
|
#include <stdio.h>
main()
{
FILE *fp;
fp = fopen("/tmp/test.txt", "w+");
fprintf(fp, "This is testing for fprintf...\n");
fputs("This is testing for fputs...\n", fp);
fclose(fp);
}
|
When the above code is compiled and executed, it creates a new file test.txt
in
/tmp directory and writes two lines using two different functions. Let us read
this
file in the next section.
Reading a File
Given below is the simplest function to read a single character from a file:
The fgetc() function
reads a character from the input file referenced by fp. The
return value is the character read, or in case of any error, it returns EOF.
The
following function allows to read a string from a stream:
|
char *fgets( char *buf, int n, FILE *fp );
|
The functions fgets() reads up to n - 1 characters from the input stream
referenced by fp. It copies the read string into the buffer buf,
appending
a null character to terminate the string.
If this function encounters a newline character '\n' or the end of the file EOF
before they have read the maximum number of characters, then it returns only
the characters read up to that point including the new line character. You can
also use int fscanf(FILE *fp, const char *format, ...) function to read
strings
from a file, but it stops reading after encountering the first space character.
|
#include <stdio.h>
main()
{
FILE *fp;
char buff[255];
fp = fopen("/tmp/test.txt", "r");
fscanf(fp, "%s", buff);
printf("1 : %s\n", buff );
fgets(buff, 255, (FILE*)fp);
printf("2: %s\n", buff );
fgets(buff, 255, (FILE*)fp);
printf("3: %s\n", buff );
fclose(fp);
}
|
When the above code is compiled and executed, it reads the file created in the
previous section and produces the following result:
|
1 : This
2: is testing for fprintf...
3: This is testing for fputs...
|
Let's see a little more in detail about what happened here. First, fscanf() reads
just This because after that, it encountered a space, second call is
for fgets() which reads the remaining line till it encountered end of
line. Finally,
the last call fgets() reads the second line completely.
Binary I/O Functions
There are two functions that can be used for binary input and output:
|
size_t fread(void *ptr, size_t size_of_elements,
size_t number_of_elements, FILE *a_file);
size_t fwrite(const void *ptr, size_t size_of_elements,
size_t number_of_elements, FILE *a_file);
|
Both of these functions should be used to read or write blocks of memories -
usually arrays or structures.
