File in C programming language, can be anything from a disk file to a device. C language provide support for opening a file, closing a file, reading data from a file and writing data to a file through a set of standard library functions.
C file handling is designed to work with a number of devices and file formats like printer, screen, keyboard, disk file etc. C file system transforms different devices into a abstract logical device called stream. All streams behaves same irrespective of devices and files. It provides an abstract consistent interface for C program to interact by hiding the details of the device specific implementations.
Disk file read function can be used to read data from keyboard as well.
- A Stream is a the sequence of bytes of data.
- All Streams behaves similarly. Same code can be used to perform I/O operations of various streams.
- A stream hides the low level device dependent complexities from C program.
- In a C program, all input and Output is done through streams.
- Three streams gets automatically attached when a program starts execution that is
- Standard Input Stream(stdin)
- Standard Output Stream(stdout)
- Standard Error Stream(stderr)
- Keyboard is associated with stdin stream.
- Screen is associated with stdout stream.
FILE Pointer in C
A file pointer is a pointer to a structure of type FILE. A file pointer is associated with stream and manages all Input and Output operations of that stream. The FILE structure contains information which is required to perform any file I/O operations like name of the file, current location of position indicator in file, data transfer bugger, status flag etc.
A file pointer is a bridge between c program and file/stream. We can declare a FILE pointer as follows:
- To perform any I/O operation on file, we need FILE pointer.
- FILE Structure is defined in stdio.h header file.
- FILE Structure contains necessary information about a stream, which is required to perform I/O operation in that stream.
- FILE pointer is an interface for our programs to interact with files.
Opening a File in C
The stdio.h library function fopen() is used to create a new file or open an exiting file.
FILE *fopen(const char *filename, const char *mode);
Function fopen() opens the file whose name is given in the filename argument and associate it with a stream and return a FILE pointer to be used in any future I/O operations on this stream. The fopen function opens a stream in a particular mode, which defines the operations that are allowed on the stream.
Various Modes of Opening a File| Mode | Description |
|---|---|
| "r" | To read a file. Opens a file for reading. The file must exist. |
| "w" | To write on a file. Creates an empty file for writing. If a file already exists with same name, its content is removed and the file is considered as a new empty file. |
| "a" | To append data at the end of file. The file is created if it does not exist. |
| "r+" | To read and write on an existing file. Opens a file to update both reading and writing. The file must exist. |
| "w+" | To create a new file for reading and writing. |
| "a+" | To read and append data on a file. |
C Program to Open a File using fopen Function
The following program shows the use of fopen function to open a text file in read mode.
#include <stdio.h>
int main(){
FILE *file;
int ch;
/* Open a file for reading */
file = fopen("textFile.txt","r");
if(file == NULL){
perror("Error: Unable to open a file");
} else {
while(!feof(file)){
ch = fgetc(file);
printf("%c", ch);
}
fclose(file);
}
return(0);
}
Output
fopen C Standard library function
Closing a File in C
The stdio.h library function fclose() is used to close a stream that was opened by fopen() function. Before closing a stream it flushes it's buffer.
int fclose(FILE *stream);
Function fclose closes a stream whose file pointer is passed as argument. If a call to fclose is successful it returns zero, otherwise EOF is returned.
C Program to Close a File using fclose Function
The following program shows the use of fclose function to close a file(stream) after writing a sentence in it.
#include <stdio.h>
int main (){
FILE * pFile;
/* Creates a new file */
pFile = fopen ("TextFile.txt","w");
/* Write a sentence in a file */
fprintf (pFile, "fclose C standard library function");
/* Close a file */
fclose (pFile);
return 0;
}
Output
fclose C standard library function
Reading from a File in C
The stdio.h standard library provides a number of functions to read data from a stream.
Functions to read a single character from a file| Function | Description |
|---|---|
| fgetc() | Reads a character from the given stream. |
| getc() | Reads a single character from given stream. |
| getchar() | Reads a single character from stdin stream. |
| Function | Description |
|---|---|
| fgets() | Reads a line from given stream and stores it into a character array. |
| gets() | Reads a line from stdin and stores it into given character array. |
| Function | Description |
|---|---|
| fscanf() | Read formatted data from given stream. |
| scanf() | Reads formatted data from stdin. |
C Program to read formatted data from a file using fscanf function
The following program shows the use of fscanf function to read formatted data from a stream.
#include <stdio.h>
int main()
{
char string[50];
int val;
float fval;
FILE *file;
file = fopen ("textFile.txt", "w+");
fprintf(file, "%d %f %s", 5, 5.5, "TechCrashCourse.com");
rewind(file);
fscanf(file, "%d %f %s", &val, &fval, string);
printf("Integer : %d\n", val);
printf("Floating point number : %f\n", fval);
printf("String : %s\n", string);
fclose(file);
return(0);
}
Output
Integer : 5 Floating point number : 5.500000 String : TechCrashCourse.com
Writing to a File in C
The stdio.h standard library provides a number of functions to write data on a stream.
Functions to write a single character on a file| Function | Description |
|---|---|
| fputc() | Writes a character to the given stream. |
| putc() | Writes a character to the given stream. |
| putchar() | Writes a character to stdout stream. |
| Function | Description |
|---|---|
| fputs() | Writes a string to the given stream excluding the null terminating character. |
| puts() | Writes a string to stdout stream excluding null terminating character. |
| Function | Description |
|---|---|
| fprintf() | Writes formatted output to a stream. |
| printf() | Print formatted data to stdout. |
| vfprintf() | Writes formatted data to a stream using an argument list. |
| vprintf() | Print formatted data to stdout using an argument list. |
| vsprintf() | Writes formatted data to a string using an argument list. |
C Program to write formatted data on a file
#include <stdio.h>
int main (){
int c, counter = 0;
FILE *file;
char *string = "fprintf C Standard Library function";
/* Create a temporary file */
file = fopen("textFile.txt", "w");
fprintf(file, "%s", string);
fclose(file);
/* Opening textFile.txt and printing it's content */
file = fopen("textFile.txt", "r");
while(!feof(file)){
c = fgetc(file);
printf("%c", c);
}
return(0);
}
Output
fprintf C Standard Library function
Standard Library function for File Handling in C
There are many file handling functions defined in stdio.h header file. Here is the list of frequently used file handling functions:
| Function | Description |
|---|---|
| clearerr() | Clears error indicators associated with a given stream. |
| fclose() | Closes the stream and flushes buffers associated with the given stream. |
| feof() | Checks the end-of-file indicator of the given stream. |
| ferror() | Checks the error indicator of the given stream. |
| fflush() | Flushes the content of the given stream. |
| fgetc() | Gets the next character from the given stream. |
| fgetpos() | Gets current position of the given stream. |
| fgets() | Reads a line from given stream and stores it into a character array. |
| fopen() | Opens a file in the given mode. |
| fprintf() | Writes formatted output to a stream. |
| fputc() | Writes a character to the given stream. |
| fputs() | Writes a string to the given stream excluding the null terminating character. |
| fread() | Reads data from the given stream and stores it into an array. |
| freopen() | Reopens a stream with different file or mode. |
| fscanf() | Read formatted data from given stream. |
| fseek() | Changes the position indicator of the given stream. |
| fsetpos() | Sets the position indicator of the given stream. |
| ftell() | Returns the current position of the given stream. |
| fwrite() | Writes data from an array to the given stream. |