File Input Output in C Programming

File Handling in C

File in C programming language, can be anything from a disk file to a device like. 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.

Points to Remember about Streams
  • 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:

FILE *file;
Facts about FILE Structure
  • 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);
}

Program 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;
}

Program 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.
Functions to Read a String from a File
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.
Functions to Read Formatted Data from a File
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);
}

Program 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.
Functions to write a string on a file
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.
Functions to write formatted data on a file
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);
}

Program 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.