Advertisement

---

Top

< Prev

In our last article, we have explained how to append new data to a file on the disk. In this tutorial, we are going to explain how to modify data of an existing file on disk. For this, the C language provides us a structure called FILE, which is defined in the header file <conio.h>. The FILE structure allows us to create, read, write/modify the file on disk.

The FILE structure provides us a function named fopen(), using which we could provide location of the file stored on the disk and in return(if the file is found), the fopen() function open the file and gives us a pointer which is pointed at either the first character position or the last character position in the file, depending on the mode in which we open this file. Let's read the file mode to use to modify the content of a file.

• Syntax of fopen() function




FILE *fopen(const char *filename, const char *mode)


where filename is the name of the file being searched and mode defines the mode in which the file is opened. As, we have to open a file for writing, hence this mode will take a string value "r+".



File Mode	Description
"r+"	Searches for the file and opens the file(if the file is found). If the file is not found, NULL is returned and no new file is created. Allows you to read, write and modify the content of file..


Using this mode, if this file does not exist on the disk then it will be created and if it already exists this function returns the pointer to the first character position in the file to allow us to read, modify its old content and to write new content.






• Modifying the content a file by using r+ mode


Let's say that we already have an existing file on disk, named File2.txt, with content :



Hello, how are you going? Have a pleasant day ahead!


Now let's try to write and append data to this file by taking input from an already initialized char[] array, This file will be searched in the current directory(the same directory where we are going to store the upcoming C program.).




#include<stdio.h>
#include<conio.h>
#include<stdlib.h>

int main()
{

/* This will look the file in the directory where this C program file is stored */
FILE *frplus = fopen("File2.txt", "r+"); /* Opens the file for reading/writing/modification */

char ch;

/* If the searched file is not found, the program exits */
if(frplus==NULL)
{
        printf("can not open target file\n");
        exit(1);
}

/* If the file is found */
while(1)
{
	ch = fgetc(frplus);
	if(ch=='Z')
	{
		printf("%c", ch); 
		fseek(frplus, -1, SEEK_CUR);
		fputc('e', frplus);
		fseek(frplus, 0, SEEK_CUR);	
	}
	if(ch == EOF)
		break;
}
fclose(frplus);

return 0;
}





Output


Executing this program will read the file named File2.txt in the current directory and modify its content, such as -



H1llo, how ar1 you going? Hav1 a pl1asant day ah1ad!






Advertisement






Program Analysis


As you may see in the output of the code, the program looks for a file named File2.txt in the current directory where this C program is stored and if the file is found, it reads its content and modify it. To do this -



• We have used called function fgetc() in an indefinite while loop. This function takes a FILE pointer and returns the char value in the first position pointed by this pointer and on its next call it returns the char value in the next position pointed by pointer and so on, until the EOF is reached.




• Next, we have called another library function fseek(), which sets the file position to the given offset. Let's see the declaration of fseek() function.
  
  

  int fseek(FILE * file, long int offset, int source)

  
  
  • file − pointer to a FILE object.
  • offset − number of bytes to offset from source.
  • source − the position from where offset is added.
  
  
  The source may take one of the following macro values :
  
  

  Macros	Description
  SEEK_SET	Beginning of file
  SEEK_CUR	Current position in the file
  SEEK_END	End of file

  
  
  Q: Why use fseek() function?
  
  The use of fseek() is of paramount importance when working with a file in r+ mode because using this mode we can not only read the file but also modify its old content and write some new content. Hence, when you switch between read, modify and writing mode, then you must use the file positioning function, fseek().
  
  
  
• Finally, just after we have concluded reading the file, we have used another function named fclose(), which closes the file being opened for reading and thereby frees the memory that was used for working with this file and flushes the buffer associated with this file.







Please share this article -







< Prev
< Append to a File



Advertisement