In C language, use the fopen() function to open an existing file in write mode

The fopen() method in C is used to open the specified file.

Let’s take an example to understand the problem

Syntax

FILE *fopen(filename, mode)

The following are valid modes for opening files using fopen(): 'r', 'w', 'a', 'r ', 'w ', 'a '. For more information please visit C library functions - fopen()

fopen() to open an existing file in write mode

If the file to be opened does not exist in the current directory, it will be created New empty file, using write mode.

If the file to be opened exists in the current directory and is opened using 'w' / 'w ', the contents will be deleted before writing.

Example

Program example illustrating how our solution works

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

int main(){

   FILE *opFile = fopen("test.txt", "w");
   if (opFile == NULL){

      puts("Couldn&#39;t open file");
      exit(0);
   }
   else{

      fputs("includehelp", opFile);
      puts("Write operation successful");
      fclose(opFile);
   }
   return 0;
}

Output

Write operation successful

Initial contents of the file - C Programming Language

Contents after append operation - Includes help

The write operation will do its job but delete any content that was present in the file before the write operation was performed . To solve this problem, the C programming language has been updated with two different methods that programmers can use depending on the requirements of the program.

• 'a' (append) mode - This mode appends new content to the end of what has already been written in the file.

• 'wx' mode - Returns NULL if the file already exists in the directory.

Example

A program that demonstrates writing to an existing file using 'a' mode

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

int main(){

   FILE *opFile = fopen("test.txt", "a");
   if (opFile == NULL){

      puts("Couldn&#39;t open file");
      exit(0);
   }
   else{

      fputs("includehelp", opFile);
      puts("Write operation successful");
      fclose(opFile);
   }
   return 0;
}

Output

Write operation successful

File Initial content − C Programming Language

Content after append operation− C Programming Language includehelp

Example

Use ' Program for writing on existing files in wx' mode

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

int main(){

   FILE *opFile = fopen("test.txt", "wx");
   if (opFile == NULL){

      puts("Couldn&#39;t open file");
      exit(0);
   }
   else{

      fputs("includehelp", opFile);
      puts("Write operation successful");
      fclose(opFile);
   }
   return 0;
}

Output

Write operation successful

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