Help replacing words in a file

I'm trying to write a program that will open an existing file supplied by the command line argument and then replace words with "We" or "we" by "I" and "a" or "A" by "The". When I run the program it reads the file, changes the word but re writes it on a new line with only the replaced words not the whole sentence. Would anyone be able to help me?

Code:

---


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

#define BUFFERSIZE      4096

 main(int ac, char *av[])
{
        int    in_fd, n_chars;
        char    buf[BUFFERSIZE];
        char x;
                                        /* check args  */
        if ( ac != 2 ){
                fprintf( stderr, "usage: %s source destination\n", *av);
                exit(1);
        }
                                                /* open files  */

        if ( (in_fd=open(av[1], O_RDWR)) == -1 )
                printf("Cannot open ", av[1]);


        while ( (n_chars = read(in_fd , buf, BUFFERSIZE)) > 0 ){


        for  (x=0; x<n_chars; x++)
        {


                if (buf[x] ==  'W') if (buf[x+1] == 'e') if (buf[x+2]=' ')
                        {
                                lseek(in_fd, 0, SEEK_CUR);
                                write(in_fd, "I", strlen("I"));
                        }
                if (buf[x] == 'w') if (buf[x+1] == 'e') if (buf[x+2]=' ')
                        {
                                lseek(in_fd, 0, SEEK_CUR);
                                write(in_fd, "I", strlen("I"));
                        }
                if (buf[x] == 'A') if (buf[x+1] == ' ')
                        {
                                lseek(in_fd, 0, SEEK_CUR);
                                write(in_fd, "The", strlen("The"));
                        }
                if (buf[x] == 'a') if (buf[x+2] == ' ')
                        {
                                lseek(in_fd, 0, SEEK_CUR);
                                write(in_fd, "the", strlen("the"));
                        }       
                write( in_fd, &buf[x], 1 );                       
        }

        }
        if ( n_chars == -1 )
                printf("Read error from ", av[1]);
        if ( close(in_fd) == -1)
                printf("Error closing files","");




}

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> if ( (in_fd=open(av[1], O_RDWR))
Open two different files, one for input, one for output.
You're NEVER going to do an in-file search/replace for words which are of different lengths.

Here,

Code:

---

#include <stdio.h>
int main ( ) {
  char buff[BUFSIZ];
  FILE *in, *out;
  in = fopen( "file-in.txt", "r" );
  out= fopen( "file-out.txt", "w" );
  while ( fgets( buff, sizeof buff, in ) != NULL ) {
    fputs ( buff, out );
  }
  fclose( in );
  fclose( out );
  return 0;
}

---

This code simply makes a copy of the file.
Just before the fputs() call, add your code to perform all the substitutions.