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Writing to certain position in a file

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Writing to certain position in a file

Jun 29th, 2006

Hello
I am trying to write some data to a certain position in a file, but cannot get it to work
I want to write "XX" to position 6 of a specified file, but the data is just being written to the end of file.
see below

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include<stdlib.h> 
 
main() 
 
{ 
FILE *stream; 
char buffer[20]="XX"; 
fpos_t file_pos; 
 
stream = fopen("ste.txt", "a+" ); 
 
fseek(stream, 10, SEEK_SET ); 
 
fgetpos(stream, &file_pos ); 
fsetpos(stream, &file_pos ); 
printf("File Pos :%ld\n", file_pos ); 
 
fwrite(&buffer,1,file_pos,stream ); 
 
fclose(fp); 
 
exit(1); 
}
#include<stdlib.h> 

main() 

{ 
FILE *stream; 
char buffer[20]="XX"; 
fpos_t file_pos; 

stream = fopen("ste.txt", "a+" ); 

fseek(stream, 10, SEEK_SET ); 

fgetpos(stream, &file_pos ); 
fsetpos(stream, &file_pos ); 
printf("File Pos :%ld\n", file_pos ); 

fwrite(&buffer,1,file_pos,stream ); 

fclose(fp); 

exit(1); 
}

Last edited by Dave Sinkula; Jun 29th, 2006 at 10:04 am.

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Re: Writing to certain position in a file

Jun 29th, 2006

The "a" mode of fopen always appends, regardless of any seeking you do afterward. You want to open the file with "w+b" for write/update access in binary. You have to use binary because text doesn't allow arbitrary seeking like you're trying to do.

You may also want to read up on how file streams work in C...

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Re: Writing to certain position in a file

Jun 29th, 2006

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Originally Posted by Narue

The "a" mode of fopen always appends, regardless of any seeking you do afterward. You want to open the file with "w+b" for write/update access in binary. You have to use binary because text doesn't allow arbitrary seeking like you're trying to do.

You may also want to read up on how file streams work in C...

the file i am updating already exists, with data, when i try the w+b option, the file is nulled

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Re: Writing to certain position in a file

Jun 29th, 2006

My brain hasn't been working to well lately, sorry about that. :p You want "r+b" because the "w" mode always empties the file if it already exists.

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Re: Writing to certain position in a file

Jun 29th, 2006

I thanks

I have tried that but its putting the data to the end of the file, and not the position i want

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Re: Writing to certain position in a file

Jun 29th, 2006

Then post your current code, the file you're using, and tell use what compiler and OS you're using.

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Re: Writing to certain position in a file

Jun 29th, 2006

I don't do much in the way of I/O using C. However, this reference:

http://www.cppreference.com/

indicates that

"rb+"

opens a binary file for read/write operations.

Good luck.

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Re: Writing to certain position in a file

Jun 29th, 2006

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Originally Posted by Narue

Then post your current code, the file you're using, and tell use what compiler and OS you're using.

SunOS jcmunx3 5.9 Generic_118558-23 sun4u sparc SUNW,Sun-Fire-V240

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include<stdio.h>     
#include<stdlib.h>    
 
main()                
 
 
{                                          
 
   FILE *fp;                               
   char buffer[]="STEPHEN";                
   fpos_t file_pos;                        
   char c;                                 
 
   fp = fopen("ste.txt", "rb+" );          
 
   fseek(fp, 20, SEEK_SET );               
 
   fgetpos(fp, &file_pos );                
 
   printf("File Pos :%ld\n", file_pos );   
 
   c = getc(fp);                           
   printf("Character is : %c\n", c );      
 
   fwrite(buffer,sizeof(buffer),1,fp);     
 
   fclose(fp);                             
 
}
#include<stdio.h>     
#include<stdlib.h>    
                      
main()                
                                       
 
{                                          
                                           
   FILE *fp;                               
   char buffer[]="STEPHEN";                
   fpos_t file_pos;                        
   char c;                                 
                                           
   fp = fopen("ste.txt", "rb+" );          
                                           
   fseek(fp, 20, SEEK_SET );               
                                           
   fgetpos(fp, &file_pos );                
                                           
   printf("File Pos :%ld\n", file_pos );   
                                           
   c = getc(fp);                           
   printf("Character is : %c\n", c );      
                                           
   fwrite(buffer,sizeof(buffer),1,fp);     
                                           
   fclose(fp);                             
                                           
}

Last edited by Dave Sinkula; Jun 29th, 2006 at 10:35 am.

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ste.txt (924 Bytes, 5 views)

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Re: Writing to certain position in a file

Jun 29th, 2006

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include<stdio.h>
#include<stdlib.h>
 
main()
 
 
{
 
FILE *fp;
char buffer[]="STEPHEN";
fpos_t file_pos;
char c;
 
fp = fopen("ste.txt", "rb+" );
 
fseek(fp, 18, SEEK_SET );
fwrite(buffer, sizeof(buffer), 1, fp);
 
fgetpos(fp, &file_pos );
 
printf("File Pos :%ld\n", file_pos );
 
c = getc(fp);
printf("Character is : %c\n", c );
 
 
fclose(fp);
#include<stdio.h>
#include<stdlib.h>

main()


{

FILE *fp;
char buffer[]="STEPHEN";
fpos_t file_pos;
char c;

fp = fopen("ste.txt", "rb+" );

fseek(fp, 18, SEEK_SET );
fwrite(buffer, sizeof(buffer), 1, fp);

fgetpos(fp, &file_pos );

printf("File Pos :%ld\n", file_pos );

c = getc(fp);
printf("Character is : %c\n", c );


fclose(fp);

Last edited by andor; Jun 29th, 2006 at 10:38 am.

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Re: Writing to certain position in a file

#10

Jun 29th, 2006

http://c-faq.com/stdio/fupdate.html

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