Re: Reading and writing to/from a file in C++

[Mihail Mihaylov]

Hi,

Right now I experience some difficulties using int write( int, char*, int );
In the simple program that I have attached I first move to a location within
a file using long lseek(int, long, int) and then I write some dummy data 
using the int write( int, char*, int ) function. 
I am doing that under Solaris, and for unknown reason the dummy string 
that I am trying to write goes not at the pre-set location, but instead 
goes to the very beginning of the file. The file size though is updatedA 
correctly, if I move to a location that is currently outside the file 
range.

Thanks,
mich


On Thu, 19 Nov 1998, Glynn Clements wrote:

> 
> Mihail Mihaylov wrote:
> 
> > Is there a way in C++ ( using the iostream.h ) or in C ( using the 
> > stdio.h and the like ) to control precisely how 
> > data gets saved to a file. What I mean by this is, can I control exactly 
> > at which offset within the file my data blocks will go.
> 
> Sure. Use fseek() to set the offset before writing the data.
> 
> If you're not writing any data sequentially, it's more efficient to
> either use the Unix functions (open, lseek, write, close), or to
> disable buffering with e.g. setbuf(fp, NULL).
> 
> -- 
> Glynn Clements <[EMAIL PROTECTED]>
> 

#include <sys/file.h>
#include <sys/fcntl.h>
#include <fcntl.h>
#include <iostream.h>
#include <unistd.h>



int main( void ) 
{
  int fd ;
  long location ;
  long offset = 512;
  

  if ( ( fd = open( "save.txt",  O_WRONLY | O_CREAT,
                     0644 )  ) < 0 ) {
    cerr<<"ClientGlobals; Error openening file: " << "save.txt" << endl;
    exit( 1 );
  }
  
        
  char buf[17]; 
  lseek( fd, offset, L_SET);
  write( fd, "string1", 7 );

  close( fd );
 

  return 0;
}