From mboxrd@z Thu Jan  1 00:00:00 1970
From: LDB <thesource@ldb-jab.org>
Subject: Re: How to lseek the larger file > 2GB under linux
Date: Sun, 17 May 2009 14:10:50 -0400
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To: Nicle <ynicle@gmail.com>
Cc: linux-c-programming@vger.kernel.org

Nicle wrote:
> Hi all,
> 
>  I have a file > 2GB, and my job is seeking the file to pos: 2.1G.
> But, it seems that the lseek64 doesn't work.
> Here is the sample code:
> 
> #define _LARGEFILE64_SOURCE
> #include ...
> 
> int main()
> {
>  int fd = -1;
>  long long pos = (long long) 2*1024*1024*1024 + 10; // over 2G
> 
>  fd = open(FILENAME, O_WRONLY|O_LARGEFILE);
>  if (fd < 0){
>    ...
>  }
> 
>  if (lseek64(fd, pos, SEEK_SET) < 0) {
>      fprintf(stderr, "Failed seeking to %lld, %s\n", pos, strerror(errno));
>  }
> 
>  return 0;
> }
> 
>  Then the building cmd:
>     gcc -o test test.c -D_FILE_OFFSET_BITS=64
> 
>  Output:
>     Failed seeking to 2147483658, Success.
> 
> The return val of lseek64  was "<0", but the strerror told me "Success".
> --
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> 
> 



I changed the below a little, but when I seeked to 2.1GB,
it worked fine. I am not writing like you, but reading.

#define _LARGEFILE64_SOURCE

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/stat.h>
#include <fcntl.h>

int main()
{
 int fd, ret;
 size_t count = 200;
 long long pos = (long long) 2*1024*1024*1024 + 10; // over 2G
 char buf[count];

 fd = open("/tmp/fish", O_RDONLY|O_LARGEFILE);
 if (fd < 0){
   fprintf(stderr, "%s\n", strerror(errno));
   exit(1);
 }

 if (lseek64(fd, pos, SEEK_SET) < 0) {
   fprintf(stderr, "Failed seeking to %lld, %s\n", pos, strerror(errno));
   exit(1);
 }

 if ((ret = read(fd, buf, count)) < 0) {
   fprintf(stderr, "Failed reading: %s\n", strerror(errno));
   exit(1);
 }

 buf[ret] = 0x00;
 printf("%s\n", buf);

 close(fd);

 return 0;
}