sendfile64 only copy about 2GB

I need to use sendfile64 to copy about 16GB of files. What I have achieved so far is

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

int main (int argc, char** argv)
{
  long long src;
  long long dest;
  struct stat64 stat_buf;
  off64_t offset = 0LL;
  long long rc;

  if (argc != 3) {
    fprintf(stderr, "usage: %s <source> <destination>\n", argv[0]);
    exit(1);
  }

  src = open64(argv[1], O_RDONLY);
  if (src == -1) {
    fprintf(stderr, "unable to open '%s': %s\n", argv[1], strerror(errno));
    exit(1);
  }

  fstat64(src, &stat_buf);

  dest = open64(argv[2], O_WRONLY|O_CREAT, stat_buf.st_mode);
  if (dest == -1) {
    fprintf(stderr, "unable to open '%s': %s\n", argv[2], strerror(errno));
    exit(1);
  }

 /* copy file using sendfile */
 rc = sendfile64 (dest, src, &offset, stat_buf.st_size);
 if (rc == -1) {
    fprintf(stderr, "error from sendfile: %s\n", strerror(errno));
    exit(1);
 }
 if (rc != stat_buf.st_size) {
   fprintf(stderr, "incomplete transfer from sendfile: %lld of %lld bytes\n",
           rc,
           (long long)stat_buf.st_size);
   exit(1);
 }

 /* clean up and exit */
 close(dest);
 close(src);

 return 0;
}

I have compiled using

g++ BigCopy2.cpp -o BigCopy2 -D_FILE_OFFSET_BITS=64 -DLARGEFILE64_SOURCE

The problem is I still can't copy more than 2GB of file.

Can someone point me where is my mistake?

You should use a loop to copy it all, sendfile() might, for various reasons, not copy all the data with one call. As janneb points out, the return value of sendfile64 is a ssize_t, so we should not pass in more than SSIZE_MAX to sendfile, moreover the last argument to sendfile is a size_t which would be 32 bit on 32 bit platforms.

 /* copy file using sendfile */
while (offset < stat_buf.st_size) {
  size_t count;
  off64_t remaining = stat_buf.st_size- offset;
  if (remaining > SSIZE_MAX)
      count = SSIZE_MAX;
   else 
      count = remaining;
  rc = sendfile64 (dest, src, &offset, count);
  if (rc == 0) {
     break;
  }
  if (rc == -1) {
     fprintf(stderr, "error from sendfile: %s\n", strerror(errno));
     exit(1);
  }
}

 if (offset != stat_buf.st_size) {
   fprintf(stderr, "incomplete transfer from sendfile: %lld of %lld bytes\n",
           rc,
           (long long)stat_buf.st_size);
   exit(1);
 }

Note that you can replace all your 64 bit variants, off64_t, stat64, sendfile64, with off_t, stat, sendfile. As long as you have the -D_FILE_OFFSET_BITS=64 flag, that define will do the right thing and transform off_t to off64_t, sendfile to sendfile64 and so on if those types and functions are not already 64 bits (such as on 32 bit architectures).