Re: [Boost-users] Can a mapped_file larger than 2GB be random seeked?

Subject: Re: [Boost-users] Can a mapped_file larger than 2GB be random seeked?
From: james (dirtydroog_at_[hidden])
Date: 2015-05-05 03:47:07

In the original code you're seeking to FILESIZE - 1, but in this code
you're seeking to FILESIZE - 2

On Tuesday, May 5, 2015, Narcélio Filho <narcelio_at_[hidden]> wrote:

> Hi Michael!
>
> >> I'm compiling under Ubuntu 14.10 (amd64):
> >>
> >> $ g++ boost-mmap.cpp -lboost_iostreams
> >> $ ./a.out
> >> 0
> >> 2147493647
> >> Segmentation fault (core dumped)
> >>
> >> Everything is 64-bits:
>
> > You are certain that 'everything' is 64-bits? Without diving too deep
> > into any code, this sounds vaguely, suspiciously like a 32-bit
> > addressing issue. It could even be something as 'simple' as underlying
> > file API, or other, code using shorts, ints, instead of longs, for
> > legacy purposes. Or layers built on said underlying API (i.e. boost
> > wrappers).
> >
> >> $ file a.out
> >> a.out: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically
> >> linked (uses shared libs), for GNU/Linux 2.6.32,
> >> BuildID[sha1]=d39e3dc0c99e666631747e496bf798b92e4d4c71, not stripped
>
> Yes, I'm pretty sure that all my system and boost libs are all 64 bits:
>
> $ dpkg --get-selections | grep boost-iostream
> libboost-iostreams-dev:amd64                    install
> libboost-iostreams1.55-dev:amd64                install
> libboost-iostreams1.55.0:amd64                  install
>
> $ file /usr/lib/x86_64-linux-gnu/libboost_iostreams.so.1.55.0
> /usr/lib/x86_64-linux-gnu/libboost_iostreams.so.1.55.0: ELF 64-bit LSB
> shared object, x86-64, version 1 (SYSV), dynamically linked,
> BuildID[sha1]=a4839c7904aba3cb68a66739aa801e4d5d50c14f, stripped
>
>
> I'm using just boost API, that calls mmap(). Is this a boost bug?
>
> Using mmap() directly, it works as expected:
>
>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <sys/mman.h>
>
> #include <iostream>
> #include <fstream>
> #include <cassert>
>
> using namespace std;
>
> int main(int argc, char *argv[])
> {
>     const size_t GIGA = 1024l*1024l*1024l;
>     const size_t FILESIZE = 2l * GIGA + 10000;
>
>     ofstream block("file", ios::binary | ios::out);
>     assert(block.good());
>
>     block.seekp(FILESIZE);
>     block.write("", 1);
>     block.close();
>
>     auto fd = open("file", O_RDWR);
>
>     if (fd == -1) {
>         perror("Error opening file for reading");
>         exit(EXIT_FAILURE);
>     }
>
>     char *map = (char*)mmap(0, FILESIZE, PROT_WRITE, MAP_SHARED, fd, 0);
>
>     if (map == MAP_FAILED) {
>         close(fd);
>         perror("Error mmapping file");
>         exit(EXIT_FAILURE);
>     }
>
>     char buffer[10] = {0};
>
>     cout << 0 << endl;
>     lseek(fd, 0, 0);
>     write(fd, buffer, 1);
>
>     cout << FILESIZE - 2 << endl;
>     lseek(fd, FILESIZE - 2, 0);
>     write(fd, buffer, 1);
>
>     cout << 1l*GIGA << endl;
>     lseek(fd, 1l*GIGA, 0);
>     write(fd, buffer, 1);
>
>     if (munmap(map, FILESIZE) == -1) {
>         perror("Error un-mmapping the file");
>     }
>
>     close(fd);
>
>     return 0;
> }
>
>
> $ g++ --std=c++11 ex.cpp
> $ ./a.out
> 0
> 2147493646
> 1073741824
>
>
> --
> []s, Narcélio.
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