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

5 May 2015

      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 
#include 
#include 
#include 
#include 

#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.

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

Narcélio Filho