Btukfyl

I'm trying to read from a mapped file in chunks (hence the buffer_size and file_offset arguments) but the buffer is always NULL. The file is opened successfully. My code is the following:

#include <boost/iostreams/device/mapped_file.hpp>

#include <iostream>

#include <boost/filesystem.hpp>



namespace filesystem = boost::filesystem;



using std::ios;

using std::ifstream;



char *read_mapped_bytes(const char *file_path, boost::iostreams::mapped_file mapped_file,

                        unsigned long long int buffer_size, unsigned long long int file_offset) {

    const filesystem::path memory_dump_file{file_path};

    mapped_file.open(memory_dump_file, boost::iostreams::mapped_file::readonly, buffer_size,

                     static_cast<boost::iostreams::stream_offset>(file_offset));

    if (mapped_file.is_open()) {

        auto read_buffer = mapped_file.data();

        if (read_buffer == nullptr) {

            printf("The mapped file buffer was NULL");

            exit(-1);

        }

        return read_buffer;

    } else {

        printf("Failed to open mapped file");

        exit(-1);

    }

}



int main(int argc, char *argv) {

    boost::iostreams::mapped_file mapped_file;

    auto buffer = read_mapped_bytes((const char *) "/mnt/d/Cpp/MyProject/myFile.bin", mapped_file,

                                    (unsigned long long int) 1000, (unsigned long long int) 0);



    exit(EXIT_SUCCESS);

}

Note that the file path is correct since otherwise it would cause an exception:

terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<std::ios_base::failure[abi:cxx11]> >'

  what():  failed opening file: No such file or directory: iostream error

The code has been ran on Ubuntu 18.04.01 LTS using WSL (Windows Subsystem for Linux).

The strace (strace -ff -o test.txt ./BoostTesting) of the code execution is the following:

execve("./BoostTesting", ["./BoostTesting"], 0x7fffc2c75168 /* 17 vars */) = 0

brk(NULL)                               = 0x7fffc5980000

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3

fstat(3, {st_mode=S_IFREG|0644, st_size=48784, ...}) = 0

mmap(NULL, 48784, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f4759dab000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/usr/lib/x86_64-linux-gnu/libboost_iostreams.so.1.65.1", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF2113>1200300"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0644, st_size=105032, ...}) = 0

mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4759da0000

mmap(NULL, 2200080, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f47597e0000

mprotect(0x7f47597f8000, 2097152, PROT_NONE) = 0

mmap(0x7f47599f8000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x18000) = 0x7f47599f8000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/usr/lib/x86_64-linux-gnu/libboost_system.so.1.65.1", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF2113>100030"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0644, st_size=18544, ...}) = 0

mmap(NULL, 2113856, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f47595d0000

mprotect(0x7f47595d4000, 2093056, PROT_NONE) = 0

mmap(0x7f47597d3000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x3000) = 0x7f47597d3000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/usr/lib/x86_64-linux-gnu/libstdc++.so.6", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF21133>136030310"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0644, st_size=1594832, ...}) = 0

mmap(NULL, 3702816, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f4759240000

mprotect(0x7f47593b9000, 2097152, PROT_NONE) = 0

mmap(0x7f47595b9000, 49152, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x179000) = 0x7f47595b9000

mmap(0x7f47595c5000, 12320, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f47595c5000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libgcc_s.so.1", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF2113>1300*"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0644, st_size=96616, ...}) = 0

mmap(NULL, 2192432, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f4759020000

mprotect(0x7f4759037000, 2093056, PROT_NONE) = 0

mmap(0x7f4759236000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x16000) = 0x7f4759236000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF21133>1260342"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0755, st_size=2030544, ...}) = 0

mmap(NULL, 4131552, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f4758c20000

mprotect(0x7f4758e07000, 2097152, PROT_NONE) = 0

mmap(0x7f4759007000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e7000) = 0x7f4759007000

mmap(0x7f475900d000, 15072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f475900d000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libz.so.1", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF2113>122037"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0644, st_size=116960, ...}) = 0

mmap(NULL, 2212016, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f4758a00000

mprotect(0x7f4758a1c000, 2093056, PROT_NONE) = 0

mmap(0x7f4758c1b000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b000) = 0x7f4758c1b000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libbz2.so.1.0", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF2113>1`23"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0644, st_size=66800, ...}) = 0

mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4759d90000

mmap(NULL, 2161864, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f47587f0000

mprotect(0x7f47587ff000, 2093056, PROT_NONE) = 0

mmap(0x7f47589fe000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xe000) = 0x7f47589fe000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF2113>1000b"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0755, st_size=144976, ...}) = 0

mmap(NULL, 2221184, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f47585d0000

mprotect(0x7f47585ea000, 2093056, PROT_NONE) = 0

mmap(0x7f47587e9000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x19000) = 0x7f47587e9000

mmap(0x7f47587eb000, 13440, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f47587eb000

close(3)                                = 0

access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libm.so.6", O_RDONLY|O_CLOEXEC) = 3

read(3, "177ELF21133>1200272"..., 832) = 832

fstat(3, {st_mode=S_IFREG|0644, st_size=1700792, ...}) = 0

mmap(NULL, 3789144, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f4758230000

mprotect(0x7f47583cd000, 2093056, PROT_NONE) = 0

mmap(0x7f47585cc000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x19c000) = 0x7f47585cc000

close(3)                                = 0

mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4759d80000

arch_prctl(ARCH_SET_FS, 0x7f4759d811c0) = 0

mprotect(0x7f4759007000, 16384, PROT_READ) = 0

mprotect(0x7f47585cc000, 4096, PROT_READ) = 0

mprotect(0x7f47587e9000, 4096, PROT_READ) = 0

mprotect(0x7f47589fe000, 4096, PROT_READ) = 0

mprotect(0x7f4758c1b000, 4096, PROT_READ) = 0

mprotect(0x7f4759236000, 4096, PROT_READ) = 0

mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4759d70000

mprotect(0x7f47595b9000, 40960, PROT_READ) = 0

mprotect(0x7f47597d3000, 4096, PROT_READ) = 0

mprotect(0x7f47599f8000, 4096, PROT_READ) = 0

mprotect(0x7f475a008000, 4096, PROT_READ) = 0

mprotect(0x7f4759c27000, 4096, PROT_READ) = 0

munmap(0x7f4759dab000, 48784)           = 0

set_tid_address(0x7f4759d81490)         = 1491

set_robust_list(0x7f4759d814a0, 24)     = 0

rt_sigaction(SIGRTMIN, {sa_handler=0x7f47585d5cb0, sa_mask=, sa_flags=SA_RESTORER|SA_SIGINFO, sa_restorer=0x7f47585e2890}, NULL, 8) = 0

rt_sigaction(SIGRT_1, {sa_handler=0x7f47585d5d50, sa_mask=, sa_flags=SA_RESTORER|SA_RESTART|SA_SIGINFO, sa_restorer=0x7f47585e2890}, NULL, 8) = 0

rt_sigprocmask(SIG_UNBLOCK, [RTMIN RT_1], NULL, 8) = 0

prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=8192*1024}) = 0

brk(NULL)                               = 0x7fffc5980000

brk(0x7fffc59a1000)                     = 0x7fffc59a1000

futex(0x7f47595c607c, FUTEX_WAKE_PRIVATE, 2147483647) = 0

futex(0x7f47595c6088, FUTEX_WAKE_PRIVATE, 2147483647) = 0

openat(AT_FDCWD, "/mnt/d/Cpp/MyProject/myFile.bin", O_RDONLY) = 3

mmap(NULL, 1000, PROT_READ, MAP_SHARED, 3, 0) = 0x7f4759db6000

fstat(1, {st_mode=S_IFCHR|0660, st_rdev=makedev(4, 1), ...}) = 0

ioctl(1, TCGETS, {B38400 opost isig icanon echo ...}) = 0

write(1, "The mapped file buffer was NULL", 31) = 31

exit_group(-1)                          = ?

+++ exited with 255 +++

Note that when compiling for Windows the same problem arises so it's most likely a bug in my code or Boost.

It turns out I passed bad parameters. Also, the offset must be a multiple of the page size. Here is the platform independent example source code demonstrating iteration over a file in chunks using mapped_file:

#include <boost/iostreams/device/mapped_file.hpp>

#include <iostream>

#include <boost/filesystem.hpp>



#ifdef _WIN32



#include <sysinfoapi.h>



#endif



namespace filesystem = boost::filesystem;

namespace io_streams = boost::iostreams;



using std::ios;

using std::ifstream;



unsigned long long int get_page_size() {

#ifdef _WIN32

    SYSTEM_INFO sysInfo;

    GetSystemInfo(&sysInfo);

    return (unsigned long long int) sysInfo.dwPageSize;

#else

    return (unsigned long long int) sysconf(_SC_PAGE_SIZE);

#endif

}



unsigned long long int get_file_size(const char *file_path) {

    filesystem::path file{file_path};

    auto generic_path = file.generic_path();

    return filesystem::file_size(generic_path);

}



std::vector<char> read_bytes(const char *file_path,

                             unsigned long long int offset,

                             unsigned long long int length) {

    io_streams::mapped_file_params parameters;

    parameters.path = file_path;

    parameters.length = static_cast<size_t>(length);

    parameters.flags = io_streams::mapped_file::mapmode::readonly;

    parameters.offset = static_cast<boost::iostreams::stream_offset>(offset);



    boost::iostreams::mapped_file_source file;



    file.open(parameters);



    if (file.is_open()) {

        auto buffer = file.data();

        std::vector<char> vector_buffer;

        for (auto buffer_index = 0; buffer_index < length; buffer_index++) {

            auto byte = buffer[buffer_index];

            vector_buffer.push_back(byte);

        }



        return vector_buffer;

    } else {

        printf("Failed to open filen");

        exit(EXIT_FAILURE);

    }

}



int main(int argc, char *argv) {

#ifdef _WIN32

    auto file_path = (const char *) R"(D:CppBoostTestingtest.bin)";

#else

    auto file_path = (const char *) "/mnt/d/Cpp/BoostTesting/test.bin";

#endif



    unsigned long long int offset = 0;

    unsigned long long int page_size = get_page_size();

    auto buffer_size = page_size;

    auto total_length = get_file_size(file_path);



    while (offset < total_length) {

        auto remaining_bytes = total_length - offset;

        if (buffer_size > remaining_bytes) {

            buffer_size = remaining_bytes;

        }



        auto buffer = read_bytes(file_path, offset, buffer_size);

        for (auto byte : buffer) {

            printf("%i ", byte);

        }



        offset += buffer_size;

    }



    return EXIT_SUCCESS;

}