# FBB::SharedStream(3bobcat)

## I/O on shared memory
(libbobcat-dev_6.07.01)

## 2005-2025

## NAME

FBB::SharedStream - I/O operations on shared memory

## SYNOPSIS

#include <bobcat/sharedstream>
Linking option: -lpthread -lbobcat

## DESCRIPTION

This class combines the features of the std::istream and std::ostream classes, operating on shared memory. As with std::fstream objects, a seekp or seekg member call is required to switch from writing to reading or v.v.

As with std::fstream objects, FBB::SharedStream objects do not keep separate offsets for reading and writing: the seek-members always refer to the (single) offset maintained by the FBB::SharedMemory object to which the SharedStream object interfaces.

So, although tellg and tellp return identical values, tellg should not be called after writing to a SharedStream object, and tellp should not be called after reading from a SharedStream object, as calling members related to reading (tellg) after writing and v.v. put the stream in its fail state.

## NAMESPACE

FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

## INHERITS FROM

FBB::SharedBuf (private inheritance),
std::istream,
std::ostream,
FBB::SharedEnum__ (cf. sharedmemory(3bobcat) for a description of this base class).

## SIZEUNIT ENUMERATION

The enum SizeUnit defines the following symbolic constants:

- kB, representing 1024 (2**10) bytes of memory;
- MB, representing 1048576 (2**20 bytes of memory;
- GB, representing 1073741824 (2**30) bytes of memory )

  ## CONSTRUCTORS

  - SharedStream():
    The default constructor defines a stub a SharedStream object that cannot immediately be used to access shared memory. To use it, its member open must first be called.

  - SharedStream(size_t maxSize, SizeUnit sizeUnit, std::ios::openmode openMode = std::ios::in | std::ios::out, size_t access = 0600):
    This constructor creates a stream inheriting the facilities of an std::istream and std::ostream that interfaces to a shared memory segment having a capacity of at least maxSize * sizeUnit bytes.

    By default, the shared memory segment is opened for reading and writing. Different from the open modes used for file streams, creating a shared memory stream with open modes ios::in | ios::out is OK. In this case the shared memory segment is created and once information has been written to the shared memory it can also be read again.

    The shared memory's access rights are defined by the access parameter, interpreted as an octal value, using the well-known (chmod(1)) way to define the access rights for owner, group and others.

    If construction fails, an FBB::Exception is thrown.

  - SharedStream(int id, std::ios::openmode openMode = std::ios::in | std::ios::out):
    This constructor creates a stream inheriting the facilities of an std::istream and std::ostream that connects to a shared memory segment having ID id.

    Specifying the ios::trunc flag immediately clears the content of the shared memory.

    An FBB::Exception is thrown if construction fails (e.g., no shared memory segment having ID id exists),

  Copy and move constructors (and assignment operators) are not available.

  ## MEMBER FUNCTIONS

  All members of std::istream and std::ostream and the enum values kB, MB, and GB, defined by FBB::SharedEnum__ are available.

  - FBB::SharedCondition attachSharedCondition(std::ios::off_type offset, std::ios::seekdir origin):
    Returns an FBB::SharedCondition(3) object, interfacing to a shared condition variable located at offset offset (relative to origin) in the SharedMemory object to which the SharedStream object interfaces. This member does not alter the value returned by the stream's tellg and tellp members.

    An FBB::Exception is thrown if the FBB::SharedCondition object could not be constructed.

  - FBB::SharedCondition createSharedCondition():
    Returns an FBB::SharedCondition(3) object, interfacing to a newly created shared condition variable which is created at the current offset of the SharedMemory object to which the SharedStream object interfaces (or at the first offset of the next physical shared memory data block, cf. sharedcondition(3bobcat))
  . Creating a SharedCondition object does not alter the value returned by the stream's tellg and tellp members.

  An FBB::Exception is thrown if the FBB::SharedCondition object could not be constructed.

- int id() const:
  The ID of the shared memory segment is returned.

- void kill():
  Without locking the shared memory first, all shared memory is returned to the operating system. The FBB::SharedStream object is unusable after returning from kill. Other processes that were using the shared memory can continue to do so.

- void memInfo(std::ostream &out, char const *end = "\n"):
  Information about the SharedMemory object is inserted into the provide ostream object. The IDs of the shared segments, their sizes, the maximum number of shared memory segments, the number of bytes that can be read from the shared memory, and its actual storage capacity, etc., are displayed. Following the information about the shared memory, end is inserted into out.

- void open(size_t maxSize, SizeUnit sizeUnit, std::ios::openmode openMode = std::ios::in | std::ios::out, size_t access = 0600):
  This member creates a shared memory segment having a capacity of at least maxSize * sizeUnit bytes, and connects the shared memory segment to the FBB::SharedStream. A matching close member does not exist and is not required.

  By default, the shared memory segment is opened for reading and writing. Different from the open modes used for file streams, creating a shared memory stream with open modes ios::in | ios::out is OK. In this case the shared memory segment is created and once information has been written to the shared memory it can also be read again.

  The shared memory's access rights are defined by the access parameter, interpreted as an octal value, using the well-known (chmod(1)) way to define the access rights for owner, group and others.

  If opening fails, an FBB::Exception is thrown.

- void open(int id, std::ios::openmode openMode = std::ios::in | std::ios::out):
  This member connects the FBB::SharedStream object to a shared memory segment having ID id. A matching close member does not exist and is not required. If opening succeeds the shared memory is ready for use.

  Specifying the ios::trunc flag immediately clears the content of the shared memory.

  An FBB::Exception is thrown if opening fails (e.g., no shared memory segment having ID id exists).

- void remove():
  The shared memory is first locked. Next, all shared memory is returned to the operating system. The FBB::SharedStream object is unusable after returning from remove. Other processes that were using the shared memory can continue to do so.

- bool truncate(std::streamsize offset):
  If offset is not exceeding the value returned by seekg(0, std::ios::end), then this latter value is changed to offset and true is returned. Otherwise false is returned, and the value returned by seekg is not altered. If the value returned by tellg exceeded offset, tellg's return value it is reduced to offset as well. Subsequent read operations on the shared memory can only succeed as long as tellg's return value hasn't reached the value offset.

## EXAMPLE

    #include <iostream>
    #include <string>
    #include <ostream>
    #include <istream>

    #include <bobcat/exception>
    #include <bobcat/sharedstream>
    #include <bobcat/sharedcondition>

    using namespace std;
    using namespace FBB;

    int main()
    {
        SharedStream sharedStream;

        int id = -1;

        while (true)
        {
            cout <<
                    "\n"
                    " K             kill (no lock) existing shared segment\n"
                    " S             show stats of current shared segment\n"
                    " L <id>        Load segment <id>\n"
                    " C             Install a SharedCondition at offset 0\n"
                    " c            create new shared memory (sets id)\n"
    //                " l            lock segment id until key pressed\n"
    //                " p <x> c      put char c at offset x\n"
                    " q            quit\n"
    //                " r <x> <n>    read n chars from offset x\n"
    //                " w <x> args   write all args at offset x\n"
                      " i            insert lines (until empty) at the current "
                                                                    "offset\n"

                      " x            extract lines (until EOF) from the current "
                                                                    "offset\n"
                      " X            extract the next line from the current "
                                                                    "offset\n"
                      "              when nodified via 'N'\n"
                      " N            notify a waiting X\n"
                      " s <x>        seek (abs) offset x\n"
                    "? ";

            char ch;
            cin >> ch;

            ios::off_type offset;

            cout << "Requested: " << ch << '\n';

            sharedStream.clear();

            switch (ch)
            {
                case 'c':
                {
                    sharedStream.open(1, SharedStream::kB);

                    id = sharedStream.id();
                    cout << "id = " << id << '\n';
                    sharedStream.memInfo(cout);
                    cout << '\n';
                }
                break;

                case 'C':
                {
                    sharedStream.seekp(0);
                    SharedCondition cond(sharedStream.createSharedCondition());
                    sharedStream.seekg(cond.width());
                    break;
                }

                case 'K':         // delete segment
                {
                    if (id == -1)
                    {
                        cout << "No segment loaded\n";
                        continue;
                    }

                    cout << "Removing segment id = " << id << '\n';
                    if (ch == 'R')
                        sharedStream.remove();
                    else
                        sharedStream.kill();

                    id = -1;
                }
                break;

                case 'L':
                    cin >> id;
                    cout << "Loading segment " << id << '\n';
                    sharedStream.open(id);
                    sharedStream.memInfo(cout);
                    cout << '\n';
                break;

                case 'S':
                    if (id == -1)
                    {
                        cout << "No segment loaded\n";
                        continue;
                    }
                    sharedStream.memInfo(cout);
                    cout << '\n';
                break;

                case 's':
                {
                    size_t offset;
                    if (id == -1)
                    {
                        cout << "No segment loaded\n";
                        continue;
                    }
                    cin >> offset;
                    sharedStream.seekg(offset);
                    sharedStream.seekp(offset);
                    cout << "tellg: " << sharedStream.tellg() << ", "
                            "tellp: " << sharedStream.tellp() << '\n';
                }
                break;

                case 'i':
                {
                    string line;
                    getline(cin, line);
                    sharedStream.seekp(0, ios::cur);
                    while (true)
                    {
                        cout << "? ";
                        if (not getline(cin, line) || line.empty())
                            break;
                        sharedStream << line << endl;
                        cout <<
                                "   tellp: " << sharedStream.tellp() << '\n';
                    }
                        cout << // "   tellg: " << sharedStream.tellg() << ", "
                                "   tellp: " << sharedStream.tellp() << '\n';
                }
                break;

                case 'x':
                {
                    string line;
                    sharedStream.seekg(0, ios::cur);

                    while (true)
                    {
                        cout << ": ";
                        if (not getline(sharedStream, line))
                            break;
                        cout << line << "\n"
                                "   tellg: " << sharedStream.tellg() << ", "
                                "   tellp: " << sharedStream.tellp() << '\n';
                    }
                }
                break;

                case 'X':
                {
                    SharedCondition cond(sharedStream.attachSharedCondition(0));

                    string line;
                    sharedStream.seekg(cond.width());

                    cond.lock();

                    while (true)
                    {
                        cond.wait();

                        cout << ": ";
                        if (not getline(sharedStream, line))
                            break;

                        cout << line << "\n"
                                "   tellg: " << sharedStream.tellg() << ", "
                                "   tellp: " << sharedStream.tellp() << '\n';
                    }
                    cout << "All done\n";
                    cond.unlock();
                }
                break;

                case 'N':
                {
                    string line;
                    getline(cin, line);

                    SharedCondition cond(sharedStream.attachSharedCondition(0));
                    while (true)
                    {
                        cout << "'enter' or 'q'? ";
                        getline(cin, line);

                        if (line == "q")
                            break;

                        cond.lock();
                        cond.notify();
                        cond.unlock();
                    }
                }
                break;

                case 'p':           // put a char behind the last written
                {
                    if (id == -1)
                    {
                        cout << "No segment loaded\n";
                        continue;
                    }

                    cin >> offset >> ch;
                    if (!cin)
                        throw Exception() << "cmd specification error";

                    sharedStream.seekp(offset);
                    cout << "Segment id = " << id << " at write offset " <<
                                                        sharedStream.tellp() << '\n';
                    sharedStream.put(ch);
                }
                break;

                case 'r':           // put a char behind the last written
                {
                    if (id == -1)
                    {
                        cout << "No segment loaded\n";
                        continue;
                    }

                    int n;

                    cin >> offset >> n;
                    if (!cin)
                        throw Exception() << "cmd specification error";

                    char buf[n];

                    sharedStream.seekg(offset);
                    cout << "Segment id = " << id << " at offset " <<
                             sharedStream.tellg() << ", to read " << n << " bytes\n";

                    n = sharedStream.read(buf, n).gcount();

                    if (n < 0)
                        cout << "No data at " << offset << '\n';
                    else
                    {
                        cout << "Retrieved " << n << " bytes, containing `";
                        cout.write(buf, n);
                        cout << "'\n";

                        for (auto ch: buf)
                            cout << static_cast<int>(ch) << ' ';
                        cout << '\n';
                    }
                }
                break;

                case 'w':           // write chars at offset
                {
                    if (id == -1)
                    {
                        cout << "No segment loaded\n";
                        continue;
                    }

                    string line;

                    cin >> offset;
                    getline(cin, line);
                    if (!cin)
                        throw Exception() << "cmd specification error";

                    streampos pos = sharedStream.seekp(offset).tellp();

                    cout << "Segment id = " << id << " at offset " <<
                             pos << ", to write " << line.length() << " bytes\n";

                    sharedStream.write(line.data(), line.length());

                    if (!sharedStream)
                        cout << "No room left to write any bytes\n";
                    else
                        cout << "Wrote " << (sharedStream.tellp() - pos) << " bytes\n";
                }
                break;

                case 'q':
                return 0;

                default:
                    cout << "request not implemented: " << ch << '\n';
                break;
            }
        }
    }

## FILES

bobcat/sharedstream - defines the class interface

## SEE ALSO

bobcat(7), chmod(1), isharedstream(3bobcat), osharedstream(3bobcat), sharedblock(3bobcat), sharedcondition(3bobcat), sharedmemory(3bobcat) sharedmutex(3bobcat), sharedpos(3bobcat), sharedsegment(3bobcat), sharedbuf(3bobcat)

## BUGS

Note that by default exceptions thrown from within a std::stream object are caught by the stream object, setting its ios::failbit flag. To allow exceptions to leave a stream object, its exceptions member can be called, e.g., using:

        myStream.exceptions(ios::failbit | ios::badbit | ios::eofbit);

## BOBCAT PROJECT FILES

- https://fbb-git.gitlab.io/bobcat/: gitlab project page;

Debian Bobcat project files:

- libbobcat6: debian package containing the shared library, changelog and copyright note;
- libbobcat-dev: debian package containing the static library, headers, manual pages, and developer info;

## BOBCAT

Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

## COPYRIGHT

This is free software, distributed under the terms of the GNU General Public License (GPL).

## AUTHOR

Frank B. Brokken (f.b.brokken@rug.nl).