9. pipestream Classes

pipestream stream classes provide the services of the UNIX system calls pipe and socketpair and the C library function popen. ipipestream, opipestream, and iopipestream are obtained by simply deriving from isockstream, osockstream and iosockstream respectively. See section 8. sockstream Classes for details.

In what follows,

• ip is an ipipestream object
• op is an opipestream object
• iop is an iopipestream object
• cmd is a char* denoting an executable like "wc"
• ty is of type sockbuf::type indicating the type of the connection
• proto is an int denoting a protocol number

ipipestream ip(cmd)

construct an ipipestream object ip such that the output of the command cmd is available as input through ip.

opipestream op(cmd)

construct an opipestream object op such that the input for the command cmd can be send through op.

iopipestream iop(cmd)

construct an iopipestream object iop such that the input and the output to the command cmd can be sent and received through iop.

iopipestream iop(ty, proto)

construct a iopipestream object iop whose socket is a socketpair of type ty with protocol number proto. ty defaults to sockbuf::sock_stream and proto defaults to 0. Object iop can be used either as a pipe or as a socketpair.

iop.pid ()

return the process id of the child if the current process is the parent or return 0. If the process has not forked yet, return -1.

iopipestream::fork ()

fork() is a static function of class iopipestream. fork() forks the current process and appropriately sets the cpid field of the iopipestream objects that have not forked yet.

9.1 pipestream as pipe

pipe is used to communicate between parent and child processes in the unix domain.

The following example illustrates how to use iopipestream class as a pipe. The parent sends the string "I am the parent" to the child and receives the string "I am the child" from child. The child, in turn, receives the string "I am the parent" from parent and sends the string "I am the child" to the parent. Note the same iopipestream object is used for input and output in each process.

#include <pipestream.h>

int main()
{
        iopipestream p;
        if ( p.fork() ) {
                char buf[128];
                p << "I am the parent\n" << flush;
                cout << "parent: ";
                while(p >> buf)
                        cout << buf << ' ';
                cout << endl;
        }else {
                char buf[128];
                p.getline(buf, 127);
                cout << "child: " << buf << endl;
                p << "I am the child\n" << flush;
        }
        return 0;
}

9.2 pipestream as socketpair

Like pipes, socketpairs also allow communication between parent and child processes. But socketpairs are more flexible than pipes in the sense that they let the users choose the socket type and protocol.

The following example illustrates the use of iopipestream class as a socketpair whose type is sockbuf::sock_dgram. The parent sends the string "I am the parent" to the child and receives the string "I am the child" from the child. The child, in turn, receives and sends the strings "I am the parent" and "I am the child" respectively from and to the parent. Note in the following example that the same iopipestream object is used for both the input and the output in each process.

#include <pipestream.h>

int main()
{
        iopipestream p(sockbuf::sock_dgram);
        if ( iopipestream::fork() ) {
                char buf[128];
                p << "I am the parent\n" << flush;
                p.getline(buf, 127);
                cout << "parent: " << buf << endl;
        }else {
                char buf[128];
                p.getline(buf, 127);
                cout << "child: " << buf << endl;
                p << "I am the child\n" << flush;
        }
        return 0;
}

9.3 pipestream as popen

popen is used to call an executable and send inputs and outputs to that executable. For example, the following example executes "/bin/date", gets its output, and prints it to stdout.

#include <pipestream.h>

int main ()
{
    char buf[128];
    ipipestream p("/bin/date");

    p.getline (buf, 127);
    cout << buf << endl;
    return 0;
}

Here is an example that prints "Hello World!!" on stdout. It uses opipestream object.

#include <pipestream.h>

int main ()
{
    opipestream p("/bin/cat");
    p << "Hello World!!\n" << endl;
    return 0;
}

The following example illustrates the use of iopipestream for both input and output.

#include <pipestream.h>

int main()
{
        char buf[128];
        iopipestream p("lpc");
        p << "help\nquit\n" << flush;
        while ( p.getline(buf, 127) ) cout << buf << endl;
        return 0;
}