PStreams Reference

A C++ IOStream interface to POSIX Process I/O.

The library is a work in progress. It is intended to provide a C++ re-implementation of the POSIX.2 functions popen(3) and pclose(3), using IOStreams to read from and write to the opened process.

The advantages over the standard popen() function are:

• Standard C++ interface. PStreams classes can be used in any code that expects an IOStream class, giving all the advantages of type-safety and localisation that IOStreams have over the printf()/scanf() functions in standard C.
• Bidirectional I/O. Implementations of popen() vary between systems. Some systems use bidirectional pipes, allowing reading and writing on the same stream, but this is not supported everywhere. Because PStreams doesn't use popen() but re-implements it at a lower level, bidirectional I/O is available on all systems.
• Reading from the process' stderr. Input PStreams can read from the process' stderr as well as stdout.
• More flexible interface. In addition to handling a shell command in the same way as popen() the PStreams classes can open a process specified by a filename and a vector of arguments, similar to the execv() function.
• Signals. The child process can be sent any signal, which can be used to kill or otherwise control it.

The library is available under the GNU Lesser General Public License

To help improve PStreams see the SourceForge project page.

Current status

Version 0.42

Working ipstream and opstream classes for ISO C++-compliant compilers. The classes are fully functional and the public interfaces should be stable for all except the pstreambuf class, which may be extended to add new features.

The stream buffer class, pstreambuf, doesn't use popen(). It uses up to three pipes shared with the associated process, giving access to any combination of the process' stdin, stdout and stderr streams. I/O operations on these pipes are buffered to avoid making a system call for every character written or read.

Another class, rpstream (Restricted PStream) is similar to pstream except that the child process' stdout and stderr cannot be read directly from an rpstream. To read from the process you must call either rpstream::out() or rpstream::err() to obtain a reference to an istream that reads from the process' corresponding output stream. This class is not as well tested as the others (i.e. it's hardly tested at all).

No code-conversion is performed on multi-byte character streams. It should be possible to use the PStreams classes templatized with character types other than char (e.g. basic_pstream<int>) but note that characters are transfered in a bytewise manner, so it is the programmer's responsibility to interpret the resulting character strings. Since the classes are intended to be used to read/write data between processes, which will usually share an internal character representation, rather than to/from files, this behaviour should be sufficient.

Usage

Please refer to the doxygen-generated documentation, accessible through the links at the top of the page.

Using the PStreams classes is similar to using a std::fstream, except that a shell command is given rather than a filename:

// print names of all header files in current directory
redi::ipstream in("ls ./*.h");
std::string str;
while (in >> str) {
    std::cout << str << std::endl;
}

The command argument is a pointer to a null-terminated string containing a shell command line. This command is passed to /bin/sh using the -c flag; Alias and wildcard interpretation, if any, is performed by the shell.

Alternatively, the process can be started with a vector of arguments:

// remove some files, capturing any error messages
std::vector<std::string> argv;
std::vector<std::string> errors;
argv.push_back("rm");
argv.push_back("-rf");
argv.push_back("./foo.txt");
argv.push_back("./bar.html");
argv.push_back("./fnord/");
redi::ipstream in("rm", argv, pstreambuf::pstderr);
std::string errmsg;
while (std::getline(in, errmsg)) {
    errors.push_back(errmsg);
}

If this form of initialisation is used and the file argument doesn't contain a slash then the actions of the shell will be duplicated in searching for an executable in PATH. The shell will not interpret the other arguments, so wildcard expansion will not take place if this interface is used.

If an rpstream was used in the example above it would be necessary to replace the while condition like so:

while (std::getline(in.err(), errmsg)) {
    errors.push_back(errmsg);
}

This form can also be used with the unrestricted pstream and ipstream classes, but it is not strictly necessary.

The latest version of PStreams can be found at http://pstreams.sf.net

Generated on Mon Jul 5 10:39:09 2004 for PStreams by 1.3.2