panthema / 2010 / 0718-stx-execpipe-0.7.0
Execution pipe with exec() bubbles

Published stx-execpipe 0.7.0 - STX Execution Pipe C++ Library

Posted on 2010-07-18 23:31 by Timo Bingmann at Permlink with 0 Comments. Tags: c++

The STX C++ library series has been extended today with a new installation: the STX Execution Pipe library, in short STX ExecPipe. It is the solution to an issue that I encountered in writing a small backup application. This backup tool collects some file to backup and then calls tar and xz followed by ncftpput.

However, I could not find any useful C++ library that allows convenient chaining of external programs like used in everyday shell piping. This pipe line functionality is based on the system calls fork(), exec() and pipe(), which are not easy to use correctly. After writing some ad-hoc functions to call one or two external programs, I decided to tackle this basic problem once and for all. The result is the stx-execpipe library.

Using the library a C++ program can build a sequence of external programs with command line parameters. These programs are connected by the library just like a shell pipe: stdout of the preceding stage goes into stdin of the next one. The input and output of the whole pipeline can be redirected to a plain fd, a file or saved in a std::string.

One very interesting feature of the library is to insert intermediate processing functions into a pipe line. The data can be intercepted and passed back to the parent process for manipulation or just inspection. This was necessary to calculate the SHA1 digest of a backup tarball simultaneously to uploading it.

For more information and the source code see the stx-execpipe web page.

The following small code snippet exemplifies the flexibility of the stx-execpipe solution:

stx::ExecPipe ep;

// first stage calls tar
std::vector<std::string> tarargs;
tarargs.push_back("tar");
tarargs.push_back("--create");
tarargs.push_back("--verbose");
tarargs.push_back("--no-recursion");
tarargs.push_back("/path/to/some/files");
tarargs.push_back("/path/to/more/files");
ep.add_execp(&tarargs);

// second stage compresses the tarball
ep.add_execp("xz", "-9");

// third stage intercepts data for a SHA1 digest
Sha1Function sha1tar;
ep.add_function(&sha1tar);

// fourth stage sends the tarball via FTP
std::vector<std::string> ftpargs;
ftpargs.push_back("ncftpput");
ftpargs.push_back("-c");
ftpargs.push_back("ftp.upload-to-host.net");
ftpargs.push_back("/path/to/ftpfile.tar.gz");
ep.add_execp(&ftpargs);

if (ep.run().all_return_codes_zero()) {
    std::cout << "Backup upload complete." << std::endl
}
else {
    // error processing...
}

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