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xqt_util - Shell Execution Utilities

The Shell Execution Utilities provide a means for a program to execute host system commands, e.g., shell commands under UNIX and DCL commands under VMS. The utilities create a subprocess (by forking under UNIX and spawning under VMS) to execute the commands; pipes (mailboxes under VMS) are used to connect the parent program to the subprocess's standard input and output streams.

To execute a single shell command and to read the output of the command, the following code could be used:

    #include  "xqt_util.h"		-- Shell execution utilities.
    char  buffer[132] ;
    XqtStream  stream ;
    xqtOpen (NULL, &stream) ;
    xqtWrite (stream, "%s", "shell command") ;
    xqtRead (stream, sizeof buffer, buffer) ;
    ... process result string in buffer ...
    xqtClose (stream) ;
The code above executes only one command; you can actually have any number of xqtWrite()s and xqtRead()s between the opening and the closing of the execution stream.

Note that you must have some way of knowing how many lines of output to read from an executed command. One means of ensuring that the correct number of lines is read is as follows:

  1. Issue the command to be executed.
  2. Issue a command that generates known output.
  3. Read each line of output until the known output is encountered.
Under VMS, for example, the following code fragment reads a variable-length directory listing:

    char  filename[256] ;
    xqtWrite (stream, "%s", "DIRECTORY/NOHEADING/NOTRAILING") ;
    xqtWrite (stream, "%s", "SHOW SYMBOL $STATUS") ;
    while (!xqtRead (stream, sizeof filename, filename)) {
        if (strncmp (filename, "  $STATUS ==", 12) == 0)  break ;
        ... process file name ...
    ... end of directory listing ...
DCL's $STATUS variable holds the completion status of the last command executed (the directory command, in this case). The SHOW SYMBOL $STATUS command outputs the completion status in the following format:

                      $STATUS == "%XcompletionStatus"
The completion status is a VMS error code (SS$_, etc.) displayed in hexadecimal form. A useful thing to do is to examine the value of the $STATUS variable after every command executed. UNIX shells generally have a similar means of determining the completion status of a command.

Public Procedures

xqtClose() - closes a shell execution stream.
xqtFd() - returns the file descriptor for input from a shell execution stream.
xqtOpen() - opens a shell execution stream.
xqtPoll() - checks if any output from a shell execution stream is waiting to be read.
xqtRead() - reads a line of output from a shell execution stream.
xqtWrite() - writes a line of input to a shell execution stream.

Source Files


(See libgpl for the complete source, including support routines and build files.)

Alex Measday  /  E-mail