16.4 How to pass command lines longer than 126 characters

Q: Can I invoke my program with a command line longer than the infamous DOS 126-character limit?

Q: I have a Makefile of Unix origin which contains some very long command lines. Will it work with DJGPP?

A: Yes and yes. DJGPP supports several methods of passing command-line arguments which allow it to work around the DOS 126-character limit. These are:

• The !proxy method. If you invoke the program from within another DJGPP program (like Make or Gcc compilation driver), it gets the address of the memory block where the actual command line is stored. The start-up code will detect this and use that info to retrieve the command-line arguments.

  This method is suitable only for invoking DJGPP programs from other DJGPP programs. You don't have to do anything special to use this method, it is all done automagically for you by the library functions from the spawnXX and execXX family on the parent program side, and by the start-up code on the child side²⁷.

• The environment method. This is the same as the !proxy method above, but the information about the long command line is stored in an environment variable called " !proxy" (with the leading blank and in lower-case). The reason for two similar, but different methods is that command-line arguments passed by system library functions should be globbed by the child, while the arguments passed by spawnXX and execXX family of functions should not; thus the former uses the environment method while the latter use the !proxy method.

  This method is used only by the system library function, and only when the command line is longer than the DOS 126-character limit.

• The response file method. Any argument which starts with a @ character (like in myprog @file) will cause the named file to be read and its contents to be used as command-line arguments, like in many DOS-based compilers and linkers. If you invoke your DJGPP program from the DOS command line, this would be the only method available for you to pass long command lines (like when calling Gawk or SED without the -f option).

  This method is not used by the DJGPP library functions, but you can use it explicitly in your application code. It is designed for invoking non-DJGPP programs that support response files.

  Note that this method makes @ special when it is the first (or the only) character of a command-line argument, which should be protected with quotes if you want to use it verbatim (see how to pass the @ character).

Of course, if the DJGPP start-up code doesn't see any of the above methods, it will use the DOS command line by default.

Since the long command lines are a very important feature, DJGPP's version of the system library function avoids calling the DOS command processor, COMMAND.COM, unless it needs to run a batch file or an internal command of COMMAND.COM. Other features of the command processor, like redirection and pipes, are emulated internally by system. See the library reference for system, for more details about its extended functionality.