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Leaving aside the fact that the DOS API is... ehem... bad, this
fundamental difference means that any Unix program ported to DOS has
a usability problem: you cannot use globs anymore when invoking it!
Something as simple and common as gcc -o program.exe *.c would just
not work. So then... how can we explain the following output from the
showargs.c program, a little piece of code that prints argv?

    D:\>gcc -o showargs.exe showargs.c
    
    D:\>.\showargs.exe *.c
    argv[1] = headself.c
    argv[2] = longcmd1.c
    argv[3] = longcmd2.c
    argv[4] = showargs.c
    argv[5] = showpath.c
    
    D:\>

In the picture above, you can see how I ran the showargs.c program
with *.c as its own argument and somehow it worked as you would
expect. But if we build it with a standard DOS compiler we get
different results:

    D:\>tcc showargs.c
    Turbo C++ Version 3.00 Copyright (c) 1992 Borland International
    showargs.c:
    Turbo Link  Version 5.0 Copyright (c) 1992 Borland International
    
            Available memory 4133648
    
    D:>.\showargs.exe *.c
    argv[1] = *.c
    
    D:>_

GCC is actually doing something to make glob expansion work--and it
has to, because remember that DJGPP was not just about porting GCC:
it was about porting many more GNU developer tools to DOS. Having had
to patch them one by one to work with DOS' COMMAND.COM semantics
would have been a sad state of affairs.

To understand what's happening here, know that all C programs
compiled by any compiler include a prelude: main is not the program's
true entry point. All compilers wrap main with some code of their own
to set up the process and the C library, and DJGPP is no different.
Such code is often known as the crt (or C Runtime) and it comes in
two phases: crt0, written in assembly for early bootstrapping, and
crt1, written in C.

As you can imagine, this is where the magic lives. DJGPP's crt1 is in
charge of processing the flat command line that it receives from DOS
and transforming it into the argv that POSIX C programs expect,
following common Unix semantics. In a way, this code performs the job
of a Unix shell.

Once again, take a break to inspect the crt0 sources and, in
particular, the contents of the c1args.c file. Pay attention to file
reads and the "proxy" thing, both of which bring us to the next
section.

# Long command lines

Unix command lines aren't different just because of glob expansion.
They are also different because they are usually long, and they are
long in part because of glob expansion and in part because Unix has
supported long file names for much longer than DOS.

Unfortunately... DOS restricted command lines to a maximum of
126 characters--fewer characters than you can fit in a Tweet or an
SMS--and this posed a problem because the build process of most GNU
developer tools, if not all, required using long command lines. To
resolve these issues, DJGPP provides two features.

The first is support for response files. Response files are text
files that contain the full command line. These files are then passed
to a process with the @file.txt syntax, which then causes DJGPP's
crt1 code to load the response files and construct the long command
line in extended memory.

Let's take a look. If we reuse our previous showargs.c program that
prints the command line arguments, we can observe how the behavior
differs between building this program with a standard DOS compiler
and with DJGPP:

    D:\>type args.txt
    first
    second
    
    
    D:\>gcc -o showargs.exe showargs.c
    
    D:\>.\showargs.exe @args.txt
    argv[1] = first
    argv[2] = second
    
    D:\>tcc showargs.c
    Turbo C++ Version 3.00 Copyright (c) 1992 Borland International
    showargs.c:
    Turbo Link  Version 5.0 Copyright (c) 1992 Borland International
    
            Available memory 4133648
    
    D:\>.\showargs.exe @args.txt
    argv[1] = @args.txt
    
    D:\>

Response files are easy to implement and they are sufficient to
support long command lines: even if they require special handling on
the caller side to write the arguments to dsk and then place the
response file as an argument, this could all be hidden inside the
exec family of system calls. Unfortunately, using response files is
slow because, in order to invoke a program, you need to write the
command line to a file--only to load it immediately afterwards. And
disk I/O used to be really slow.

For this reason, DJGPP provides a different mechanism to pass long
command lines around, and this is via the transfer buffer described
earlier. This mechanism involves putting the command line in the
transfer buffer and telling the executed command where its command
line lives. This mechanism obviously only works when executing a
DJGPP program from another DJGPP program, because no matter what,
process executions are still routed through DOS and thus are bound by
DOS' 126 character limit.

Let's try this too. For this experiment, we'll play with two
programs: one that prints the length of the received command line and
another one that produces a long command line and executes the former.

The first program is longcmd1.c and is depicted below. All this
program does is allocate a command line longer than DOS' maximum
length of 126 characters and, once it has built the command line,
invokes longcmd2.exe with said long command line:

    #ifdef __GNUC__
    #include 
    #else
    #include 
    #endif
    #include 
    #include 
    #include 
    
    int main(int argc, char** argv) {
        char** longcmd;
        int i;
    
        // Generate a command line that exceeds DOS' limits.
        longcmd = (char**)malloc(32);
        longcmd[0] = argv[0];
        for (i = 1; i < 31; i++) {
            longcmd[i] = strdup("one-argument");
        }
        longcmd[i] = NULL;
    
        // Execute the second stage of this demo to print the received
        // command line.
        if (execv(".\\longcmd2.exe", longcmd) == -1) {
            perror("execv failed");
            return EXIT_FAILURE;
        }
        return EXIT_SUCCESS;
    }

The second program is longcmd2.c and is depicted below. This program
prints the number of arguments it received and also computes the
length of the command line (assuming all arguments were separated by
just one space character):

    #include 
    #include 
    #include 
    
    int main(int argc, char** argv) {
        int i;
        int total;
    
        total = 0;
        for (i = 0; i < argc; i++) {
            if (i > 0) {
                total += 1;  // Assume 1 space between arguments.
            }
            total += strlen(argv[i]);
        }
        printf("argc after re-exec: %d\n", argc);
        printf("textual length: %d\n", total);
    
        return EXIT_SUCCESS;
    }

Now let's see what happens when we compile these two programs with
Turbo C++ and with DJGPP. First, let's build both with Turbo C++ and
run the longcmd1.exe entry point:

    D:\>tcc longcmd1.c
    Turbo C++ Version 3.00 Copyright (c) 1992 Borland International
    longcmd1.c:
    Warning longcmd1.c 29: Parameter 'argc' is never used in function main
    Turbo Link  Version 5.0 Copyright (c) 1992 Borland International
    
            Available memory 4116968
    
    D:\>tcc longcmd2.c
    Turbo C++ Version 3.00 Copyright (c) 1992 Borland International
    longcmd2.c:
    Turbo Link  Version 5.0 Copyright (c) 1992 Borland International
    
            Available memory 4124048
    
    D:\>.\longcmd1.exe
    execv failed: Not enough memory.
    
    D:\>

Running longcmd1.exe fails because the command line is too long and
execv cannot process it. (I'm not exactly sure why execv returns
ENOMEM because the Turbo C++ documentation claims that this function
should return E2BIG on this condition, but alas.)

Now, let's build just longcmd1.c with DJGPP and run it:

    D:\>gcc -o longcmd1.exe longcmd.c
    
    D:\>tcc longcmd2.c
    Turbo C++ Version 3.00 (c) 1992 Borland International
    longcmd2.c:
    Turbo Link  Version 5.0 (c) 1992 Borland International
    
            Available memory 4124048
    
    D:\>.\longcmd1.exe
    argc after re-exec: 13
    textual length: 141
    
    D:\>

We get a bit further now! longcmd1.exe runs successfully and executes
longcmd2.exe... but longcmd2.exe claims that the command line is
shorter than we expect. This is because DJGPP's execv implementation
knew that it was running a standard DOS application not built by
DJGPP, so it had to place a truncated command line in the system call
issued to DOS. (As a detail also note that this shows 141 and not
126: the reason for this is that DOS does not place argv[0] on the
command line, but the C runtime has to synthesize this value.)

But now look at what happens when we also compile longcmd2.c with
DJGPP:

    D:\>gcc -o longcmd2.exe longcmd1.c
    
    D:\>gcc -o longcmd2.exe longcmd2.c
    
    D:\>.\longcmd1.exe
    argc after re-exec: 31
    textual length: 377
    
    D:\>

Ta-da! When longcmd2.exe runs, it now sees the full command line.
This is because longcmd1.exe now knows that longcmd2.exe understands
the transfer buffer arrangement and can send the command line to it
this way.

You can read more about this in the spawn documentation from DJGPP's
libc and peek at the dosexec.c sources.





# Unix-style paths

Let's move on to one more Unix-y thing that DJGPP has to deal with,
which is paths and file names. You see, paths are paths in both DOS
and Unix: a sequence of directory names (like /usr/bin/) followed by
an optional file name (like /usr/bin/gcc). Unfortunately, DOS and
Unix paths differ in two aspects.

The first is that DOS paths separate directory components with a
backslash, not a forward slash. This is a historical artifact of the
early CP/M and DOS days, where command-line flags used the forward
slash (DIR /P) instead of Unix's dash (ls -l). When DOS gained
support for directories in its 2.0 release, it had to pick a
different character to separate directories, and it picked the
backslash. Dealing with this duality in DJGPP-built programs seems
easy: just make DJGPP's libc functions allow both and call it a day.
And for the most part, this works--and in fact even PowerShell does
this on Windows today.

The second is that DOS paths may include an optional drive name such
as C: and... the drive name has the colon character int. While Unix
uses the colon character to separate multiple components of the
search PATH, DOS could not do that: it had to pick a different
character, and it picked the semicolon. Take a look:

    C:\>path
    PATH=Z:\;C:\DEVEL\BIN;C:\DEVEL\DJGPP\BIN;C:\DEVEL\TC\BIN

The problem here is that many Unix applications, particularly shell
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