From:	CRDGW2::CRDGW2::MRGATE::"SMTP::CRVAX.SRI.COM::RELAY-INFO-VAX" 20-APR-1989 21:01
To:	MRGATE::"ARISIA::EVERHART"
Subj:	Mixing C and Pascal params

Message-Id:  <8904210054.AA27402@crdgw1.ge.com>
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Date: 20-APR-1989 20:09:21.00
From: RSILVERMAN@EAGLE.WESLEYAN.EDU
Subject: Mixing C and Pascal params
To: INFO-VAX@EAGLE.WESLEYAN.EDU

Bill Smith writes:

>  I have a problem.  I am trying to call a pascal subroutine from C and
> pass a char string.  The following is the C program:
>     
>main()
>{
>  char test[80] = "abcdefghijklmnop\0";
>  cls();
>  printf("%s  \n",test);
>  index(test);
>  printf("%s  \n",test);
>}
>    Below is the Pascal Module I am linking in with the C program:
>     
>MODULE CLEAR(OUTPUT);
>  [GLOBAL] PROCEDURE CLS;
>    BEGIN
>      WRITELN(CHR(27),'[H',CHR(27),'[J');
>    END;
>  [GLOBAL] PROCEDURE INDEX(i : varying[A] of char);
>    BEGIN
>	 WRITELN(I);
>    END;
>END.

(and the program crashes...)
     
Bill,
	The problem is that Pascal expects the argument to be passed in a
different way than that in which C is providing it.  The code generated by the
Pascal compiler for your routine 'INDEX' expects to find in the argument list a
pointer to a string descriptor, which in turn points to a 'varying of char'
record type.  Instead, you provide just the address of the beginning of the
character string.  When the Pascal runtime tries to read the 'descriptor' at
that address, it finds garbage instead, interprets it as an address, and you get
your access violation.  Here is a version of the calling C routine that works:

#include descrip

$DESCRIPTOR( test, "\5\0Hello" );

main()
{
  cls();
  index(&test);
}

Notice that the varying of char type contains a 16-bit count of the current
length of the string, followed by the string characters themselves; hence the
'\0\5' == short 5 in the string.

VAX Pascal is a nightmare when it comes to this sort of thing; I much prefer the
simple, straightforward conventions in C.

                                                Richard Silverman

arpa:	rsilverman@eagle.wesleyan.edu           Computing Center
bitnet:	rsilverman@wesleyan                     Wesleyan University
CIS:	[72727,453]                             Middletown, CT 06457