#What is this line of Turbo C (v1.0) supposed to do?

34 messages · Page 1 of 1 (latest)

empty spire
#

Hello,

I've been reading the C source code of the video game “Hard Drivin’” (1989) for the Atari ST, and I've stumbled upon this line of code, that doesn't compile under modern C compilers:

long* d;
int* p;

/* ... */

*d++ = *((long*)p)++;

I would like to decompose this line to make it much easier to understand. No need to write “clever” code.

Here what I've come up with, from what I understand the code is trying to do (since it doesn't compile):

long tmp = (long)*p;
++tmp;
*d = tmp;
++d;

If you need to have more context, the line in question in the full source code is visible here: https://github.com/malespiaut/hard-drivin-st/blob/master/src/display.c#L894

Is this correct?

Thank you all for your time!

GitHub
hard-drivin-st/src/display.c at master · malespiaut/hard-drivin-st

Leaked source code of Hard Drivin’ (1989) for the Atari ST, and experiments around it. - malespiaut/hard-drivin-st

uneven tangleBOT
#

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nocturne spoke
#

What are p and d?

empty spire
#

What is this line of Turbo C (v1.0) supposed to do?

empty spire
nocturne spoke
#

My guess is it just reinterprets p as a long pointer, copies its value to where p points to, then increments both pointers by a long's size.

slow pilot
#

why not make p a char* and add sizeof(long) ?

long* d;
char* p;

/* ... */

*d++ = (long) *(p += sizeof(long));
nocturne spoke
#

Hard to know without at least seeing more context.

slow pilot
#

true true

#

or you could do the char thing even without altering the type of p:

#include <stdio.h>

int src_p[] = {1,2,3};
long src_d[] = {0,0,0};

long *d = src_d;
int *p = src_p;
char **c = (char**) &p;

int main()
{
    printf( "addr of p: %08x\n", (int) p);
    
    *(d++) = (long) *(*c+=sizeof(long));
    
    printf( "addr of p: %08x\n", (int) p);

    return 0;
}
empty spire
empty spire
slow pilot
empty spire
#

What does *(d++) means? @slow pilot
Does it mean

*d = /* ... */ ;
++d;

or does it mean

++d;
*d = /* ... */ ;
slow pilot
#

I think the first one

#

i just put the parenthesis for no reason ||bit of an addiction||

plain needle
#

Yes, it means the first one

empty spire
slow pilot
#

hm I see the problem. maybe try *(++d) = (long) *(*c+=sizeof(long)); as you are assigning the next value to the still not updated d in the above example

verbal wadi
#

or maybe valid in a compiler where operator precedence was different

#

my guess is that the intent is *d++ = (*((long*)p))++;

empty spire
empty spire
verbal wadi
#

precedence in standard C

image.png
#

precedence in non-standard turbo C

image.png
#

since in turbo C post-increment has the same precedence as dereferencing, the code is legal and parses as (*((long*)p))++

#

but in C suffix unary operators always come first, so it's parsed as *(((long*)p)++) and it doesn't make sense to increment an lvalue

verbal wadi
#

as you can imagine that was a bit of a nightmare

opal briar
opal briar
#

but that seems like a reasonable possibility