Some questions regarding old if.c Post: 302930433

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Top Forums Programming Some questions regarding old if.c Post 302930433 by achenle on Sunday 4th of January 2015 01:41:56 AM

01-04-2015

Registered User

Quote:

Originally Posted by ongoto

I meant no offense.
I'm just going along with what you said. The questions you raised supports the fact that it doesn't make any sense, right?

if (exp()) is asking if the function exists; it's not calling that function.
...

The more I think about it, the less sure I am about that. As K&R C treats pretty much all functions as variable-argument, how could the compiler know what you meant? There's no way for a compiler to know if you're doing an existence check which would simply evaluate to the address of the function, or call the function with zero arguments.

I think there's a good chance that code that you identified actually does make the call to exp(), with unknown data on the stack. I think to just check if the function exists, the code would be if ( exp ).

Either way, anyone who writes code like that without comments on WHAT is being done is being incompetent, in my opinion.

When you go out into the esoteric edges of a programming language like that, even you are likely to not remember exactly what you did later on. And everyone else who didn't write the code is almost certainly going to be stumped for a good bit. Production or library code is not the place to compete in obscure coding ego wars.

And I freely admit that calling a vararg function with zero arguments makes no sense - with undefined junk on the stack, there's no way to know what would happen. That's another reason the code should be commented. To figure out what exactly is going on requires breaking out the C standard and maybe even hardware-specific behavior because "putting variables on the stack" is highly hardware-specific.

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LEARN ABOUT SUNOS

_stack_grow

_stack_grow(3C) 					   Standard C Library Functions 					   _stack_grow(3C)

NAME

       _stack_grow - express an intention to extend the stack

SYNOPSIS

       #include <ucontext.h>

       void *_stack_grow(void *addr);

DESCRIPTION

       The  _stack_grow()  function indicates to the system that the stack is  about to be extended to the address specified by addr. If extending
       the stack to this address would violate the stack boundaries as retreived by stack_getbounds(3C), a SIGSEGV is raised.

       If the disposition of SIGSEGV is SIG_DFL, the process is terminated and a core dump is generated.  If the application has installed its own
       SIGSEGV handler to run on the alternate signal stack, the signal information passed to the handler will be such that a call to stack_viola-
       tion(3C) with these parameters returns 1.

       The addr argument is a biased stack pointer value. See the Solaris 64-bit Developer's Guide.

       This function has no effect if the specified address, addr, is within the bounds of the current stack.

RETURN VALUES

       If the _stack_grow() function  succeeds and does not detect a stack violation, it returns addr.

ERRORS

       No errors are defined.

USAGE

       The _stack_grow() function does not actually adjust the stack pointer register. The  caller  is	responsible  for  manipulating	the  stack
       pointer register once _stack_grow() returns.

       The  _stack_grow()  function  is typically invoked by code created by the compilation environment prior to executing code that modifies the
       stack pointer. It can also be used by hand-written assembly routines to allocate stack-based storage safely.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Evolving			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Async-Signal-Safe 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       stack_getbounds(3C), stack_inbounds(3C), stack_violation(3C), attributes(5)

       Solaris 64-bit Developer's Guide

SunOS 5.10							    18 Jul 2002 						   _stack_grow(3C)