fgets problems Post: 302397047

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Top Forums Programming fgets problems Post 302397047 by CRGreathouse on Saturday 20th of February 2010 02:29:33 PM

02-20-2010

Registered User

Quote:

Originally Posted by Corona688

Memory is memory. It doesn't have to track the contents of the pari block to notice clobbering happening outside it.

Sure -- but that's not the problem (and it won't be the problem). Every time the library uses the stack it ensures that there's room for what it's creating. Also, the stack is very large -- at least 20 MB and sometimes as big as 1 GB, with its pointer avma moving by [only] several bytes per call.

The real problem with memory management is leaving garbage on the stack (another possibility is returning corrupted objects). The usual technique for is

Code:

GEN genericfunction(----) {
	pari_sp ltop = avma;	// Keep a pointer to the top of the stack's current location
	GEN r;	// Return value (assigned below)

	// ...
	
	r = gerepileupto(ltop, r);	// copy r to the top of the stack and move avma just above it
	return r;
}

which ensures that you're not leaving garbage on the stack. But this is somewhat more complicated by returning complicated objects like vectors. For example:

Code:

GEN vector = cgetg(4, t_VEC); // Make a vector
gel(vector, 1) = gen_2;
int i = 2;
for(; i < 4; i++)
  gel(vector, i) = addii(gel(vector, i-1), mulii(gel(vector, i-1), gel(vector, i-1))); // v[i] = v[i-1] + v[i-1]^2

Memory diagram:

(vector with pointers to its members A, B, and C)(A)(garbage)(B)(garbage)(C)

CRGreathouse

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LEARN ABOUT NETBSD

stack

STACK(9)						   BSD Kernel Developer's Manual						  STACK(9)

NAME

     STACK -- stack macros

SYNOPSIS

     #include <sys/param.h>

     type
     STACK_ALLOC(sp, size);

     type
     STACK_MAX(sp, size);

     type
     STACK_ALIGN(sp, bytes);

     type
     STACK_GROW(sp, size);

     type
     STACK_SHRINK(sp, size);

DESCRIPTION

     A stack is an area of memory with a fixed origin but with a variable size.  A stack pointer points to the most recently referenced location
     on the stack.  Initially, when the stack has a size of zero, the stack pointer points to the origin of the stack.	When data items are added
     to the stack, the stack pointer moves away from the origin.

     The STACK_ALLOC() macro returns a pointer to allocated stack space of some size.  Given the returned pointer sp and size, STACK_MAX() returns
     the maximum stack address of the allocated stack space.  The STACK_ALIGN() macro can be used to align the stack pointer sp by the specified
     amount of bytes.

     Two basic operations are common to all stacks: a data item is added (``push'') to the location pointed by sp or a data item is removed
     (``pop'') from the stack.	The stack pointer must be subsequently adjusted by the size of the data item.  The STACK_GROW() and STACK_SHRINK()
     macros adjust the stack pointer sp by given size.

     A stack may grow either up or down.  The described macros take this into account by using the __MACHINE_STACK_GROWS_UP preprocessor define.

SEE ALSO

     param(3), queue(3)

BSD
								   April 8, 2011							       BSD

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