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Full Discussion: Deallocating memory in multi-threaded environment.
Top Forums Programming Deallocating memory in multi-threaded environment. Post 302518400 by DGPickett on Friday 29th of April 2011 01:56:10 PM
Old 04-29-2011
Threads is having two cooks in the kitchen, but as far as free(), same old problem. Once you free(), nobody should use the pointer or any derived pointer, and nice people set it to null for testing, and start it out that way, too. Threads adds a race condition, where only a mutex and similar IPC-ish mechanisms can tell another thread anything, so you needs to be sure they will never reference the space before your free. Your scheme has the controlling mutex in the deallocated space. It is a bit like an elevator with no outside door. You do not want to test for the car with your foot, as a passing car will cut your leg off.

The garbage collection languages free things when all references are gone. You put your finger on a big language limitation/feature. Early JAVA would be fooled by data that looked like a pointer to the object, and not always free the object!

You can manage the deallocation in a stable way in many applications, like a buffering input reader, which hangs read data on a linked list that it creates, and there is a consumer-owned-flag so when it has been consumed, the reader knows it can reuse it. The consumer might just null the pointer in an indirect linked list, and free() the space so it could return to the arena, but the arena might become fragmented (hint: bsdmalloc()!). If the buffers can be reused, it might be good for them all to be the same size, but if not, they can be just big enough. The critical part is that the references are stable, the reader creating, writing and destroying everything except turning on the consumed flag.
 

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LEARN ABOUT SUNOS

plimit

plimit(1)							   User Commands							 plimit(1)

NAME

       plimit - get or set the resource limits of running processes

SYNOPSIS

       plimit [-km] pid...

       plimit {-cdfnstv} soft,hard... pid...

DESCRIPTION

       If  one	or more of the cdfnstv options is specified, plimit sets the soft (current) limit and/or the hard (maximum) limit of the indicated
       resource(s) in the processes identified by the process-ID list, pid. Otherwise plimit reports the resource limits of the processes  identi-
       fied by the process-ID list, pid.

       Only  the  owner  of a process or the super-user is permitted either to get or to set the resource limits of a process. Only the super-user
       can increase the hard limit.

OPTIONS

       The following options are supported:

       -k	On output, show file sizes in kilobytes (1024 bytes) rather than in 512-byte blocks.

       -m	On output, show file and memory sizes in megabytes (1024*1024 bytes).

       The remainder of the options are used to change specified resource limits. They each accept an argument of the form:

	      soft,hard

       where soft specifies the soft (current) limit and hard specifies the hard (maximum) limit. If the hard limit is not  specified,	the  comma
       may  be	omitted. If the soft limit is an empty string, only the hard limit is set. Each limit is either the literal string unlimited, or a
       number, with an optional scaling factor, as follows:

       nk	n kilobytes

       nm	n megabytes (minutes for CPU time)

       nh	n hours (for CPU time only)

       mm:ss	minutes and seconds (for CPU time only)

       The soft limit cannot exceed the hard limit.

       -c soft,hard    Set core file size limits (default unit is 512-byte blocks).

       -d soft,hard    Set data segment (heap) size limits (default unit is kilobytes).

       -f soft,hard    Set file size limits (default unit is 512-byte blocks).

       -n soft,hard    Set file descriptor limits (no default unit).

       -s soft,hard    Set stack segment size limits (default unit is kilobytes).

       -t soft,hard    Set CPU time limits (default unit is seconds).

       -v soft,hard    Set virtual memory size limits (default unit is kilobytes).

OPERANDS

       The following operands are supported.

       pid	Process ID list.

EXIT STATUS

       plimit returns the exit value zero on success, non-zero on failure (such as no such process, permission denied, or invalid option).

FILES

       /proc/pid/*     process information and control files

ATTRIBUTES

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

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWesu			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       ulimit(1), proc(1), getrlimit(2), setrlimit(2), proc(4), attributes(5),

SunOS 5.10							    8 Jun 1998								 plimit(1)
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