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Full Discussion: maximum number of times - a file can be opened
Top Forums Programming maximum number of times - a file can be opened Post 302068858 by Perderabo on Tuesday 21st of March 2006 10:24:16 AM
Old 03-21-2006
If you don't have root, ask your system administrator what nfile is. Be sure to explain that you plan to exploit a possible os weakness. We sysadms just love to hear that. I decline to comment on what result would ensue if you wrap the counter. (Which I doubt is possible.)

If I execute a single open(), followed by a dup(), I now have two open files but only one file table entry. (I did say this might be cheating.) If you doubt that this represents 2 open files, try it and run lsof on your program. Or suppose I do this and then I call exit(). All opened files are closed by exit(). How many times will exit invoke close()? Or leave your limit at 256 and open a file and dup it 256 times...do you succeed? And dup() is not the only way to duplicate an fd. There is even a way to "dup" an fd from one process to another unrelated process. Also only the original dup is constrained to the lowest available fd. It is better to control the desired fd. The original dup is really still around for compatibility. On most OS's, dup is just a wrapper around fcntl(). I know that duped fd's all link to the same file table entry...that is the whole point. It circumvents the nfile limit. And I point out one more time that I did say it might be cheating.

Max pid may or may not relate strongly to max number of processes. Max processes is another kernel tunable called NPROC but there may be a limit how high it can be set. If you want to achieve the max number of times that a file can be opened, you will need to take extraordinary steps and this includes tuning the kernel to max out various limits. (Sadly, few kernels have been tuned for this particular activity...) This in turn may cause you to need a lot of physical memory. Setting records is never easy.
 

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LEARN ABOUT OSX

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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