Atomicity Post: 50847

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Top Forums Programming Atomicity Post 50847 by S.P.Prasad on Tuesday 4th of May 2004 07:53:17 AM

05-04-2004

Registered User

Thanks for the information, Driver.

Quote:

Process 2 now gets to execute, finds the shared variable zero too and writes its PID to it. Then process 1 gets to execute again and overwrites the PID of the other process.

The second if takes care such that only one process has the right to execute code:

Quote:

if (shared_variable_One = = getpid ( ) )
{
if ( ++ shared_variable_two > 1 ) /* Variable Shared Across Processes Initially initialized as 0*/
goto RERUN:
/* Set the bit value */
shared_variable_One = 0 ;
shared_variable_two = 0 ;
}

I hope that I am clear.

Quote:

I'm now sure why shared libraries come into play now ...

Shared libraries (also called dynamic libraries) are linked into the program in two stages. First, during compile time, the linker verifies that all the symbols (again, functions, variables and the like) required by the program, are either linked into the program, or in one of its shared libraries. However, the object files from the dynamic library are not inserted into the executable file. Instead, when the program is started, a program in the system (called a dynamic loader) checks out which shared libraries were linked with the program, loads them to memory, and attaches them to the copy of the program in memory.

The complex phase of dynamic loading makes launching the program slightly slower, but this is a very insignificant drawback, that is out-weighted by a great advantage - if a second program linked with the same shared library is executed, it can use the same copy of the shared library, thus saving a lot of memory. Only one copy of the library is stored in memory at any given time. This means we can use far less memory to run our programs, and the executable files are much smaller, thus saving a lot of disk space as well.

Hence if I write LOCK() and REL() subroutines as a part of Shared Library and link it to the executable, will not aid me to make the operations atomic. I can easily validate in the subroutines which address is requested for locking and which for release.

Thanks in advance.

I personally thank Driver and Perderabo for their interest shown to solve the issue. Please continue aiding me till we conclude to a unanimous decision.

S.P.Prasad

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pldd

PLDD(1) 							 Linux User Manual							   PLDD(1)

NAME

       pldd - display dynamic shared objects linked into a process

SYNOPSIS

       pldd pid
       pldd option

DESCRIPTION

       The  pldd  command  displays a list of the dynamic shared objects that are linked into the process with the specified process ID.  The list
       includes the libraries that have been dynamically loaded using dlopen(3).

OPTIONS

       -?, --help
	      Display program help message.

       --usage
	      Display a short usage message.

       -V, --version
	      Display the program version.

EXIT STATUS

       On success, pldd exits with the status 0.  If the specified process does not exist, the user does not have permission to access its dynamic
       shared  object  list, or no command-line arguments are supplied, pldd exists with a status of 1.  If given an invalid option, it exits with
       the status 64.

VERSIONS

       pldd is available since glibc 2.15.

CONFORMING TO

       The pldd command is not specified by POSIX.1.  Some other systems have a similar command.

NOTES

       The command

	   lsof -p PID

       also shows output that includes the dynamic shared objects that are linked into a process.

       The gdb(1) info shared command also shows the shared libraries being used by a process, so that one can obtain similar output to pldd using
       a command such as the following (to monitor the process with the specified pid):

	   $ gdb -ex "set confirm off" -ex "set height 0" -ex "info shared" 
		   -ex "quit" -p $pid | grep '^0x.*0x'

BUGS

       Since glibc 2.19, pldd is broken: it just hangs when executed.  It is unclear if it will ever be fixed.

EXAMPLE

       $ echo $$	       # Display PID of shell
       1143
       $ pldd $$	       # Display DSOs linked into the shell
       1143:	 /usr/bin/bash
       linux-vdso.so.1
       /lib64/libtinfo.so.5
       /lib64/libdl.so.2
       /lib64/libc.so.6
       /lib64/ld-linux-x86-64.so.2
       /lib64/libnss_files.so.2

SEE ALSO

       ldd(1), lsof(1), dlopen(3), ld.so(8)

COLOPHON

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

GNU
								    2017-09-15								   PLDD(1)