Hi friends,
I have written this small program using the concept of semaphores. I am a bit confused, hope u can help me with it.
The idea is that
1. Two threads are created
2. First will be displaying 0(zero) on the screen
3. Second will be displaing 1(one) on the screen
4. This process will go on upto three times for each thread, it would look like
But the program that i have written prints like this
0
0
0
1
1
1
I don't really know what I am missing here, could you pin point it to me, I think my concept of semaphores is corrct.
You can have a look at my code
But the same program works when I put a sleep(1) in the middle like this
But I want to achieve what I want to achieve by using only semaphores, no sleep function,
Good day, everyone!
Could anybody explain me the following situation.
If I'm running similar script:
Var="anna.kurnikova"
Var2="Anna Kurn"
echo $Var | tr -t "$Var" "$Var2"
Why the output is :
anna KurniKova
instead of Anna Kurnikova?
:confused:
Thank you in advance for any... (2 Replies)
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I have printer old about 5 year , but I can not determine which driver to use
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ls -al /usr/share/lib/terminfo/h
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-rw-r--r-- 1 root bin 1002 Jan 22 2005 h1420
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Hi friends,
I am using semaphores in my program, but when I compile the program, it gives the following error
$ gcc sem.c -o sem -lpthread
Undefined first referenced
symbol in file
sem_init ... (1 Reply)
Hello All,
I have a problem in counting number of process getting run with my current script name..
Here it is
ps -ef | grep $0 | grep -v grep
This display just one line with the PID, PPID and other details when i print it in the script.
But when I want to count the numbers in my... (11 Replies)
hallo!
if there are many processes running and I initialize the semaphore like this:
my_sem = sem_open(SEM_NAME, O_CREAT | O_RDWR,
S_IRUSR | S_IWUSR, 10);
(the last argument is 10)
and then i use
sem_wait(my_sem);
sleep(5);
sem_post;
Will 10 processes be able to access the... (1 Reply)
Is wc -c and wc -m same ?
Shellscript::cat file1
hello
Shellscript::cat file1 | wc -c
6
Shellscript::cat file1 | wc -m
6
Shellscript::file file1
file1: ASCII text
Shellscript::uname -a
Linux was85host 2.6.27.45-0.1-vmi #1 SMP 2010-02-22 16:49:47 +0100 i686 i686 i386 GNU/LinuxAtleast... (5 Replies)
Discussion started by: shellscripting
5 Replies
LEARN ABOUT X11R4
sem_overview
SEM_OVERVIEW(7) Linux Programmer's Manual SEM_OVERVIEW(7)NAME
sem_overview - overview of POSIX semaphores
DESCRIPTION
POSIX semaphores allow processes and threads to synchronize their actions.
A semaphore is an integer whose value is never allowed to fall below zero. Two operations can be performed on semaphores: increment the
semaphore value by one (sem_post(3)); and decrement the semaphore value by one (sem_wait(3)). If the value of a semaphore is currently
zero, then a sem_wait(3) operation will block until the value becomes greater than zero.
POSIX semaphores come in two forms: named semaphores and unnamed semaphores.
Named semaphores
A named semaphore is identified by a name of the form /somename; that is, a null-terminated string of up to NAME_MAX-4 (i.e., 251)
characters consisting of an initial slash, followed by one or more characters, none of which are slashes. Two processes can operate
on the same named semaphore by passing the same name to sem_open(3).
The sem_open(3) function creates a new named semaphore or opens an existing named semaphore. After the semaphore has been opened,
it can be operated on using sem_post(3) and sem_wait(3). When a process has finished using the semaphore, it can use sem_close(3)
to close the semaphore. When all processes have finished using the semaphore, it can be removed from the system using
sem_unlink(3).
Unnamed semaphores (memory-based semaphores)
An unnamed semaphore does not have a name. Instead the semaphore is placed in a region of memory that is shared between multiple
threads (a thread-shared semaphore) or processes (a process-shared semaphore). A thread-shared semaphore is placed in an area of
memory shared between the threads of a process, for example, a global variable. A process-shared semaphore must be placed in a
shared memory region (e.g., a System V shared memory segment created using shmget(2), or a POSIX shared memory object built created
using shm_open(3)).
Before being used, an unnamed semaphore must be initialized using sem_init(3). It can then be operated on using sem_post(3) and
sem_wait(3). When the semaphore is no longer required, and before the memory in which it is located is deallocated, the semaphore
should be destroyed using sem_destroy(3).
The remainder of this section describes some specific details of the Linux implementation of POSIX semaphores.
Versions
Prior to kernel 2.6, Linux supported only unnamed, thread-shared semaphores. On a system with Linux 2.6 and a glibc that provides the NPTL
threading implementation, a complete implementation of POSIX semaphores is provided.
Persistence
POSIX named semaphores have kernel persistence: if not removed by sem_unlink(3), a semaphore will exist until the system is shut down.
Linking
Programs using the POSIX semaphores API must be compiled with cc -pthread to link against the real-time library, librt.
Accessing named semaphores via the filesystem
On Linux, named semaphores are created in a virtual filesystem, normally mounted under /dev/shm, with names of the form sem.somename.
(This is the reason that semaphore names are limited to NAME_MAX-4 rather than NAME_MAX characters.)
Since Linux 2.6.19, ACLs can be placed on files under this directory, to control object permissions on a per-user and per-group basis.
NOTES
System V semaphores (semget(2), semop(2), etc.) are an older semaphore API. POSIX semaphores provide a simpler, and better designed inter-
face than System V semaphores; on the other hand POSIX semaphores are less widely available (especially on older systems) than System V
semaphores.
EXAMPLE
An example of the use of various POSIX semaphore functions is shown in sem_wait(3).
SEE ALSO sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7), shm_over-
view(7)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/.
Linux 2017-05-03 SEM_OVERVIEW(7)