sem_wait(3) Library Functions Manual sem_wait(3)NAME
sem_wait, sem_trywait - Performs (or conditionally performs) a semaphore lock (P1003.1b)
LIBRARY
Realtime Library (librt.so, librt.a)
SYNOPSIS
#include <semaphore.h>
int sem_wait (
sem_t *sem);
int sem_trywait (
sem_t *sem);
PARAMETERS
sem Specifies a pointer to the semaphore to be locked.
DESCRIPTION
The sem_wait function locks the semaphore referenced by sem by performing a semaphore lock operation on it. If the semaphore value is zero,
the sem_wait function blocks until it either locks the semaphore or is interrupted by a signal.
The sem_trywait function locks a semaphore only if the semaphore is currently not locked. If the semaphore value is zero, the sem_trywait
function returns without locking the semaphore.
These functions help ensure that the resource associated with the semaphore cannot be accessed by other processes. The semaphore remains
locked until the process unlocks it with a call to the sem_post function.
Use the sem_wait function instead of the sem_trywait function if the process should wait for access to the semaphore.
RETURN VALUES
If the sem_wait or sem_trywait function returns successfully, a value of 0 (zero) is returned and the function executes the semaphore lock
operation.
On an unsuccessful call, a value of -1 is returned and errno is set to indicate that an error occurred. The state of the semaphore remains
unchanged.
ERRORS
The sem_wait and sem_trywait functions fail under the following conditions:
[EAGAIN] The semaphore was already locked and cannot be locked by the sem_trywait operation (sem_trywait only).
[EINTR] A signal interrupted this function.
[EINVAL] The sem argument does not refer to a valid semaphore.
RELATED INFORMATION
Functions: sem_post(3)
Guide to Realtime Programming delim off
sem_wait(3)
Check Out this Related Man Page
SEM_WAIT(3) BSD Library Functions Manual SEM_WAIT(3)NAME
sem_wait, sem_trywait -- decrement (lock) a semaphore
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <semaphore.h>
int
sem_wait(sem_t *sem);
int
sem_trywait(sem_t *sem);
DESCRIPTION
The sem_wait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-
zero and the value can be decremented.
The sem_trywait() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero. Otherwise, the semaphore is not
decremented and an error is returned.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The sem_wait() and sem_trywait() functions will fail if:
[EINVAL] The sem argument points to an invalid semaphore.
Additionally, sem_wait() will fail if:
[EINTR] A signal interrupted this function.
Additionally, sem_trywait() will fail if:
[EAGAIN] The semaphore value was zero, and thus could not be decremented.
SEE ALSO sem_getvalue(3), sem_post(3), sem_timedwait(3)STANDARDS
The sem_wait() and sem_trywait() functions conform to ISO/IEC 9945-1:1996 (``POSIX.1'').
BSD April 16, 2013 BSD
Hi All,
I am going through the semaphore concept and have a doubt regarding the same and hope to get a resolution here.
I have a file which has a number of records.
I want to write an application (in C) which will be able to do concurrent read/write on these records.
Of what I have... (8 Replies)
hi.
i am writing a c program under bash shell.
i would like to use semaphore functions like sem_wait(), sem_post()
and i included <semaphore.h> and it compailes fine
but when i try to run it gives me an error "undefined reference to sem_wait() , sem_post() , sem_init()"
what have i missed... (2 Replies)
hi Guys,
I just wondering how I can check and ensure a file is not locked by another process. I need to modify a file using sed but I need to ensure that is not being modified by another process at the same time. Thanks.
Harby. (2 Replies)
I am investigating some locking scheme using semaphores. To evaluate basic system speed I run a loop of getting some semaphore info and display it:
while : ; do ./semshow; done > res.txt
I ran this on 3 boxes - two similar modern HP XEON boxes, one running SCO OpenServer 5, the other is... (46 Replies)
I have a situation where I have created a semaphore and set it's value to 10. I am using this semaphore to control access to a shared memory location. I can have 10 processes simultaneously read from the shared memory location, process 11 would get locked out. My question is, is there a way I... (6 Replies)
we developed a set of system V semphore interface for our application, in general, all of them work normal, seldom cause the deadlock. Here are some important sem_wait and sem_post interface, pls point some suggestion to fixed the deadlock problem:
int sem_wait_V(int id, struct sembuf *sem_pv)... (1 Reply)
I wanted to use the pthread read/write locks in an application, unfortunately my task requires that readers perform very well, but writers are never starved. I have found that the AIX implementation, while having excellent reader performance, heavily starves writers.
To test this, I wrote an... (8 Replies)
If say a process creates a semaphore/mutex etc then will this semaphore get created in its address space?
If yes then how an another process which wants to acquire this semaphore(created by the first process) will access the other
process address space to acquire the semaphore?
Where I mean in... (2 Replies)
dear all~
I am just new here.I am very happy to see such a forum.
I am very puzzled by the difference between signal and semaphore.
What's the difference between signal() wait() and P V operation?
the linux kernel have implemented a good semaphore system,why we need the system V IPC... (3 Replies)
How to can find all locked files and release the locks in Unix?
We have an issue were an ftp job lands files under /home/loginid at 5:30 pm and at 6 pm we run a get file on the mvs mainframe machine ftp the file from the server to the mainframe. We get an error file is in use eventhu it is not !!... (4 Replies)
Hi friends,
I hope you guys are doing well. I am facing this problem, hope you can help me with it. I am trying to create a semaphore inside a structure. The idea is that I am creating a shared memory space between two processes. The reason for a semaphore is that I want some kind of... (2 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)