Sponsored Content
Top Forums Programming how to prevent deadlock on this... Post 86150 by Perderabo on Tuesday 11th of October 2005 03:34:42 PM
Old 10-11-2005
Post your code for this section.." if(FD_ISSET(fd, &rfds)),' does not sound right

Do a second select before you do the second read. Do a select before each read.
 

10 More Discussions You Might Find Interesting

1. HP-UX

how to check whether a script is running actively or hanged/ in deadlock)

Hi I have a doubt regarding process states in HP unix system. Is there a way to check whether a process is hanged or still actively running? I have few scripts which run for a long time. but sometimes these are getting hanged. But i'm never sure whether they are running or waiting in kind of... (4 Replies)
Discussion started by: truth
4 Replies

2. Programming

How to prevent a class from inheretance?

:(Hi, There is a class in C++ called "CL". It should not participate in inheretance. If some body inherit it it should give errors.....:( (0 Replies)
Discussion started by: krishna_sicsr
0 Replies

3. UNIX for Dummies Questions & Answers

Please help about Unix Deadlock

hello guy, i really have a hard time complete one of my school paper. Does anyone how Unix deal with Deadlock situation. ><; Description or theory is good enough. ^^". Thank you for your kindness, Jaideej (1 Reply)
Discussion started by: jaideej
1 Replies

4. UNIX for Dummies Questions & Answers

Deadlock

Hi All, how to find which all processes cause deadlock into the system and how we can resolve in Unix platform. (1 Reply)
Discussion started by: ravi.sadani19
1 Replies

5. UNIX for Advanced & Expert Users

Parallel access - how to prevent

I have one shell script which is being accessed by many jobs at same time. I want to make the script such that , other job should wait for the script if script is being used by some other job. Is there any way to implement it in script level ? Gops (1 Reply)
Discussion started by: Gopal_Engg
1 Replies

6. UNIX for Dummies Questions & Answers

How to prevent queues from disabling themselves

I understand that on my HP-UX 11.31 system when print queues can no longer communicate with remote printers, the queue disables itself. How can I configure it to stop disabling itself, or alternatively, to re-enable itself when the remote printer comes back online? I have users in warehouses who... (6 Replies)
Discussion started by: EatenByAGrue
6 Replies

7. Shell Programming and Scripting

how to prevent process from being killed

Hi,all.Well,I know someone has already asked this question before,however,It's too long before.So i post a new thread here. Here is the issue.I have a shell script that use awk to calculate something and the script takes about 15 mins,it will use 100% CPU,and the system automatically killed the... (2 Replies)
Discussion started by: homeboy
2 Replies

8. Shell Programming and Scripting

How to prevent command from deleted

Hi, I've been searching around for solution, hope that some gurus here can help. I'm using some commands in my shell script and I'd like to protect these command to be moved to another directory. For instance, cp currently in /bin/cp. If I move it to /bin/cpxxx, my script will not be able to... (3 Replies)
Discussion started by: gklntn
3 Replies

9. UNIX for Advanced & Expert Users

How to prevent Accidents 'rm -rf *'?

When invoking unix commands from other third party tools (IBM ETL), we run the rm / mv commands with the folder as argument been passed. Eg rm -rf {folder}/* when the parameter {folder} did not pass rightly or becomes blank, the command becomes dangerous to execute rm -rf /* How to prevent... (9 Replies)
Discussion started by: deepakwins
9 Replies

10. Emergency UNIX and Linux Support

How to prevent emails as spam?

If an email is sent from our application server(running on AIX) to an id that is outside of the organization like gmail etc, and if gmail should not treat the mail as spam, what has to be done from unix level? (7 Replies)
Discussion started by: ggayathri
7 Replies
SELECT(2)						     Linux Programmer's Manual							 SELECT(2)

NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing SYNOPSIS
/* According to POSIX 1003.1-2001 */ #include <sys/select.h> /* According to earlier standards */ #include <sys/time.h> #include <sys/types.h> #include <unistd.h> int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); int pselect(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, const struct timespec *timeout, const sigset_t *sigmask); FD_CLR(int fd, fd_set *set); FD_ISSET(int fd, fd_set *set); FD_SET(int fd, fd_set *set); FD_ZERO(fd_set *set); DESCRIPTION
The functions select and pselect wait for a number of file descriptors to change status. Their function is identical, with three differences: (i) The select function uses a timeout that is a struct timeval (with seconds and microseconds), while pselect uses a struct timespec (with seconds and nanoseconds). (ii) The select function may update the timeout parameter to indicate how much time was left. The pselect function does not change this parameter. (iii) The select function has no sigmask parameter, and behaves as pselect called with NULL sigmask. Three independent sets of descriptors are watched. Those listed in readfds will be watched to see if characters become available for read- ing (more precisely, to see if a read will not block - in particular, a file descriptor is also ready on end-of-file), those in writefds will be watched to see if a write will not block, and those in exceptfds will be watched for exceptions. On exit, the sets are modified in place to indicate which descriptors actually changed status. Four macros are provided to manipulate the sets. FD_ZERO will clear a set. FD_SET and FD_CLR add or remove a given descriptor from a set. FD_ISSET tests to see if a descriptor is part of the set; this is useful after select returns. n is the highest-numbered descriptor in any of the three sets, plus 1. timeout is an upper bound on the amount of time elapsed before select returns. It may be zero, causing select to return immediately. (This is useful for polling.) If timeout is NULL (no timeout), select can block indefinitely. sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not NULL, then pselect first replaces the current signal mask by the one pointed to by sigmask, then does the `select' function, and then restores the original signal mask again. The idea of pselect is that if one wants to wait for an event, either a signal or something on a file descriptor, an atomic test is needed to prevent race conditions. (Suppose the signal handler sets a global flag and returns. Then a test of this global flag followed by a call of select() could hang indefinitely if the signal arrived just after the test but just before the call. On the other hand, pselect allows one to first block signals, handle the signals that have come in, then call pselect() with the desired sigmask, avoiding the race.) Since Linux today does not have a pselect() system call, the current glibc2 routine still contains this race. The timeout The time structures involved are defined in <sys/time.h> and look like struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ }; and struct timespec { long tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ }; (However, see below on the POSIX 1003.1-2001 versions.) Some code calls select with all three sets empty, n zero, and a non-null timeout as a fairly portable way to sleep with subsecond preci- sion. On Linux, the function select modifies timeout to reflect the amount of time not slept; most other implementations do not do this. This causes problems both when Linux code which reads timeout is ported to other operating systems, and when code is ported to Linux that reuses a struct timeval for multiple selects in a loop without reinitializing it. Consider timeout to be undefined after select returns. RETURN VALUE
On success, select and pselect return the number of descriptors contained in the descriptor sets, which may be zero if the timeout expires before anything interesting happens. On error, -1 is returned, and errno is set appropriately; the sets and timeout become undefined, so do not rely on their contents after an error. ERRORS
EBADF An invalid file descriptor was given in one of the sets. EINTR A non blocked signal was caught. EINVAL n is negative. ENOMEM select was unable to allocate memory for internal tables. EXAMPLE
#include <stdio.h> #include <sys/time.h> #include <sys/types.h> #include <unistd.h> int main(void) { fd_set rfds; struct timeval tv; int retval; /* Watch stdin (fd 0) to see when it has input. */ FD_ZERO(&rfds); FD_SET(0, &rfds); /* Wait up to five seconds. */ tv.tv_sec = 5; tv.tv_usec = 0; retval = select(1, &rfds, NULL, NULL, &tv); /* Don't rely on the value of tv now! */ if (retval) printf("Data is available now. "); /* FD_ISSET(0, &rfds) will be true. */ else printf("No data within five seconds. "); return 0; } CONFORMING TO
4.4BSD (the select function first appeared in 4.2BSD). Generally portable to/from non-BSD systems supporting clones of the BSD socket layer (including System V variants). However, note that the System V variant typically sets the timeout variable before exit, but the BSD variant does not. The pselect function is defined in IEEE Std 1003.1g-2000 (POSIX.1g), and part of POSIX 1003.1-2001. It is found in glibc2.1 and later. Glibc2.0 has a function with this name, that however does not take a sigmask parameter. NOTES
Concerning the types involved, the classical situation is that the two fields of a struct timeval are longs (as shown above), and the struct is defined in <sys/time.h>. The POSIX 1003.1-2001 situation is struct timeval { time_t tv_sec; /* seconds */ suseconds_t tv_usec; /* microseconds */ }; where the struct is defined in <sys/select.h> and the data types time_t and suseconds_t are defined in <sys/types.h>. Concerning prototypes, the classical situation is that one should include <time.h> for select. The POSIX 1003.1-2001 situation is that one should include <sys/select.h> for select and pselect. Libc4 and libc5 do not have a <sys/select.h> header; under glibc 2.0 and later this header exists. Under glibc 2.0 it unconditionally gives the wrong prototype for pselect, under glibc 2.1-2.2.1 it gives pselect when _GNU_SOURCE is defined, under glibc 2.2.2-2.2.4 it gives it when _XOPEN_SOURCE is defined and has a value of 600 or larger. No doubt, since POSIX 1003.1-2001, it should give the prototype by default. SEE ALSO
For a tutorial with discussion and examples, see select_tut(2). For vaguely related stuff, see accept(2), connect(2), poll(2), read(2), recv(2), send(2), sigprocmask(2), write(2) Linux 2.4 2001-02-09 SELECT(2)
All times are GMT -4. The time now is 07:58 PM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy