11-10-2007
Quote:
Originally Posted by
jim mcnamara
Porter is right about keeping things simple in a signal handler. But there is more - do not call most routines in the standard C library - as Porter points out fflush() is one of them.
If you absolutely must do I/O use write(). The idea is that you call something that cannot be interrupted by another signal - this is called an atomic operation.
write() is "partially" guaranteed to be atomic, but is as close as you'll get. POSIX says write() must guarantee a write the size of PIPE_BUF bytes or smaller to be atomic. There are other I/O system calls that are like this.
Are there any list of function calls that should not be called in a signal handler ?
I have been thinking write () is uninterrupted until it is done - this is something new from the statement that it is ' partially ' atomic.
Is there any way I could test this behavior by some kind of simulation ?
Here is one more point / question ,
with newer signal semantics - using sigaction
kernel guarantees the binary that when executing the handler registered for a possible signal, within that block another signal of the same kind would not be delivered until it returns - in such case why shouldn't a fflush operation be used ?
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LEARN ABOUT MOJAVE
tcl_waitpid
Tcl_DetachPids(3) Tcl Library Procedures Tcl_DetachPids(3)
__________________________________________________________________________________________________________________________________________________
NAME
Tcl_DetachPids, Tcl_ReapDetachedProcs, Tcl_WaitPid - manage child processes in background
SYNOPSIS
#include <tcl.h>
Tcl_DetachPids(numPids, pidPtr)
Tcl_ReapDetachedProcs()
Tcl_Pid
Tcl_WaitPid(pid, statusPtr, options)
ARGUMENTS
int numPids (in) Number of process ids contained in the array pointed to by pidPtr.
int *pidPtr (in) Address of array containing numPids process ids.
Tcl_Pid pid (in) The id of the process (pipe) to wait for.
int *statusPtr (out) The result of waiting on a process (pipe). Either 0 or ECHILD.
int options (in) The options controlling the wait. WNOHANG specifies not to wait when checking the process.
_________________________________________________________________
DESCRIPTION
Tcl_DetachPids and Tcl_ReapDetachedProcs provide a mechanism for managing subprocesses that are running in background. These procedures
are needed because the parent of a process must eventually invoke the waitpid kernel call (or one of a few other similar kernel calls) to
wait for the child to exit. Until the parent waits for the child, the child's state cannot be completely reclaimed by the system. If a
parent continually creates children and doesn't wait on them, the system's process table will eventually overflow, even if all the children
have exited.
Tcl_DetachPids may be called to ask Tcl to take responsibility for one or more processes whose process ids are contained in the pidPtr
array passed as argument. The caller presumably has started these processes running in background and does not want to have to deal with
them again.
Tcl_ReapDetachedProcs invokes the waitpid kernel call on each of the background processes so that its state can be cleaned up if it has
exited. If the process has not exited yet, Tcl_ReapDetachedProcs does not wait for it to exit; it will check again the next time it is
invoked. Tcl automatically calls Tcl_ReapDetachedProcs each time the exec command is executed, so in most cases it is not necessary for
any code outside of Tcl to invoke Tcl_ReapDetachedProcs. However, if you call Tcl_DetachPids in situations where the exec command may
never get executed, you may wish to call Tcl_ReapDetachedProcs from time to time so that background processes can be cleaned up.
Tcl_WaitPid is a thin wrapper around the facilities provided by the operating system to wait on the end of a spawned process and to check a
whether spawned process is still running. It is used by Tcl_ReapDetachedProcs and the channel system to portably access the operating sys-
tem.
KEYWORDS
background, child, detach, process, wait
Tcl Tcl_DetachPids(3)