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Top Forums Programming SIGCHLD interrupts its own handler Post 302404467 by jrichemont on Tuesday 16th of March 2010 02:47:58 PM
Old 03-16-2010
SIGCHLD interrupts its own handler

Hi. I have a program whose job it is to manage 15 child processes. Sometimes these children die (sometimes deliberately other times with a SEGV). This causes a SIGCHLD to be sent to my program which uses waitpid() in the signal handler to gather information and, in most cases, restart the child.
The problem I am having I that under very high loads I am seeing SIGCHLDs sent while the first is still being processed. I can see from my log that the intention was to restart both but only the last one actually gets restarted leading to a gradual haemorrhage of children which eventually causes the whole system to stop responding.

I am using posix threads and have omitted any mutex in the signal handler because I thought it would be atomic. Obviously not. I am a bit scared to put a mutex there; what happens if the same thread is interrupted again while the mutex is held? Deadlock would be worse than the current situation.

I don't want to ignore signals while in the handler either; it is most important that all SIGCHLDs are honoured and the child restarted. I have seen a deferred solution where one thread is dedicated to catching signals and the main program looks at this from time to time. I don't think this will work too well though because I need to call waitpid straight after I get the signal; it needs to wait for the right child status after all.

Any pointers in the right direction would be most welcome.

Cheers;
Jeremy
 

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waitpid(3C)                                                Standard C Library Functions                                                waitpid(3C)

NAME
waitpid - wait for child process to change state SYNOPSIS
#include <sys/types.h> #include <sys/wait.h> pid_t waitpid(pid_t pid, int *stat_loc, int options); DESCRIPTION
The waitpid() function will suspend execution of the calling thread until status information for one of its terminated child processes is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If more than one thread is suspended in waitpid(), wait(3C), or waitid(2) awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination. If status information is available prior to the call to waitpid(), return will be immediate. The pid argument specifies a set of child processes for which status is requested, as follows: o If pid is equal to (pid_t)-1, status is requested for any child process. If pid is greater than (pid_t)0, it specifies the process ID of the child process for which status is requested. o If pid is equal to (pid_t)0 status is requested for any child process whose process group ID is equal to that of the calling process. o If pid is less than (pid_t)-1, status is requested for any child process whose process group ID is equal to the absolute value of pid. One instance of a SIGCHLD signal is queued for each child process whose status has changed. If waitpid() returns because the status of a child process is available, and WNOWAIT was not specified in options, any pending SIGCHLD signal associated with the process ID of that child process is discarded. Any other pending SIGCHLD signals remain pending. If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN and the process has no unwaited children that were transformed into zombie processes, it will block until all of its children terminate, and waitpid() will fail and set errno to ECHILD. If waitpid() returns because the status of a child process is available, then that status may be evaluated with the macros defined by wait.h(3HEAD) If the calling process had specified a non-zero value of stat_loc, the status of the child process will be stored in the location pointed to by stat_loc. The options argument is constructed from the bitwise-inclusive OR of zero or more of the following flags, defined in the header <sys/wait.h>: WCONTINUED The status of any continued child process specified by pid, whose status has not been reported since it continued, is also reported to the calling process. WNOHANG The waitpid() function will not suspend execution of the calling process if status is not immediately available for one of the child processes specified by pid. WNOWAIT Keep the process whose status is returned in stat_loc in a waitable state. The process may be waited for again with identi- cal results. WUNTRACED The status of any child processes specified by pid that are stopped, and whose status has not yet been reported since they stopped, is also reported to the calling process. WSTOPPED is a synonym for WUNTRACED. RETURN VALUES
If waitpid() returns because the status of a child process is available, it returns a value equal to the process ID of the child process for which status is reported. If waitpid() returns due to the delivery of a signal to the calling process, -1 is returned and errno is set to EINTR. If waitpid() was invoked with WNOHANG set in options, it has at least one child process specified by pid for which status is not available, and status is not available for any process specified by pid, then 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The waitpid() function will fail if: ECHILD The process or process group specified by pid does not exist or is not a child of the calling process or can never be in the states specified by options. EINTR The waitpid() function was interrupted due to the receipt of a signal sent by the calling process. EINVAL An invalid value was specified for options. USAGE
With options equal to 0 and pid equal to (pid_t)-1, waitpid() is identical to wait(3C). The waitpid() function is implemented as a call to the more general waitid(2) function. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |Async-Signal-Safe | +-----------------------------+-----------------------------+ SEE ALSO
intro(2), exec(2), exit(2), fork(2), pause(2), sigaction(2), ptrace(3C), signal(3C), siginfo.h(3HEAD), wait(3C), waitpid(3C), wait.h(3HEAD), attributes(5), standards(5) SunOS 5.10 19 Jun 2003 waitpid(3C)
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