05-27-2009
No, your sleep does not block the FTP command. Let's sidetrack into kernel process management for a bit:
Since their invention CPU cores can only execute one process at a time. This meant that a single process waiting for user input could block the whole system. So someone came up with the idea of time slices. Each process is allowed execution for a certain time. After that time it returns control to the kernel. All was good until some processes didn't return control on purpose.
Most modern kernels use preempting instead. Again, each process is allocated execution time. After that time the OS kernel is woken via a timer interrupt (or sooner if the process gives it's rights back because it's waiting for something), the first process is sent into a sleep mode and another is given CPU time. The first process will continue when it's its turn.
Now back to your case: you're starting the FTP process and send it into the background. It gets its time-slices just like any other process, but it's not executing the whole time. Sometimes other processes are executed, so ftp is marked as 'sleeping' or, better said, waiting for its turn again. If you only have one CPU core and get the current list with ps, all processes except for ps will (most likely) be marked as 'sleeping', since ps is currently using its allocated time to gather the stats.
My suggestion for your problem would be:
- ping the host in question (maybe mark those pingable accordingly and skip the others)
- test if the FTP port is open. You can either use netcat (eg: netcat -z $host 21 and check $?) or parse the output of nmap)
- create a small checkfile on the FTP server, and fetch that first. For example, start the FTP command for that in the background. If it's still running after 5 seconds, consider the host dead.
- if all (necessary) checks are good, continue with the large transfer
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LEARN ABOUT MOJAVE
tcl_detachpids
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)