Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>There are a great number of ways to do this, all involving some form of inter-process communication. Which one you choose will depend on many factors, but some are:</p> <ul> <li>shared memory.</li> <li>pipes (<code>popen</code> and such).</li> <li>sockets.</li> </ul> <p>In general, I would probably <code>popen</code> a number of communications sessions in the parent before spawning the children; the parent will know about all five but each child can be configured to use only one.</p> <p>Shared memory is also a possibility, although you'd probably have to have a couple of values in it per child to ensure communications went smoothly:</p> <ul> <li>a value to store the variable and return value.</li> <li>a value to store the state (0 = start, 1 = variable ready for child, 2 = variable ready for parent again).</li> </ul> <p>In all cases, you need a way for the children to only pick up their values, not those destined for other children. That may be as simple as adding a value to the shared memory block to store the PID of the child. All children would scan every element in the block but would only process those where the state is 1 and the PID is their PID.</p> <p>For example:</p> <ul> <li>Main creates shared memory for five children. Each element has state, PID and value.</li> <li>Main sets all states to "start".</li> <li>Main starts five children who all attach to the shared memory.</li> <li>Main stores all their PIDs.</li> <li>All children start scanning the shared memory for state = "ready for child" and their PID.</li> <li>Main puts in first element (state = "ready for child", PID = pid1, value = 7).</li> <li>Main puts in second element (state = "ready for child", PID = pid5, value = 9).</li> <li>Child pid1 picks up first element, changes value to 49, sets state to "ready for parent"), goes back to monitoring.</li> <li>Child pid5 picks up second element, changes value to 81, sets state to "ready for parent"), goes back to monitoring.</li> <li>Main picks up pid5's response, sets that state back to "start.</li> <li>Main picks up pid1's response, sets that state back to "start.</li> </ul> <p>This gives a measure of parallelism with each child continuously monitoring the shared memory for work it's meant to do, Main places the work there and periodically receives the results.</p>
    singulars
    1. This table or related slice is empty.
    1. POHow to make multiple `fork()`-ed processes comunicate using shared memory?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USpaxdiablo
    1. USpaxdiablo
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COi'd be using shared memory
      singulars
      1. PO
      1. USkylex
    2. COYou probably don't need to do the scanning; the children get a copy of the parent process data, so (unlike the vfork() example) the children could look at a counter that tells them which entry is theirs. You could preload the memory with the random values,; then the children know the data is ready.
      singulars
      1. PO
      1. USJonathan Leffler
    3. COThat'll work if you just fork (with parent and child code in the same executable) but not if you exec to bring in the child: the process space is overwritten (including a counter). Not execing generally leaves a lot of dangerous things about that are shared between both forks. Still, it's an option.
      singulars
      1. PO
      1. USpaxdiablo
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload