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Diffstat (limited to 'system/doc/reference_manual/code_loading.xml')
| -rw-r--r-- | system/doc/reference_manual/code_loading.xml | 55 |
1 files changed, 25 insertions, 30 deletions
diff --git a/system/doc/reference_manual/code_loading.xml b/system/doc/reference_manual/code_loading.xml index e9da40e02f..f6fd2911fa 100644 --- a/system/doc/reference_manual/code_loading.xml +++ b/system/doc/reference_manual/code_loading.xml @@ -130,26 +130,6 @@ loop() -> <marker id="on_load"></marker> <title>Running a Function When a Module is Loaded</title> - <warning> - <p>The <c>on_load</c> feature is to be considered experimental - as there are a number of known weak points in current semantics, - which therefore might change in future Erlang/OTP releases:</p> - <list> - <item><p>Doing external call in <c>on_load</c> to the module itself - leads to deadlock.</p></item> - <item><p>At module upgrade, other processes calling the module - get suspended waiting for <c>on_load</c> to finish. This can be very bad - for applications with demands on realtime characteristics.</p></item> - <item><p>At module upgrade, no rollback is done if the - <c>on_load</c> function fails. - The system is left in a bad limbo state without any working - and reachable instance of the module.</p></item> - </list> - <p>The problems with module upgrade described above can be fixed in future - Erlang/OTP releases by changing the behaviour to not make the module reachable until - after the <c>on_load</c> function has successfully returned.</p> - </warning> - <p>The <c>-on_load()</c> directive names a function that is to be run automatically when a module is loaded.</p> <p>Its syntax is as follows:</p> @@ -159,20 +139,35 @@ loop() -> <p>It is not necessary to export the function. It is called in a freshly spawned process (which terminates as soon as the function - returns). The function must return <c>ok</c> if the module is to - remain loaded and become callable, or any other value if the module - is to be unloaded. Generating an exception also causes the - module to be unloaded. If the return value is not an atom, - a warning error report is sent to the error logger.</p> - - <p>A process that calls any function in a module whose <c>on_load</c> - function has not yet returned, is suspended until the <c>on_load</c> - function has returned.</p> + returns).</p> + + <p>The function must return <c>ok</c> if the module is to + become the new current code for the module and become + callable.</p> + + <p>Returning any other value or generating an exception + causes the new code to be unloaded. If the return value is not an + atom, a warning error report is sent to the error logger.</p> + + <p>If there already is current code for the module, that code will + remain current and can be called until the <c>on_load</c> function + has returned. If the <c>on_load</c> function fails, the current + code (if any) will remain current. If there is no current code for + a module, any process that makes an external call to the module + before the <c>on_load</c> function has finished will be suspended + until the <c>on_load</c> function have finished.</p> + + <note> + <p>Before OTP 19, if the <c>on_load</c> function failed, any + previously current code would become old, essentially leaving + the system without any working and reachable instance of the + module. That problem has been eliminated in OTP 19.</p> + </note> <p>In embedded mode, first all modules are loaded. Then all <c>on_load</c> functions are called. The system is terminated unless all of the <c>on_load</c> functions return - <c>ok</c></p>. + <c>ok</c>.</p> <p><em>Example:</em></p> |