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bpo-33015: Add a wrapper for thread function in PyThread_start_new_thread #6008

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Merged
vstinner merged 4 commits into from
Nov 30, 2018
Merged

bpo-33015: Add a wrapper for thread function in PyThread_start_new_thread #6008

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d3a9724
Add a wrapper for thread function in PyThread_start_new_thread
siddhesh Oct 25, 2018
4d5b9b0
Cleanup code
vstinner Nov 30, 2018
6d0bade
More cleanup
vstinner Nov 30, 2018
1499f46
fix typo in the NEWS entry
vstinner Nov 30, 2018
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Add a wrapper for thread function in PyThread_start_new_thread 
PyThread_start_new_thread accepts a function of type void (*) (void
*), which does not match with the pthread_create function callback
prototype void *(*) (void *).  This is undefined behaviour and hence
results in an invalid function cast warning with gcc8.  Fix this by
wrapping the function in an internal pthread function callback that
returns NULL.
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@siddhesh @vstinner
siddhesh authored and vstinner committed Nov 30, 2018
commit d3a972434c8aec0a47500d7b368130701b4f0a52
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Build/2018-08-24-09-48-25.bpo-33015.s21y74.rst
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@@ -0,0 +1,3 @@
Add a wrapper for the thread function argument in
`PyThread_start_new_thread` to avoid an incompatible cast warning with gcc
8.
42 changes: 39 additions & 3 deletions Python/thread_pthread.h
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Expand Up @@ -152,6 +152,31 @@ PyThread__init_thread(void)
* Thread support.
*/

/* The thread function argument passed to PyThread_start_new_thread has a
* different type compared to the thread function passed to the POSIX
* pthread_create function and the result of forcibly casting it to the latter
* type is undefined. The pythread_fn struct and the pythread_helper_fn below
* help work around this in a conforming manner by wrapping around the thread
* function.
*/
typedef struct {
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void (*func) (void *);
void *arg;
} pythread_fn;
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@vstinner vstinner Oct 29, 2018

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I suggest to rename it to pythread_callback since it's not only a function but function+arguments (or maybe pythread_func).


static void *
pythread_helper_fn(void *fn)
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@vstinner vstinner Oct 29, 2018

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nitpick: i suggest to rename it to "pythread_wrapper". To me, it's more a wrapper than an helper ;-)

{
pythread_fn *pfn = fn;

void (*func)(void *) = pfn->func;
void *arg = pfn->arg;

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@vstinner vstinner Oct 29, 2018

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nitpick: please move this empty line after the free, not before :-) to make it clear that the memory is freed when the function is called.

PyMem_RawFree((void *)fn);
func(arg);

return NULL;
}

unsigned long
PyThread_start_new_thread(void (*func)(void *), void *arg)
Expand Down Expand Up @@ -188,21 +213,32 @@ PyThread_start_new_thread(void (*func)(void *), void *arg)
pthread_attr_setscope(&attrs, PTHREAD_SCOPE_SYSTEM);
#endif

pythread_fn *fn = (pythread_fn *)PyMem_RawMalloc(sizeof (pythread_fn));
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@vstinner vstinner Oct 29, 2018

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the cast is useless, no?


if (fn == NULL)
return PYTHREAD_INVALID_THREAD_ID;
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fn->func = func;
fn->arg = arg;

status = pthread_create(&th,
#if defined(THREAD_STACK_SIZE) || defined(PTHREAD_SYSTEM_SCHED_SUPPORTED)
&attrs,
#else
(pthread_attr_t*)NULL,
#endif
(void* (*)(void *))func,
(void *)arg
pythread_helper_fn,
fn
);

#if defined(THREAD_STACK_SIZE) || defined(PTHREAD_SYSTEM_SCHED_SUPPORTED)
pthread_attr_destroy(&attrs);
#endif
if (status != 0)

if (status != 0) {
PyMem_RawFree((void *)fn);
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the cast is useless, no?

return PYTHREAD_INVALID_THREAD_ID;
}

pthread_detach(th);

Expand Down