errno is sometimes read too late after the error:  After another callmay have modified it.  Here's a patch against py3k.  Most of it or avariant applies to 2.7 too, but I haven't really looked at that.I've not looked at math code, where I don't even know what sets errno.There might also be a late errno read atModules/_ctypes/callproc.cline 794: "space[0] = errno;".  Does it want the errno from before_ctypes_get_errobj()?  I'm unsure what's going on there.Modules/_multiprocessing/semaphore.c doesn't need the patch's extravariable, it can instead be rearranged with the commented-out patch.

This stuff is hard to write automated tests for, hence there are none.The patch is mostly straightforward: capture errno with new variable err at point of possible error when intervening calculation is needed before testing the value of errno. This seems like a good idea.There is one relocation of memory freeing and the additions of+        if (res >= 0)+            break;which I cannot evaluate.

Terry J. Reedy writes:> There is one relocation of memory freeingModules/timemodule.c does '#if,if(..errno..)' after PyMem_Free(outbuf),which can overwrite the desired errno.  Instead of reading errno intoa temporary, I moved the free into both branches taken by the #if,if().> and the additions of> +        if (res >= 0)> +            break;> which I cannot evaluate.errno is only needed after an error., so I moved 'res < 0' out of the'while'.        if (res == MP_EXCEPTION_HAS_BEEN_SET) break;    } while (res < 0 && errno == EINTR && !PyErr_CheckSignals());-->        if (res == MP_EXCEPTION_HAS_BEEN_SET) break;        if (! (res < 0))                      break;    } while (errno == EINTR && !PyErr_CheckSignals());-->        if (res >= 0)                         break;        err = errno;        if (res == MP_EXCEPTION_HAS_BEEN_SET) break;    } while (err == EINTR && !PyErr_CheckSignals());

The patch seems a bit confused at times. For example, you need to restore errno before calling PyErr_SetFromErrno().Here is a new patch for py3k.

New changeset6a966179c73a by Antoine Pitrou in branch '3.2':Issue#10350: Read and save errno before calling a function which might overwrite it.http://hg.python.org/cpython/rev/6a966179c73aNew changeset8e0b2e75ca7a by Antoine Pitrou in branch 'default':Issue#10350: Read and save errno before calling a function which might overwrite it.http://hg.python.org/cpython/rev/8e0b2e75ca7a

Committed in 3.x. I won't bother backporting to 2.x. Thank you!