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In writeBytes return actual numBytesWritten #165

Job	Run time
macos (aarch64, 11, MacOSX11.0.sdk)	58s
macos (x86_64, 10.9, MacOSX10.9.sdk)	32s
windows (aarch64)	59s
windows (x86_64)	1m 3s
ubuntu (aarch64, g++-aarch64-linux-gnu)	1m 26s
ubuntu (mingw32, g++-mingw-w64-i686)	1m 31s
ubuntu (mingwaarch64, clang)	1m 5s
ubuntu (riscv64, g++-riscv64-linux-gnu)	1m 28s
ubuntu (x86, g++-multilib)	1m 10s
ubuntu (ppc64, g++-powerpc64le-linux-gnu)	1m 37s
ubuntu (x86_64, g++-)	49s
ubuntu (armhf, g++-arm-linux-gnueabihf)	49s
ubuntu (mingw64, g++-mingw-w64-x86-64)	1m 20s
	14m 47s

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