fork, vfork, posix_spawn

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1. fork, vfork, posix_spawn 20210528
2. environment linux glibc2.24+ musl
3. fork create child process copy-on-write memory need same size memory per a child process POSIX.1-2001 https://git.musl-libc.org/cgit/musl/tree/src/process/fork.c
4. vfork same as fork share memory with child process sleep parent process -> run child process -> execve POSIX.1-2001(obsolate) removed POSIX.1-2008 https://git.musl-libc.org/cgit/musl/tree/src/process/vfork.c https://git.musl-libc.org/cgit/musl/tree/src/process/x86_64/vfork.s
5. posix_spawn fork-exec before glibc2.24, fork & vfork after glibc2.24, clone(2) https://git.musl-libc.org/cgit/musl/tree/src/process/posix_spawn.c
6. clone Linux-specific with flag CLONE_VFORK = vfork https://git.musl-libc.org/cgit/musl/tree/src/thread/clone.c https://git.musl-libc.org/cgit/musl/tree/src/thread/x86_64/clone.s
7. in Java usage example(shell command for chmod, etc) https://github.com/apache/hadoop/blob/ae9630f580d73e81042f3b8b2b337aba86130653/hadoop- common-project/hadoop-common/src/main/java/org/apache/hadoop/util/Shell.java#L907 available linux & glibc2.24+ & JDK12 backported JDK11 https://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8212828
8. other languages http://www.a-k-r.org/d/2014-09.html#a2014_09_06