You have to be pretty careful when using syscalls directly, at least in the presence of some libc. For example, from what I have gathered from random tidbits he

You have to be pretty careful when using syscalls directly, at least in the presence of some libc. For example, from what I have gathered from random tidbits here and there, it's not a good idea to use any of the fork-like calls (fork, clone, etc.) if you have any threads that were created using glibc.