Some functions in linux are marked "thread safe" by _r (e.g. gmtime_r ).
But most of the syscalls are not be marked and also not mentioned in the man pages.
So my question is:
How can I know whether a linux syscall is thread safe?
Thank you!
I think you mean "library functions"; syscalls should, by virtue of operating on the thread's kernel-side data, be thread-safe.
And the answer is: check the manual pages for the functions in question. The "_r" variants are provided specifically for functions which were non-reentrant, meaning that the extra parameters passed to them were statically declared and modified in the non-"_r" versions.
Most of glibc should be, IIRC, thread-safe, but you always need to check manual pages; or, if you don't trust those, the code itself. There's no silver bullet that will remove from you the responsibility of understanding the interfaces which you are programming against.