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When not linking libc on 64-bit Linux and calling posix.setsid(), we get a type error at compile time inside of posix.errno(). This is because posix.errno()'s non-libc branch expects a usize-sized value, which is what all the error-returning os.linux syscalls return, and linux.setsid() instead returned a pid_t, which is only 32 bits wide. This and the other 3 pid-related calls just below it (getpid(), getppid(), and gettid()) are the only Linux syscall examples here that are casting their return values to pid_t. For the other 3 this makes sense: those calls are documented to have no possible errors and always return a valid pid_t value. However, setsid() actually can return the error EPERM, and therefore needs to return the raw value from syscall0 for posix.errno() to process like normal. Additionally, posix.setsid() needs an @intCast(rc) for the success case as a result, like most other such cases.