Constant SYS_ARCH_PRCTL 

pub const SYS_ARCH_PRCTL: usize = 350;

Architecture-specific process control.

• code: operation code (ARCH_SET_FS = 0x1001, ARCH_GET_FS = 0x1002)
• addr: address value (for set operations)

Used primarily to set the FS base for thread-local storage.