RASCTL(2) BSD System Calls Manual RASCTL(2)
rasctl -- restartable atomic sequences
Standard C Library (libc, -lc)
rasctl(void *addr, size_t len, int op);
Restartable atomic sequences are code sequences which are guaranteed to execute without pre-
emption. This property is assured by the kernel by re-executing a preempted sequence from
the start. This functionality enables applications to build atomic sequences which, when
executed to completion, will have executed atomically. Restartable atomic sequences are
intended to be used on systems that do not have hardware support for low-overhead atomic
The rasctl function manipulates a process's set of restartable atomic sequences. If a
restartable atomic sequence is registered and the process is preempted within the range addr
and addr+len, then the process is resumed at addr.
As the process execution can be rolled-back, the code in the sequence should have no side
effects other than a final store at addr+len-1. The kernel does not guarantee that the
sequences are successfully restartable. It assumes that the application knows what it is
doing. Restartable atomic sequences should adhere to the following guidelines:
o have a single entry point and a single exit point;
o not execute emulated instructions; and
o not invoke any functions or system calls.
Restartable atomic sequences are inherited from the parent by the child during the fork(2)
operation. Restartable atomic sequences for a process are removed during exec(3).
The operations that can be applied to a restartable atomic sequence are specified by the op
argument. Possible operations are:
RAS_INSTALL Install this sequence.
RAS_PURGE Remove the specified registered sequence for this process.
RAS_PURGE_ALL Remove all registered sequences for this process.
The RAS_PURGE and RAS_PURGE_ALL operations should be considered to have undefined behaviour
if there are any other runnable threads in the address space which might be executing within
the restartable atomic sequence(s) at the time of the purge. The caller must be responsible
for ensuring that there is some form of coordination with other threads to prevent unex-
To preserve the atomicity of sequences, the kernel attempts to protect the sequences from
alteration by the ptrace(2) facility.
Upon successful completion, rasctl() returns zero. Otherwise, -1 is returned and errno is
set to indicate the error.
The rasctl function will fail if:
[EINVAL] Invalid input was supplied, such as an invalid operation, an invalid
address, or an invalid length. A process may have a finite number of
atomic sequences that is defined at compile time.
[EOPNOTSUPP] Restartable atomic sequences are not supported by the kernel.
[ESRCH] Restartable atomic sequence not registered.
The rasctl functionality first appeared in NetBSD 2.0 based on a similar interface that
appeared in Mach 2.5.
Modern compilers reorder instruction sequences to optimize speed. The start address and
size of a RAS need to be protected against this. One level of protection is created by com-
piler dependent instructions, abstracted from user level code via the following macros:
RAS_DECL(name) Declares the start and end labels used internally by the other macros to
mark a RAS. The name uniquely identifies the RAS.
RAS_START(name) Marks the start of the code. Each restart returns to the instruction fol-
lowing this macro.
RAS_END(name) Marks the end of the restartable code.
RAS_ADDR(name) Returns the start address of a RAS and is used to create the first argument
RAS_SIZE(name) Returns the size of a RAS and is used as second argument to rasctl.
Recent versions of gcc(1) require the -fno-reorder-blocks flag to prevent blocks of code
wrapped with RAS_START/RAS_END being moved outside these labels. However, be aware that
this may not always be sufficient to prevent gcc(1) from generating non-restartable code
within the RAS due to register clobbers. It is, therefore, strongly recommended that
restartable atomic sequences are coded in assembly. RAS blocks within assembly code can be
specified by using the following macros:
RAS_START_ASM(name) Similar to RAS_START but for use in assembly source code.
RAS_END_ASM(name) Similar to RAS_END but for use in assembly source code.
RAS_START_ASM_HIDDEN(name) Similar to RAS_START_ASM except that the symbol will not be
placed in the dynamic symbol table.
RAS_END_ASM_HIDDEN(name) Similar to RAS_END_ASM except that the symbol will not be placed
in the dynamic symbol table.
BSD April 29, 2008 BSD