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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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PR_SET_SYSCALL_USER_DISPATCH(2const) PR_SET_SYSCALL_USER_DISPATCH(2const)
PR_SET_SYSCALL_USER_DISPATCH - set the system-call user dispatch
mechanism for the calling thread
Standard C library (libc, -lc)
#include <linux/prctl.h> /* Definition of PR_* constants */
#include <sys/prctl.h>
int prctl(PR_SET_SYSCALL_USER_DISPATCH, long op, ...);
int prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON,
unsigned long off, unsigned long size, int8_t *switch);
int prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_OFF, 0L, 0L, 0L);
Configure the Syscall User Dispatch mechanism for the calling
thread. This mechanism allows an application to selectively
intercept system calls so that they can be handled within the
application itself. Interception takes the form of a thread-
directed SIGSYS signal that is delivered to the thread when it
makes a system call. If intercepted, the system call is not
executed by the kernel.
PR_SYS_DISPATCH_ON
Enable this mechanism.
Once enabled, further system calls will be selectively
intercepted, depending on a control variable provided by
user space. In this case, off and size respectively
identify the offset and size of a single contiguous memory
region in the process address space from where system calls
are always allowed to be executed, regardless of the
control variable. (Typically, this area would include the
area of memory containing the C library.)
switch points to a variable that is a fast switch to
allow/block system call execution without the overhead of
doing another system call to re-configure Syscall User
Dispatch. This control variable can either be set to
SYSCALL_DISPATCH_FILTER_BLOCK to block system calls from
executing or to SYSCALL_DISPATCH_FILTER_ALLOW to
temporarily allow them to be executed. This value is
checked by the kernel on every system call entry, and any
unexpected value will raise an uncatchable SIGSYS at that
time, killing the application.
When a system call is intercepted, the kernel sends a
thread-directed SIGSYS signal to the triggering thread.
Various fields will be set in the siginfo_t structure (see
sigaction(2)) associated with the signal:
• si_signo will contain SIGSYS.
• si_call_addr will show the address of the system call
instruction.
• si_syscall and si_arch will indicate which system call
was attempted.
• si_code will contain SYS_USER_DISPATCH.
• si_errno will be set to 0.
The program counter will be as though the system call
happened (i.e., the program counter will not point to the
system call instruction).
When the signal handler returns to the kernel, the system
call completes immediately and returns to the calling
thread, without actually being executed. If necessary
(i.e., when emulating the system call on user space.), the
signal handler should set the system call return value to a
sane value, by modifying the register context stored in the
ucontext argument of the signal handler. See sigaction(2),
sigreturn(2), and getcontext(3) for more information.
PR_SYS_DISPATCH_OFF
Syscall User Dispatch is disabled for that thread.
The setting is not preserved across fork(2), clone(2), or
execve(2).
On success, 0 is returned. On error, -1 is returned, and errno is
set to indicate the error.
EFAULT switch is an invalid address.
EINVAL op is PR_SYS_DISPATCH_ON and the memory range specified is
outside the address space of the process.
EINVAL op is invalid.
Linux. x86 only.
Linux 5.11 (x86).
prctl(2)
For more information, see the kernel source file Documentation/
admin-guide/syscall-user-dispatch.rst
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-0P5R-_1S7ET_SYSCALL_USER_DISPATCH(2const)
Pages that refer to this page: prctl(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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