.\" Copyright, the authors of the Linux man-pages project
.\"
.\" %%%LICENSE_START(FREELY_REDISTRIBUTABLE)
.\" may be freely modified and distributed
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.TH FUTEX_TRYLOCK_PI 2const 2025-05-30 "Linux man-pages 6.15"
.SH NAME
FUTEX_TRYLOCK_PI \- try to lock a priority-inheritance futex
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR "#include <linux/futex.h>" "  /* Definition of " FUTEX_* " constants */"
.BR "#include <sys/syscall.h>" "  /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.P
.BI "long syscall(SYS_futex, uint32_t *" uaddr ", FUTEX_TRYLOCK_PI);"
.fi
.SH DESCRIPTION
This operation tries to acquire the lock at
.IR uaddr .
It is invoked when a user-space atomic acquire did not
succeed because the futex word was not 0.
.P
Because the kernel has access to more state information than user space,
acquisition of the lock might succeed if performed by the
kernel in cases where the futex word
(i.e., the state information accessible to use-space) contains stale state
.RB ( FUTEX_WAITERS
and/or
.BR FUTEX_OWNER_DIED ).
This can happen when the owner of the futex died.
User space cannot handle this condition in a race-free manner,
but the kernel can fix this up and acquire the futex.
.\" Paraphrasing a f2f conversation with Thomas Gleixner about the
.\" above point (Aug 2015): ###
.\"	There is a rare possibility of a race condition involving an
.\"	uncontended futex with no owner, but with waiters.  The
.\"	kernel-user-space contract is that if a futex is nonzero, you must
.\"	go into kernel.  The futex was owned by a task, and that task dies
.\"	but there are no waiters, so the futex value is non zero.
.\"	Therefore, the next locker has to go into the kernel,
.\"	so that the kernel has a chance to clean up. (CMXCH on zero
.\"	in user space would fail, so kernel has to clean up.)
.\" Darren Hart (Oct 2015):
.\"     The trylock in the kernel has more state, so it can independently
.\"     verify the flags that user space must trust implicitly.
.\"
.SH RETURN VALUE
On error,
\-1 is returned,
and
.I errno
is set to indicate the error.
.P
On success,
.B FUTEX_TRYLOCK_PI
Returns 0 if the futex was successfully locked.
.SH ERRORS
See
.BR futex (2).
.TP
.B EAGAIN
The futex owner thread ID of
.I uaddr
is about to exit,
but has not yet handled the internal state cleanup.
Try again.
.TP
.B EDEADLK
The futex word at
.I uaddr
is already locked by the caller.
.TP
.B EINVAL
The kernel detected an inconsistency between the user-space state at
.I uaddr
and the kernel state.
This indicates either state corruption
or that the kernel found a waiter on
.I uaddr
which is waiting via
.BR FUTEX_WAIT (2const)
or
.BR FUTEX_WAIT_BITSET (2const).
.TP
.B ENOMEM
The kernel could not allocate memory to hold state information.
.TP
.B ENOSYS
A run-time check determined that the operation is not available.
The PI-futex operations are not implemented on all architectures and
are not supported on some CPU variants.
.TP
.B EPERM
The caller is not allowed to attach itself to the futex at
.IR uaddr .
(This may be caused by a state corruption in user space.)
.TP
.B ESRCH
The thread ID in the futex word at
.I uaddr
does not exist.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.6.18.
.\" commit c87e2837be82df479a6bae9f155c43516d2feebc
.SH SEE ALSO
.BR futex (2)