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.\" $FreeBSD: head/share/man/man9/stack.9 301719 2016-06-09 06:55:00Z trasz $
.\"
.Dd September 10, 2015
.Dt STACK 9
.Os
.Sh NAME
.Nm stack
.Nd kernel thread stack tracing routines
.Sh SYNOPSIS
.In sys/param.h
.In sys/stack.h
.Pp
In the kernel configuration file:
.Cd "options DDB"
.Cd "options STACK"
.Pp
.Ft struct stack *
.Fn stack_create "void"
.Ft void
.Fn stack_destroy "struct stack *st"
.Ft int
.Fn stack_put "struct stack *st" "vm_offset_t pc"
.Ft void
.Fn stack_copy "const struct stack *src" "struct stack dst"
.Ft void
.Fn stack_zero "struct stack *st"
.Ft void
.Fn stack_print "const struct stack *st"
.Ft void
.Fn stack_print_ddb "const struct stack *st"
.Ft void
.Fn stack_print_short "const struct stack *st"
.Ft void
.Fn stack_print_short_ddb "const struct stack *st"
.Ft void
.Fn stack_sbuf_print "struct sbuf sb*" "const struct stack *st"
.Ft void
.Fn stack_sbuf_print_ddb "struct sbuf sb*" "const struct stack *st"
.Ft void
.Fn stack_save "struct stack *st"
.Ft void
.Fn stack_save_td "struct stack *st" "struct thread *td"
.Ft int
.Fn stack_save_td_running "struct stack *st" "struct thread *td"
.Sh DESCRIPTION
The
.Nm
KPI allows querying of kernel stack trace information and the automated
generation of kernel stack trace strings for the purposes of debugging and
tracing.
To use the KPI, at least one of
.Cd "options DDB"
and
.Cd "options STACK"
must be compiled into the kernel.
.Pp
Each stack trace is described by a
.Vt "struct stack" .
Before a trace may be created or otherwise manipulated, storage for the trace
must be allocated with
.Fn stack_create ,
which may sleep.
Memory associated with a trace is freed by calling
.Fn stack_destroy .
.Pp
A trace of the current kernel thread's call stack may be captured using
.Fn stack_save .
.Fn stack_save_td
and
.Fn stack_save_td_running
can also be used to capture the stack of a caller-specified thread.
Callers of these functions must own the thread lock of the specified thread.
.Fn stack_save_td
can capture the stack of a kernel thread that is not running or
swapped out at the time of the call.
.Fn stack_save_td_running
can capture the stack of a running kernel thread.
.Pp
.Fn stack_print
and
.Fn stack_print_short
may be used to print a stack trace using the kernel
.Xr printf 9 ,
and may sleep as a result of acquiring
.Xr sx 9
locks in the kernel linker while looking up symbol names.
In locking-sensitive environments, the unsynchronized
.Fn stack_print_ddb
and
.Fn stack_print_short_ddb
variants may be invoked.
This function bypasses kernel linker locking, making it usable in
.Xr ddb 4 ,
but not in a live system where linker data structures may change.
.Pp
.Fn stack_sbuf_print
may be used to construct a human-readable string, including conversion (where
possible) from a simple kernel instruction pointer to a named symbol and
offset.
The argument
.Ar sb
must be an initialized
.Dv struct sbuf
as described in
.Xr sbuf 9 .
This function may sleep if an auto-extending
.Dv struct sbuf
is used, or due to kernel linker locking.
In locking-sensitive environments, such as
.Xr ddb 4 ,
the unsynchronized
.Fn stack_sbuf_print_ddb
variant may be invoked to avoid kernel linker locking; it should be used with
a fixed-length sbuf.
.Pp
The utility functions
.Nm stack_zero ,
.Nm stack_copy ,
and
.Nm stack_put
may be used to manipulate stack data structures directly.
.Sh RETURN VALUES
.Fn stack_put
returns 0 on success.
Otherwise the
.Dv struct stack
does not contain space to record additional frames, and a non-zero value is
returned.
.Pp
.Fn stack_save_td_running
returns 0 when the stack capture was successful and a non-zero error number
otherwise.
In particular,
.Er EAGAIN
is returned if the thread was running in user mode at the time that the
capture was attempted, and
.Er EOPNOTSUPP
is returned if the operation is not implemented.
.Sh SEE ALSO
.Xr ddb 4 ,
.Xr printf 9 ,
.Xr sbuf 9 ,
.Xr sx 9
.Sh AUTHORS
.An -nosplit
The
.Nm
function suite was created by
.An Antoine Brodin .
.Nm
was extended by
.An Robert Watson
for general-purpose use outside of
.Xr ddb 4 .