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.Dd March 6, 2022
.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 "int flags"
.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 int
.Fn stack_save_td "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" .
It can be declared in the usual ways, including on the stack, and optionally
initialized with
.Fn stack_zero ,
though this is not necessary before saving a trace.
It can also be dynamically allocated with
.Fn stack_create .
The
.Ar flags
argument is passed to
.Xr malloc 9 .
This dynamic allocation must be freed with
.Fn stack_destroy .
.Pp
A trace of the current thread's kernel call stack may be captured using
.Fn stack_save .
.Fn stack_save_td
can be used to capture the kernel stack of a caller-specified thread.
Callers of
.Fn stack_save_td
must own the thread lock of the specified thread,
and the thread's stack must not be swapped out.
.Fn stack_save_td
can capture the kernel stack of a running thread, though note that this is
not implemented on all platforms.
If the thread is running, the caller must also hold the process lock for the
target 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
returns 0 when the stack capture was successful and a non-zero error number
otherwise.
In particular,
.Er EBUSY
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 .