---
name: "stack"
section: "9"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/stack/9"
---

# stack(9)

kernel thread stack tracing routines

## NAME

**stack** – kernel thread stack tracing routines

## SYNOPSIS

**#include \<sys/param.h\>**

**#include \<sys/stack.h\>**

In the kernel configuration file: **options DDB** **options STACK**

*struct* *stack* *\**\

**stack\_create**(*int flags*);

*void*\

**stack\_destroy**(*struct stack \*st*);

*int*\

**stack\_put**(*struct stack \*st*, *vm\_offset\_t pc*);

*void*\

**stack\_copy**(*const struct stack \*src*, *struct stack \*dst*);

*void*\

**stack\_zero**(*struct stack \*st*);

*void*\

**stack\_print**(*const struct stack \*st*);

*void*\

**stack\_print\_ddb**(*const struct stack \*st*);

*void*\

**stack\_print\_short**(*const struct stack \*st*);

*void*\

**stack\_print\_short\_ddb**(*const struct stack \*st*);

*void*\

**stack\_sbuf\_print**(*struct sbuf \*sb*, *const struct stack \*st*);

*void*\

**stack\_sbuf\_print\_ddb**(*struct sbuf \*sb*, *const struct stack \*st*);

*void*\

**stack\_save**(*struct stack \*st*);

*int*\

**stack\_save\_td**(*struct stack \*st*, *struct thread \*td*);

## DESCRIPTION

The **stack** 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 **options DDB** and **options STACK** must be compiled into the kernel.

Each stack trace is described by a *struct stack*. It can be declared in the usual ways, including on the stack, and optionally initialized with **stack\_zero**(), though this is not necessary before saving a trace. It can also be dynamically allocated with **stack\_create**(). The *flags* argument is passed to [malloc(9)](/malloc/9.md). This dynamic allocation must be freed with **stack\_destroy**().

A trace of the current thread's kernel call stack may be captured using **stack\_save**(). **stack\_save\_td**() can be used to capture the kernel stack of a caller-specified thread. Callers of **stack\_save\_td**() must own the thread lock of the specified thread, and the thread's stack must not be swapped out. **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.

**stack\_print**() and **stack\_print\_short**() may be used to print a stack trace using the kernel [printf(9)](/printf/9.md), and may sleep as a result of acquiring [sx(9)](/sx/9.md) locks in the kernel linker while looking up symbol names. In locking-sensitive environments, the unsynchronized **stack\_print\_ddb**() and **stack\_print\_short\_ddb**() variants may be invoked. This function bypasses kernel linker locking, making it usable in [ddb(4)](/ddb/4.md), but not in a live system where linker data structures may change.

**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 *sb* must be an initialized struct sbuf as described in [sbuf(9)](/sbuf/9.md). This function may sleep if an auto-extending struct sbuf is used, or due to kernel linker locking. In locking-sensitive environments, such as [ddb(4)](/ddb/4.md), the unsynchronized **stack\_sbuf\_print\_ddb**() variant may be invoked to avoid kernel linker locking; it should be used with a fixed-length sbuf.

The utility functions **stack\_zero**, **stack\_copy**, and **stack\_put** may be used to manipulate stack data structures directly.

## RETURN VALUES

**stack\_put**() returns 0 on success. Otherwise the struct stack does not contain space to record additional frames, and a non-zero value is returned.

**stack\_save\_td**() returns 0 when the stack capture was successful and a non-zero error number otherwise. In particular, EBUSY is returned if the thread was running in user mode at the time that the capture was attempted, and EOPNOTSUPP is returned if the operation is not implemented.

## SEE ALSO

[ddb(4)](/ddb/4.md), [printf(9)](/printf/9.md), [sbuf(9)](/sbuf/9.md), [sx(9)](/sx/9.md)

## AUTHORS

The **stack** function suite was created by Antoine Brodin. **stack** was extended by Robert Watson for general-purpose use outside of [ddb(4)](/ddb/4.md).