stack_sbuf_print_ddb9
Kernel thread stack tracing routines
Part of stack(9).
Usage
In the
kernel
configuration
file:
options
DDB
options
STACK
struct stack
*
void
int
void
void
void
void
void
void
void
void
void
int
Overview
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). 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), and may sleep as a result of acquiring sx(9) 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), 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). 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), 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.
History
- FreeBSD 16.0-CURRENT (2026-10-27) to FreeBSD 14.0-RELEASE: the text shown here
- FreeBSD 13.5-RELEASE to FreeBSD 13.3-RELEASE
- FreeBSD 13.2-RELEASE to FreeBSD 13.0-RELEASE
- FreeBSD 12.4-RELEASE
- FreeBSD 12.3-RELEASE
- FreeBSD 12.2-RELEASE
- FreeBSD 12.1-RELEASE
- FreeBSD 12.0-RELEASE
- FreeBSD 11.4-RELEASE
- FreeBSD 11.3-RELEASE
- FreeBSD 11.2-RELEASE
- FreeBSD 11.1-RELEASE
- FreeBSD 11.0-RELEASE
- FreeBSD 10.4-RELEASE
- FreeBSD 10.3-RELEASE
- FreeBSD 10.2-RELEASE
- FreeBSD 10.1-RELEASE
- FreeBSD 10.0-RELEASE
- FreeBSD 9.3-RELEASE
- FreeBSD 9.2-RELEASE
- FreeBSD 9.1-RELEASE
- FreeBSD 9.0-RELEASE
- FreeBSD 8.4-RELEASE
- FreeBSD 8.3-RELEASE
- FreeBSD 8.2-RELEASE
- FreeBSD 8.1-RELEASE
- FreeBSD 8.0-RELEASE
- FreeBSD 7.4-RELEASE
- FreeBSD 7.3-RELEASE
- FreeBSD 7.2-RELEASE
- FreeBSD 7.1-RELEASE