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STACK(9) Kernel Developer's Manual STACK(9)


stackkernel thread stack tracing routines


#include <sys/param.h>
#include <sys/stack.h>

In the kernel configuration file:
options DDB
options STACK

struct stack *

stack_destroy(struct stack *st);

stack_put(struct stack *st, vm_offset_t pc);

stack_copy(const struct stack *src, struct stack dst);

stack_zero(struct stack *st);

stack_print(const struct stack *st);

stack_print_ddb(const struct stack *st);

stack_print_short(const struct stack *st);

stack_print_short_ddb(const struct stack *st);

stack_sbuf_print(struct sbuf sb*, const struct stack *st);

stack_sbuf_print_ddb(struct sbuf sb*, const struct stack *st);

stack_save(struct stack *st);

stack_save_td(struct stack *st, struct thread *td);

stack_save_td_running(struct stack *st, struct thread *td);


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. Before a trace may be created or otherwise manipulated, storage for the trace must be allocated with (), which may sleep. Memory associated with a trace is freed by calling ().

A trace of the current kernel thread's call stack may be captured using (). () and () 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. stack_save_td() can capture the stack of a kernel thread that is not running or swapped out at the time of the call. stack_save_td_running() can capture the stack of a running kernel thread.

() and () 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 () and () 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.

() 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 () 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.


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_running() returns 0 when the stack capture was successful and a non-zero error number otherwise. In particular, EAGAIN 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.


ddb(4), printf(9), sbuf(9), sx(9)


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

September 10, 2015 Debian