unw_resume man page

unw_resume -- resume execution in a particular stack frame


#include <libunwind.h>

int unw_resume(unw_cursor_t *cp);


The unw_resume() routine resumes execution at the stack frame  identified by cp. The behavior of this routine differs  slightly for local and remote unwinding.

For local unwinding, unw_resume() restores the machine state  and then directly resumes execution in the target stack frame. Thus  unw_resume() does not return in this case. Restoring the  machine state normally involves restoring the “preserved”  (callee-saved) registers. However, if execution in any of the stack  frames younger (more deeply nested) than the one identified by  cp was interrupted by a signal, then unw_resume() will  restore all registers as well as the signal mask. Attempting to call  unw_resume() on a cursor which identifies the stack frame of  another thread results in undefined behavior (e.g., the program may  crash).

For remote unwinding, unw_resume() installs the machine state  identified by the cursor by calling the access_reg and  access_fpreg accessor callbacks as needed. Once that is  accomplished, the resume accessor callback is invoked. The  unw_resume routine then returns normally (that is, unlikely  for local unwinding, unw_resume will always return for remote  unwinding).

Most platforms reserve some registers to pass arguments to exception  handlers (e.g., IA-64 uses r15-r18 for this  purpose). These registers are normally treated like “scratch”  registers. However, if libunwind is used to set an exception  argument register to a particular value (e.g., via  unw_set_reg()), then unw_resume() will install this  value as the contents of the register. In other words, the exception  handling arguments are installed even in cases where normally only the  “preserved” registers are restored.

Note that unw_resume() does not invoke any unwind  handlers (aka, “personality routines”). If a program needs this, it  will have to do so on its own by obtaining the unw_proc_info_t of each unwound frame and appropriately processing its unwind handler  and language-specific data area (lsda). These steps are generally  dependent on the target-platform and are regulated by the  processor-specific ABI (application-binary interface).

Return Value

For local unwinding, unw_resume() does not return on success.  For remote unwinding, it returns 0 on success. On failure, the  negative value of one of the errors below is returned.

Thread and Signal Safety

unw_resume() is thread-safe. If cursor cp is in the  local address-space, this routine is also safe to use from a signal  handler.



An unspecified error occurred.


A register needed by unw_resume() wasn't  accessible.


The instruction pointer identified by  cp is not valid.


The stack frame identified by  cp is not valid.

See Also

libunwind(3), unw_set_reg(3), sigprocmask(2)


David Mosberger-Tang
Email: dmosberger@gmail.com
WWW: http://www.nongnu.org/libunwind/.

Referenced By

libunwind(3), unw_create_addr_space(3).

16 August 2007 Programming Library