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).
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.
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.