unw_get_reg man page
unw_get_reg -- get register contents
int unw_get_reg(unw_cursor_t *cp, unw_regnum_t reg, unw_word_t *valp);
The unw_get_reg() routine reads the value of register reg in the stack frame identified by cursor cp and stores the value in the word pointed to by valp.
The register numbering is target-dependent and described in separate manual pages (e.g., libunwind-ia64(3) for the IA-64 target). Furthermore, the exact set of accessible registers may depend on the type of frame that cp is referring to. For ordinary stack frames, it is normally possible to access only the preserved (“callee-saved”) registers and frame-related registers (such as the stack-pointer). However, for signal frames (see unw_is_signal_frame(3)), it is usually possible to access all registers.
Note that unw_get_reg() can only read the contents of registers whose values fit in a single word. See unw_get_fpreg(3) for a way to read registers which do not fit this constraint.
On successful completion, unw_get_reg() returns 0. Otherwise the negative value of one of the error-codes below is returned.
Thread and Signal Safety
unw_get_reg() is thread-safe as well as safe to use from a signal handler.
An unspecified error occurred.
An attempt was made to read a register that is either invalid or not accessible in the current frame.
In addition, unw_get_reg() may return any error returned by the access_mem(), access_reg(), and access_fpreg() call-backs (see unw_create_addr_space(3)).
libunwind(3), libunwind-ia64(3), unw_get_fpreg(3), unw_is_signal_frame(3), unw_set_reg(3)
libunwind(3), unw_get_fpreg(3), unw_is_fpreg(3), unw_is_signal_frame(3), unw_set_reg(3).