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112 lines
4.9 KiB
Text
112 lines
4.9 KiB
Text
The central data structure of the unwind API is the unwind cursor.
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This structure tracks the frame registers and the preserved registers.
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The distinction between frame registers and preserved registers is
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important: the former represent the *current* value of a register (as
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it existed at the current IP); the latter represent the *saved* value
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of a register (i.e., the value that existed on entry to the current
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procedure). The unwind API defines a handful of well-known frame
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"registers":
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- ip: the instruction pointer (pc)
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- rp: the return pointer (rp, aka "return address" or "return link")
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- sp: the stack pointer (memory stack pointer, in the case of ia64)
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- fp: the frame pointer
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- first_ip: the starting address of the current "procedure"
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- handler: a pointer to an architecture & language-specific
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"personality" routine
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- lsda: a pointer to an architecture & language-specific
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data-area
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The API defines no well-known preserved registers. Each architecture
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can define additional registers as needed. Of course, a portable
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application may only rely on well-known registers. The names for
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preserved registers are defined in the architecture-specific header
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file <unwind-ARCH.h>. For example, to get the IA-64-specific register
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names, an application would do:
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#include <unwind-ia64.h>
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The API is designed to handle two primary cases: unwinding within the
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current (local) process and unwinding of another ("remote") process
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(e.g., through ptrace()). In the local case, the initial machine
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state is captured by an unwind context (currently the same as
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ucontext_t). In the remote case, the initial machine state is
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captured by an unwind accessor structure, which provides callback
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routines for reading/writing memory and registers and for obtaining
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unwind information.
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Once a cursor has been initialized, you can step through the call
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chain with the unw_step() routine. The frame registers and the
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preserved state can then be accessed with unw_get_reg() or modified
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with unw_set_reg(). For floating-point registers, there are separate
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unw_get_fpreg() and unw_set_fpreg() routines (on some arches, e.g.,
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Alpha, these could be just aliases for unw_{g,s}et_reg()). The
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unw_resume() routine can be used to resume execution at an arbitrary
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point in the call-chain (as identified by an unwind cursor). This is
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intended for exception handling and, at least for now, the intention
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is to support this routine only for the local case. Kevin, if you
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feel gdb could benefit from such a routine, I'd be interested to hear
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about it.
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Note that it is perfectly legal to make copies of the unwind cursor.
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This makes it possible, e.g., to obtain an unwind context, modify the
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state in an earlier call frame, and then resume execution at the point
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at which the unwind context was captured.
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Here is a quick example of how to use the unwind API to do a simple
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stack trace:
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unw_cursor_t cursor;
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unw_word_t ip, sp;
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ucontext_t uc;
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getcontext(&uc);
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unw_init_local(&cursor, &uc);
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do
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{
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unw_get_reg(&cursor, UNW_REG_IP, &ip);
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unw_get_reg(&cursor, UNW_REG_SP, &sp);
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printf ("ip=%016lx sp=%016lx\n", ip, sp);
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}
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while (unw_step (&cursor) > 0);
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Note that this particular example should work on pretty much any
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architecture, as it doesn't rely on any arch-specific registers.
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* Multiarchitecture support
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If libunwind is configured for a target other than the local (native)
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host, the library is installed as libunwind-$ARCH, where $ARCH is
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the target architecture name (e.g., ia32, ia64, or alpha). Similarly,
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the header file is installed as libunwind-$ARCH.
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With this setup, an application should:
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- include <libunwind.h>, and
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- link against -lunwind
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if the application needs to use the unwinder of the host. An
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application wanting to use the unwinder for a different target (e.g.,
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a cross-debugger) should:
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- include <libunwind-$ARCH.h>, and
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- link against -lunwind-$ARCH
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The global symbols exported by -lunwind-$ARCH are unique such that the
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same application can be linked against the separate unwind libraries
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of multiple targets. However, a single compilation unit can include
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the header file for only one target. For example, foo.c might include
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<libunwind-ia64.h> and bar.c might include <libunwind.h> and the
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entire application would have to be linked against both -lunwind and
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-lunwind-ia64.
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Note: the unwind header files of all targets have a common dependency
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on libunwind-common.h. To avoid version conflicts, it is necessary to
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ensure that the unwind libraries for all targets were derived from the
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same release of libunwind. That is, if the unwind library for one
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target is upgraded to a newer version, the libraries for all other
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targets also need to be upgraded.
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Note 2: The assumption is that a cross-unwinder can handle all
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interesting flavors of a target. For example, the unwinder for the
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ia64 target is expected to be able to handle both Linux and HP-UX.
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