uwx(3X)

NAME

uwx() — Unwind Express Library

SYNOPSIS

cc [flag... ] file... -lunwind [library] ...

#include <uwx.h> 
#include <uwx_self.h> 

DESCRIPTION

The Unwind Express Library is a lightweight library for unwinding a process stack on Integrity systems. It may be used for "self-unwinding," where a thread traces back through its own stack, or for "cross-unwinding," where a debugger or other tool can examine the stack of another target thread or process.

To use the Unwind Express Library, link your application with the -lunwind option on the compiler or linker command line. All source code should include the header file <uwx.h>. Source code that uses the library for self-unwinding should also include the header file <uwx_self.h>.

The Unwind Express Library maintains its entire state within a single unwind environment structure. To unwind a stack, the client application provides an initial context to the unwind environment, then unwinds the stack one frame at a time. After reaching the bottom of the stack, the unwind environment may be reused for a subsequent unwind operation simply by providing a new initial context, or it may be discarded, freeing the memory it was using.

A client application may create any number of separate unwind environments, and each one will operate independently of and completely unaffected by the others.

The Unwind Express Library consists of a core unwind engine that can be adapted to specific environments through the use of callback routines. For self-unwinding, a set of standard callback routines are provided as part of the library. For cross-unwinding, the client must supply callback routines to interface the unwind engine to the target thread or process.

Self-Unwinding

To prepare for self-unwinding, a client application must perform the following steps to create and initialize a new unwind environment:

1.: Call uwx_init() to create a new unwind environment. The return value is a pointer to the unwind environment.
2.: Call uwx_self_init_info() to initialize the standard callback environment. The return value is a pointer to the callback info structure.
3.: Call uwx_register_callbacks() to register the standard callbacks: uwx_self_copyin() and uwx_self_lookupip().

The steps above may be performed at any time in advance of performing a stack unwind. The resulting unwind environment can then be used repeatedly.

To perform a stack unwind from some point in the application, the client must perform the following steps:

1.: Call uwx_self_init_context() to provide the current thread's context as a starting point for unwinding. The stack frame for the caller of this function becomes the current frame.
2.: Call uwx_step() to step back one frame in the call stack. After each step, the new frame becomes the current frame, and the client may call uwx_get_reg() to obtain values from the reconstructed context for the new frame. This step can be repeated until the process reaches the bottom of the stack or a signal context frame.
3.: When a signal context frame is reached, the client may call uwx_self_do_context_frame() to provide a new starting point for unwinding based on the context at the point of interruption. The stack frame for the function interrupted by the signal becomes the current frame. Subsequent calls to uwx_step() will proceed from there.

Cross-Unwinding

To prepare for cross-unwinding, a client application must perform the following steps to create and initialize a new unwind environment:

1.: Call uwx_init() to create a new unwind environment. The return value is a pointer to the unwind environment.
2.: Call uwx_set_remote() to setup the unwind environment for cross unwinding. This call also allows the client to specify the byte order (little-endian or big-endian) of the target process.
3.: Call uwx_register_callbacks() to register the client-provided callback functions. See uwx_register_callbacks(3X) for information on writing callback routines.

The steps above may be performed at any time in advance of performing a stack unwind. The resulting unwind environment can then be used repeatedly.

To perform a stack unwind, the client must perform the following steps:

1.: Call uwx_init_context() to provide the basic initial context as a starting point for unwinding. This initial context consists of the values of ip (instruction pointer), sp (stack pointer), bsp (backing store pointer), and cfm (current frame marker). The frame described by these values becomes the current frame. If the unwind engine needs values from additional registers in the target thread's context, it will request them as needed through the callback interface.
2.: Call uwx_step() to step back one frame in the call stack. After each step, the new frame becomes the current frame, and the client may call uwx_get_reg() to obtain values from the reconstructed context for the new frame. This step can be repeated until the process reaches the bottom of the stack or a signal context frame.
3.: When a signal context frame is reached, the client may call uwx_get_abi_context_code() to obtain the abi and context parameters recorded in the unwind descriptors for the frame. This routine returns the two values in a single int as ((abi << 8) | context). It should then call uwx_init_context() again to provide a new starting point for unwinding based on the context at the point of interruption. The stack frame for the function interrupted by the signal becomes the current frame. Subsequent calls to uwx_step() will proceed from there.

Obtaining Information From the Current Frame's Context

Registers from the current frame may be read using uwx_get_reg() and uwx_get_nat().

The starting address of the current function may be obtained by calling uwx_get_funcstart().

Symbolic information may be obtained, if available, for the current frame's IP by calling uwx_get_module_info(), uwx_get_sym_info(), and uwx_get_source_info().

ERRORS

For the APIs that return pointer values, a NULL pointer indicates a memory allocation error; otherwise a pointer to an opaque object is returned.

For other APIs, an integer status code is returned. These codes are listed here.

UWX_OK: Request successfully completed.
UWX_ABI_FRAME: uwx_step() hit a signal context frame.
UWX_BOTTOM: uwx_step() hit bottom of stack.
UWX_ERR_BADCACHE: The cached information returned by the lookup-ip callback was invalid.
UWX_ERR_BADKEY: The "Lookup IP" callback returned an invalid key in the result vector.
UWX_ERR_BADREGID: The register id is invalid.
UWX_ERR_BADUDESC: The unwind information block contains an invalid descriptor.
UWX_ERR_CANTUNWIND: The unwind engine is unable to unwind from the current frame because of an unsupported descriptor.
UWX_ERR_COPYIN_MSTK: There was an error reading a value from the memory stack.
UWX_ERR_COPYIN_REG: There was an error reading a register from the initial context.
UWX_ERR_COPYIN_RSTK: There was an error reading a value from the register stack backing store.
UWX_ERR_COPYIN_UINFO: There was an error reading the unwind information block.
UWX_ERR_COPYIN_UTBL: There was an error reading the unwind table.
UWX_ERR_IPNOTFOUND: No module information was found for the current frame's IP.
UWX_ERR_LOOKUPERR: The "Lookup IP" callback encountered an error.
UWX_ERR_NOCALLBACKS: No callback routines have been registered.
UWX_ERR_NOCONTEXT: No initial context has been supplied.
UWX_ERR_NOENV: The env parameter is NULL.
UWX_ERR_NOMEM: An attempt to allocate additional memory failed.
UWX_ERR_NOSYM: No symbolic information was found.
UWX_ERR_NOUDESC: An unwind information block was found, but it does not describe the current frame's IP.
UWX_ERR_NOUENTRY: No unwind information block was found for the current frame's IP.
UWX_ERR_PROLOG_UF: The unwind information block is invalid -- the nesting of prologue regions is not balanced.
UWX_ERR_UCACCESS: An unidentified error was received from the uc_access(3) library.
UWX_ERR_UNDEFLABEL: The unwind information block is invalid -- an undefined label is referenced.

EXAMPLES

The following code fragment performs a stack unwind in the current thread:

int status; 
struct uwx_env *env; 
struct uwx_self_info *info; 
uint64_t ip; 

env = uwx_init(); 
info = uwx_self_init_info(env); 
status = uwx_register_callbacks(env, (intptr_t)info, 
                uwx_self_copyin, uwx_self_lookupip); 
status = uwx_self_init_context(env); 
for (;;) { 
        status = uwx_step(env); 
        if (status != UWX_OK) 
                break; 
        status = uwx_get_reg(env, UWX_REG_IP, &ip); 
        printf("IP: %016llx\n", ip); 
} 
uwx_self_free_info(info); 
uwx_free(env); 

LIMITATIONS

The Unwind Express Library currently does not recognize X-format unwind descriptors. These are not currently used by HP compilers.

AUTHOR

The Unwind Express Library was developed by Hewlett-Packard.