illumos: manual page: backtrace_symbols

WALKCONTEXT(3C) Standard C Library Functions WALKCONTEXT(3C)

NAME

walkcontext, addrtosymstr, printstack, backtrace, backtrace_symbols,
backtrace_symbols_fd - walk stack pointed to by ucontext

SYNOPSIS

#include <ucontext.h>

int walkcontext(const ucontext_t *uptr,
int (*operate_func)(uintptr_t, int, void *), void *usrarg);

int addrtosymstr(uintptr_t addr, char *buffer, int len);

int printstack(int fd);

#include <execinfo.h>

int backtrace(void **buffer, int size);

char **backtrace_symbols(void *const *buffer, int size);

void backtrace_symbols_fd(void *const *buffer, int size, int fd);

DESCRIPTION

The walkcontext() function walks the call stack pointed to by uptr, which
can be obtained by a call to getcontext(2) or from a signal handler
installed with the SA_SIGINFO flag. The walkcontext() function calls the
user-supplied function operate_func for each routine found on the call
stack and each signal handler invoked. The user function is passed three
arguments: the PC at which the call or signal occurred, the signal number
that occurred at this PC (0 if no signal occurred), and the third
argument passed to walkcontext(). If the user function returns a non-zero
value, walkcontext() returns without completing the callstack walk.

The addrtosymstr() function attempts to convert a PC into a symbolic
representation of the form

objname'funcname+0xoffset[0xPC]

where objname is the module in which the PC is located, funcname is the
name of the function, and offset is the offset from the beginning of the
function. The objname, funcname, and offset values are obtained from
dladdr(3C) and might not always be present. The resulting string is
written to the user-supplied buffer. Should the length of the string be
larger than the user-supplied buffer, only the portion of the string that
will fit is written and null-terminated.

The printstack() function uses walkcontext() to print a symbolic stack
trace to the specified file descriptor. This is useful for reporting
errors from signal handlers. The printstack() function uses dladdr1()
(see dladdr(3C)) to obtain symbolic symbol names. As a result, only
global symbols are reported as symbol names by printstack().

The backtrace() function uses walkcontext() to generate a stack's program
counter values for the calling thread and place these values into the
array specified by the buffer argument. The size argument specifies the
maximum number of program counters that will be recorded. This function
is provided for compatibility with the GNU libc used on Linux systems,
glibc.

The backtrace_symbols() function translates the numerical program counter
values previously recorded by a call to backtrace() in the buffer
argument, and converts, where possible, each PC to a string indicating
the module, function and offset of each call site. The number of symbols
present in the array must be passed in with the size argument.

The set of strings is returned in an array obtained from a call to
malloc(3C). It is the responsibility of the caller to pass the returned
pointer to free(). The individual strings must not be freed. Since
malloc() is used to obtain the needed space, this function is MT-Safe
rather than Async-Signal-Safe and cannot be used reliably from a signal
handler. This function is provided for glibc compatibility.

The backtrace_symbols_fd() function translates the numerical program
counter values previously recorded by a call to backtrace() in the buffer
argument, and converts, where possible, each PC to a string indicating
the module, function, and offset of each call site. These strings are
written to the file descriptor specified in the fd argument. This
function is provided for glibc compatibility.

RETURN VALUES

Upon successful completion, walkcontext() and printstack() return 0. If
walkcontext() cannot read the stack or the stack trace appears corrupted,
both functions return -1.

The addrtosymstr() function returns the number of characters necessary to
hold the entire string representation of the passed in address,
irrespective of the size of the user-supplied buffer.

The backtrace() function returns the number of stack frames captured.

The backtrace_symbols() function returns a pointer to an array containing
string representations of the calling sequence.

ERRORS

No error values are defined.

USAGE

The walkcontext() function is typically used to obtain information about
the call stack for error reporting, performance analysis, or diagnostic
purposes. Many library functions are not Async-Signal-Safe and should not
be used from a signal handler. If walkcontext() is to be called from a
signal handler, careful programming is required. In particular,
stdio(3C) and malloc(3C) cannot be used.

The walkstack(), addrtosymstr(), printstack(), backtrace(), and
backtrace_symbols_fd() functions are Async-Signal-Safe and can be called
from a signal handler. The string representation generated by
addrtosymstr() and displayed by printstack(), backtrace_symbols() and
backtrace_symbols_fd() is unstable and will change depending on the
information available in the modules that comprise the stack trace.

Tail-call optimizations on SPARC eliminate stack frames that would
otherwise be present. For example, if the code is of the form

#include <stdio.h>

main()
{
bar();
exit(0);
}

bar()
{
int a;
a = foo(fileno(stdout));
return (a);
}

foo(int file)
{
printstack(file);
}

compiling without optimization will yield a stack trace of the form

/tmp/q:foo+0x8
/tmp/q:bar+0x14
/tmp/q:main+0x4
/tmp/q:_start+0xb8

whereas with higher levels of optimization the output is

/tmp/q:main+0x10
/tmp/q:_start+0xb8

since both the call to foo() in main and the call to bar() in foo() are
handled as tail calls that perform a return or restore in the delay slot.
For further information, see The SPARC Architecture Manual.

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