.\" Copyright (c) 2007 Apple Inc.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of Apple Inc. ("Apple") nor the names of its
.\"    contributors may be used to endorse or promote products derived from
.\"    this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY APPLE AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"
.Dd March 1, 2018
.Dt backtrace 3
.Os "Darwin"
.Sh NAME
.Nm backtrace ,
.Nm backtrace_symbols ,
.Nm backtrace_symbols_fd ,
.Nm backtrace_image_offsets ,
.Nm backtrace_from_fp
.Nd call stack backtrace and display functions
.Sh SYNOPSIS
.In execinfo.h
.Ft int
.Fo backtrace
.Fa "void** array"
.Fa "int size"
.Fc
.Ft char**
.Fo backtrace_symbols
.Fa "void* const* array"
.Fa "int size"
.Fc
.Ft void
.Fo backtrace_symbols_fd
.Fa "void* const* array"
.Fa "int size"
.Fa "int fd"
.Fc
.Ft void
.Fo backtrace_image_offsets
.Fa "void* const* array"
.Fa "struct image_offset *image_offsets"
.Fa "int size"
.Fc
.Ft int
.Fo backtrace_from_fp
.Fa "void* startfp"
.Fa "void** array"
.Fa "int size"
.Fc
.Ft size_t
.Fo backtrace_async
.Fa "void** array"
.Fa "size_t size"
.Fa "uint32_t* task_id"
.Fc
.Sh DESCRIPTION
These routines provide a mechanism to examine the current thread's call stack.
.Pp
.Fn backtrace
writes the function return addresses of the current call stack to the array of
pointers referenced by
.Fa array .
At most,
.Fa size
pointers are written.  The number of pointers actually written to
.Fa array
is returned.
.Pp
.Fn backtrace_symbols
attempts to transform a call stack obtained by
.Fn backtrace
into an array of human-readable strings using
.Fn dladdr .
The array of strings returned has
.Fa size
elements.  It is allocated using
.Fn malloc
and should be released using
.Fn free .
There is no need to free the individual strings in the array.
.Pp
.Fn backtrace_symbols_fd
performs the same operation as
.Fn backtrace_symbols ,
but the resulting strings are immediately written to the file descriptor
.Fa fd ,
and are not returned.
.Pp
.Fn backtrace_image_offsets
attempts to transform a call stack obtained by
.Fn backtrace
into an array of image offsets, for deferred symbolication.  Each entry in the
array has an offset relative to the
.Li __TEXT
section of the image with the given UUID.  The results are written to
.Fa image_offsets
which should be an array of
.Fa size
length.
.Pp
.Fn backtrace_from_fp
takes a backtrace of frames starting from the given frame pointer.
.Pp
.Fn backtrace_async
behaves exactly like
.Fn backtrace
unless it is invoked from a Swift async context.  In that case, instead of
writing the return addresses of the OS call stack, the continuation addresses
of the async invocations that led to the current state are provided.  If
unwinding an async stack rather than an OS stack, the value pointed to by
.Fa task_id
will be set to a non-zero identifier that for the current process uniquely
identifies the async task currently running.  Otherwise, 0 is stored.
.Pp
Note that the continuation addresses provided by
.Fn backtrace_async
have an offset of 1 added to them.  Most symbolication engines will substract
1 from the call stack return addresses in order to symbolicate the call site
rather than the return location.  With a Swift async continuation, substracting
1 from its address would result in an address in a different function.  This
offset allows the returned addresses to be handled correctly by most existing
symbolication engines.
.Pp
.Sh EXAMPLE
.Pp
    #include <execinfo.h>
    #include <stdio.h>
    ...
    void* callstack[128];
    int i, frames = backtrace(callstack, 128);
    char** strs = backtrace_symbols(callstack, frames);
    for (i = 0; i < frames; ++i) {
        printf("%s\\n", strs[i]);
    }
    free(strs);
    ...
.Pp
.Sh HISTORY
.Fn backtrace ,
.Fn backtrace_symbols ,
and
.Fn backtrace_symbols_fd
first appeared in Mac OS X 10.5.
.Fn backtrace_image_offsets
and
.Fn backtrace_from_fp
first appeared macOS 10.14 and iOS 12.
.Fn backtrace_async
first appeared in macOS 12.
.Sh SEE ALSO
.Xr dladdr 3 ,
.Xr malloc 3