\"      $NetBSD: backtrace.3,v 1.11 2025/01/23 12:08:12 christos Exp $
\"
\" Copyright (c) 2012 The NetBSD Foundation, Inc.
\" All rights reserved.
\"
\" This code is derived from software contributed to The NetBSD Foundation
\" by Christos Zoulas
\"
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\" modification, are permitted provided that the following conditions
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\" 1. Redistributions of source code must retain the above copyright
\"    notice, this list of conditions and the following disclaimer.
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Dd January 22, 2025
Dt BACKTRACE 3
Os
Sh NAME
Nm backtrace ,
Nm backtrace_symbols ,
Nm backtrace_symbols_fd ,
Nm backtrace_symbols_fmt ,
Nm backtrace_symbols_fd_fmt
Nd fill in the backtrace of the currently executing thread
Sh LIBRARY
Lb libexecinfo
Sh SYNOPSIS
In execinfo.h
Ft size_t
Fn backtrace "void **addrlist" "size_t len"
Ft "int"
Fn backtrace_sandbox_init "void"
Ft "void"
Fn backtrace_sandbox_fini "void"
Ft "char **"
Fn backtrace_symbols "void * const *addrlist" "size_t len"
Ft int
Fn backtrace_symbols_fd "void * const *addrlist" "size_t len" "int fd"
Ft "char **"
Fn backtrace_symbols_fmt "void * const *addrlist" "size_t len" "const char *fmt"
Ft int
Fn backtrace_symbols_fd_fmt "void * const *addrlist" "size_t len" "int fd" "const char *fmt"
Sh DESCRIPTION
The
Fn backtrace
function places into the array pointed by
Fa addrlist
the array of the values of the program counter for each frame called up to
Fa len
frames.
The number of frames found (which can be fewer than
Fa len )
is returned.
Pp
The
Fn backtrace_sandbox_init
and
Fn backtrace_sandbox_fini
functions are intended to enable sandbox usage of
Nm .
The
Fn backtrace_sandbox_init
function must be called before the sandbox is entered, and will acquire any
resources needed to function in a sandbox.
The
Fn backtrace_sandbox_fini
function will release the resources it acquired.
Pp
The
Fn backtrace_symbols_fmt
function takes an array of previously filled addresses from
Fn backtrace
in
Fa addrlist
of
Fa len
elements, and uses
Fa fmt
to format them.
The formatting characters available are:
Bl -tag -width a -offset indent
It Dv a
The numeric address of each element as would be printed using %p.
It Dv n
The name of the nearest function symbol (smaller than the address element)
as determined by
Xr dladdr 3
if the symbol was dynamic, or looked up in the executable if static and
the /proc filesystem is available to determine the executable path.
It Dv d
The difference of the symbol address and the address element printed
using 0x%tx.
It Dv D
The difference of the symbol address and the address element printed using
+0x%tx if non-zero, or nothing if zero.
It Dv f
The filename of the symbol as determined by
Xr dladdr 3 .
El
Pp
The array of formatted strings is returned as a contiguous memory address which
can be freed by a single
Xr free 3 .
Pp
The
Fn backtrace_symbols
function is equivalent of calling
Fn backtrace_symbols_fmt
with a format argument of
Dq "%a <%n%D> at %f"
Pp
The
Fn backtrace_symbols_fd
and
Fn backtrace_symbols_fd_fmt
are similar to the non _fd named functions, only instead of returning
an array of strings, they print a new-line separated array of strings in
fd, and return
Dv 0
on success and
Dv \-1
on failure.
Sh RETURN VALUES
The
Fn backtrace
function returns the number of elements that were filled in the backtrace.
The
Fn backtrace_symbols
and
Fn backtrace_symbols_fmt
return a string array on success, and
Dv NULL
on failure, setting
Va errno .
Diagnostic output may also be produced by the ELF symbol lookup functions.
Sh SEE ALSO
Xr dladdr 3 ,
Xr elf 3
Sh HISTORY
The
Fn backtrace
library of functions first appeared in
Nx 7.0 .
Sh BUGS
Bl -enum
It
Errors should not be printed but communicated to the caller differently.
It
Because these functions use
Xr elf 3
this is a separate library instead of being part of libc/libutil
so that no library dependencies are introduced.
It
The Linux versions of the functions (there are no _fmt variants) use
Ft int
instead of
Ft size_t
arguments.
It
Since
Xr dladdr 3
only deals with dynamic symbols, we need to find the symbols from the main
portion of the program.
For that we need to locate the executable, and we use procfs for
finding it, which is not portable.
El

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