.\"	$NetBSD: backtrace.3,v 1.10 2017/10/22 14:07:03 abhinav Exp $
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.\" by Christos Zoulas
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.Dd November 5, 2015
.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 "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_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