.\" -*-nroff-*-

.\" Copyright (c) 2012, 2013 Petr Machata, Red Hat Inc.

.\" Copyright (c) 1997-2005 Juan Cespedes <cespedes@debian.org>

.\"

.\" This program is free software; you can redistribute it and/or

.\" modify it under the terms of the GNU General Public License as

.\" published by the Free Software Foundation; either version 2 of the

.\" License, or (at your option) any later version.

.\"

.\" This program is distributed in the hope that it will be useful, but

.\" WITHOUT ANY WARRANTY; without even the implied warranty of

.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

.\" General Public License for more details.

.\"

.\" You should have received a copy of the GNU General Public License

.\" along with this program; if not, write to the Free Software

.\" Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA

.\" 02110-1301 USA

.\"

.TH LTRACE "1" "January 2013" "" "User Commands"

.SH NAME

ltrace \- A library call tracer

.SH SYNOPSIS

.\"

.\" ---------------------------------------------------------------------------

.\"

.PP

.B ltrace

.\"

.\" What events to trace:

.\"

[\-e \fIfilter\fR|\-L] [\-l|\-\-library=\fIlibrary_pattern\fR]

[\-x \fIfilter\fR] [\-S] [\-b|\-\-no-signals]

.\"

.\" What to display with each event:

.\"

[\-i] [\-w|\-\-where=\fInr\fR] [\-r|\-t|\-tt|\-ttt] [\-T]

.\"

.\" Output formatting:

.\"

[\-F \fIpathlist\fR]

[\-A \fImaxelts\fR] [\-s \fIstrsize\fR] [\-C|\-\-demangle]

[\-a|\-\-align \fIcolumn\fR] [\-n|\-\-indent \fInr\fR]

[\-o|\-\-output \fIfilename\fR]

.\"

.\" Various:

.\"

[\-D|\-\-debug \fImask\fR] [\-u \fIusername\fR]

.\"

.\" What processes to trace:

.\"

[\-f] [\-p \fIpid\fR] [[\-\-] \fIcommand [arg ...]\fR]

.\"

.\" ---------------------------------------------------------------------------

.\"

.PP

.BR ltrace " -c"

.\"

.\" What events to trace:

.\"

[\-e \fIfilter\fR|\-L] [\-l|\-\-library=\fIlibrary_pattern\fR]

[\-x \fIfilter\fR] [\-S]

.\"

.\" Output formatting:

.\"

[\-o|\-\-output \fIfilename\fR]

.\"

.\" What processes to trace:

.\"

[\-f] [\-p \fIpid\fR] [[\-\-] \fIcommand [arg ...]\fR]

.\"

.\" ---------------------------------------------------------------------------

.\"

.PP

.BR ltrace " \-V|\-\-version"

.PP

.BR ltrace " \-h|\-\-help"

.SH DESCRIPTION

.B ltrace

is a program that simply runs the specified

.I command

until it exits.  It intercepts and records the dynamic library calls

which are called by the executed process and the signals which are

received by that process.

It can also intercept and print the system calls executed by the program.

.PP

Its use is very similar to

.BR strace(1) .

.B ltrace

shows parameters of invoked functions and system calls.  To determine

what arguments each function has, it needs external declaration of

function prototypes.  Those are stored in files called \fIprototype

libraries\fR--see ltrace.conf(5) for details on the syntax of these

files.  See the section \fBPROTOTYPE LIBRARY DISCOVERY\fR to learn how

\fBltrace\fR finds prototype libraries.

.SH OPTIONS

.PP

.IP "\-a, \-\-align \fIcolumn"

Align return values in a specific

.IR column

(default column is 5/8 of screen width).

.IP "\-A \fImaxelts"

Maximum number of array elements to print before suppressing the rest

with an ellipsis ("...").  This also limits number of recursive

structure expansions.

.IP "\-b, \-\-no-signals"

Disable printing of signals recieved by the traced process.

.IP \-c

Count time and calls for each library call and report a summary on

program exit.

.IP "\-C, \-\-demangle"

Decode (demangle) low-level symbol names into user-level names.

Besides removing any initial underscore prefix used by the system,

this makes C++ function names readable.

.IP "\-D, \-\-debug \fRmask\fI"

Show debugging output of \fBltrace\fR itself.  \fImask\fR is a number

with internal meaning that's not really well defined at all.

\fImask\fR of 77 shows all debug messages, which is what you usually

need.

.IP "\-e \fIfilter"

A qualifying expression which modifies which library calls to trace.

The format of the filter expression is described in the section

\fBFILTER EXPRESSIONS\fR.  If more than one \-e option appears on the

command line, the library calls that match any of them are traced.  If

no \-e is given, \fB@MAIN\fR is assumed as a default.

.IP \-f

Trace child processes as they are created by

currently traced processes as a result of the fork(2)

or clone(2) system calls.

The new process is attached immediately.

.IP "\-F \fIpathlist"

Contains a colon-separated list of paths.  If a path refers to a

directory, that directory is considered when prototype libraries are

searched (see the section \fBPROTOTYPE LIBRARY DISCOVERY\fR).  If it refers to

a file, that file is imported implicitly to all loaded prototype

libraries.

.IP "\-h, \-\-help"

Show a summary of the options to ltrace and exit.

.IP \-i

Print the instruction pointer at the time of the library call.

.IP "\-l, \-\-library \fIlibrary_pattern"

Display only calls to functions implemented by libraries that match

.I library_pattern.

Multiple library patters can be specified with several instances of

this option.  Syntax of library_pattern is described in section

\fBFILTER EXPRESSIONS\fR.

Note that while this option selects calls that might be directed to

the selected libraries, there's no actual guarantee that the call

won't be directed elsewhere due to e.g. LD_PRELOAD or simply

dependency ordering.  If you want to make sure that symbols in given

library are actually called, use \fB-x @\fIlibrary_pattern\fR instead.

.IP \-L

When no -e option is given, don't assume the default action of

\fB@MAIN\fR.

.IP "\-n, \-\-indent \fInr"

Indent trace output by \fInr\fR spaces for each level of call

nesting. Using this option makes the program flow visualization easy

to follow.  This indents uselessly also functions that never return,

such as service functions for throwing exceptions in the C++ runtime.

.IP "\-o, \-\-output \fIfilename"

Write the trace output to the file \fIfilename\fR rather than to

stderr.

.IP "\-p \fIpid"

Attach to the process with the process ID \fIpid\fR and begin tracing.

This option can be used together with passing a command to execute.

It is possible to attach to several processes by passing more than one

option \-p.

.IP \-r

Print a relative timestamp with each line of the trace.  This records

the time difference between the beginning of successive lines.

.IP "\-s \fIstrsize"

Specify the maximum string size to print (the default is 32).

.IP \-S

Display system calls as well as library calls

.IP \-t

Prefix each line of the trace with the time of day.

.IP \-tt

If given twice, the time printed will include the microseconds.

.IP \-ttt

If given thrice, the time printed will include the microseconds and

the leading portion will be printed as the number of seconds since the

epoch.

.IP \-T

Show  the  time  spent inside each call. This records the time difference

between the beginning and the end of each call.

.IP "\-u \fIusername"

Run command with the userid, groupid and supplementary groups of

.IR username .

This option is only useful when running as root and enables the

correct execution of setuid and/or setgid binaries.

.IP "\-w, --where \fInr"

Show backtrace of \fInr\fR stack frames for each traced function. This

option enabled only if libunwind support was enabled at compile time.

.IP "\-x \fIfilter"

A qualifying expression which modifies which symbol table entry points

to trace.  The format of the filter expression is described in the

section \fBFILTER EXPRESSIONS\fR.  If more than one \-x option appears

on the command line, the symbols that match any of them are traced.

No entry points are traced if no \-x is given.

.IP "\-V, \-\-version"

Show the version number of ltrace and exit.

.SH FILTER EXPRESSIONS

Filter expression is a chain of glob- or regexp-based rules that are

used to pick symbols for tracing from libraries that the process uses.

Most of it is intuitive, so as an example, the following would trace

calls to malloc and free, except those done by libc:

-e malloc+free-@libc.so*

This reads: trace malloc and free, but don't trace anything that comes

from libc.  Semi-formally, the syntax of the above example looks

approximately like this:

{[+-][\fIsymbol_pattern\fR][@\fIlibrary_pattern\fR]}

\fISymbol_pattern\fR is used to match symbol names,

\fIlibrary_pattern\fR to match library SONAMEs.  Both are implicitly

globs, but can be regular expressions as well (see below).  The glob

syntax supports meta-characters \fB*\fR and \fB?\fR and character

classes, similarly to what basic bash globs support.  \fB^\fR and

\fB$\fR are recognized to mean, respectively, start and end of given

name.

Both \fIsymbol_pattern\fR and \fIlibrary_pattern\fR have to match the

whole name.  If you want to match only part of the name, surround it

with one or two *'s as appropriate.  The exception is if the pattern

is not mentioned at all, in which case it's as if the corresponding

pattern were \fB*\fR.  (So \fBmalloc\fR is really \fBmalloc@*\fR and

\fB@libc.*\fR is really \fB*@libc.*\fR.)

In libraries that don't have an explicit SONAME, basename is taken for

SONAME.  That holds for main binary as well: \fB/bin/echo\fR has an

implicit SONAME of \fBecho\fR.  In addition to that, special library

pattern \fBMAIN\fR always matches symbols in the main binary and never

a library with actual SONAME \fBMAIN\fR (use e.g. \fB^MAIN\fR or

\fB[M]AIN\fR for that).

If the symbol or library pattern is surrounded in slashes (/like

this/), then it is considered a regular expression instead.  As a

shorthand, instead of writing \fB/x/@/y/\fR, you can write

\fB/x@y/\fR.

If the library pattern starts with a slash, it is not a SONAME

expression, but a path expression, and is matched against the library

path name.

The first rule may lack a sign, in which case \fB+\fR is assumed.  If,

on the other hand, the first rule has a \fB-\fR sign, it is as if

there was another rule \fB@\fR in front of it, which has the effect of

tracing complement of given rule.

The above rules are used to construct the set of traced symbols.  Each

candidate symbol is passed through the chain of above rules.

Initially, the symbol is \fIunmarked\fR.  If it matches a \fB+\fR

rule, it becomes \fImarked\fR, if it matches a \fB-\fR rule, it

becomes \fIunmarked\fR again.  If, after applying all rules, the

symbol is \fImarked\fR, it will be traced.

.SH PROTOTYPE LIBRARY DISCOVERY

When a library is mapped into the address space of a traced process,

ltrace needs to know what the prototypes are of functions that this

library implements.  For purposes of ltrace, prototype really is a bit

more than just type signature: it's also formatting of individual

parameters and of return value.  These prototypes are stored in files

called prototype libraries.

After a library is mapped, ltrace finds out what its SONAME is.  It

then looks for a file named SONAME.conf--e.g. protolib for libc.so.6

would be in a file called libc.so.6.conf.  When such file is found

(more about where ltrace looks for these files is below), ltrace reads

all prototypes stored therein.  When a symbol table entry point (such

as those traced by -x) is hit, the prototype is looked up in a

prototype library corresponding to the library where the hit occured.

When a library call (such as those traced by -e and -l) is hit, the

prototype is looked up in all prototype libraries loaded for given

process.  That is necessary, because a library call is traced in a PLT

table of a caller library, but the prototype is described at callee

library.

If a library has no SONAME, basename of library file is considered

instead.  For the main program binary, basename is considered as well

(e.g. protolib for /bin/echo would be called echo.conf).  If a name

corresponding to soname (e.g. libc.so.6.conf) is not found, and the

module under consideration is a shared library, ltrace also tries

partial matches.  Ltrace snips one period after another, retrying the

search, until either a protolib is found, or X.so is all that's left.

Thus libc.so.conf would be considered, but libc.conf not.

When looking for a prototype library, ltrace potentially looks into

several directories.  On Linux, those are $XDG_CONFIG_HOME/ltrace,

$HOME/.ltrace, \fIX\fR/ltrace for each \fIX\fR in $XDG_CONFIG_DIRS and

/usr/share/ltrace.  If the environment variable XDG_CONFIG_HOME is not

defined, ltrace looks into $HOME/.config/ltrace instead.

There's also a mechanism for loading legacy config files.  If

$HOME/.ltrace.conf exists it is imported to every loaded prototype

library.  Similarly for /etc/ltrace.conf.  If both exist, both are

imported, and $HOME/.ltrace.conf is consulted before /etc/ltrace.conf.

If -F contains any directories, those are searched in precedence to

the above system directories, in the same order in which they are

mentioned in -F.  Any files passed in -F are imported similarly to

above legacy config files, before them.

See ltrace.conf(5) for details on the syntax of ltrace prototype

library files.

.SH BUGS

It has most of the bugs stated in

.BR strace(1) .

.LP

It only works on Linux and in a small subset of architectures.

.LP

.PP

If you would like to report a bug, send a message to the mailing list

(ltrace-devel@lists.alioth.debian.org), or use the

.BR reportbug(1)

program if you are under the Debian GNU/Linux distribution.

.SH FILES

.TP

.I /etc/ltrace.conf

System configuration file

.TP

.I ~/.ltrace.conf

Personal config file, overrides

.I /etc/ltrace.conf

.SH AUTHOR

Juan Cespedes <cespedes@debian.org>

.br

Petr Machata <pmachata@redhat.com>

.SH "SEE ALSO"

.BR ltrace.conf(5),

.BR strace(1) ,

.BR ptrace(2)