LTRACE(1) User Commands LTRACE(1)NAMEltrace - A library call tracer
SYNOPSISltrace [-e filter|-L] [-l|--library=library_pattern] [-x filter] [-S]
[-b|--no-signals] [-i] [-w|--where=nr] [-r|-t|-tt|-ttt] [-T] [-F path‐
list] [-A maxelts] [-s strsize] [-C|--demangle] [-a|--align column]
[-n|--indent nr] [-o|--output filename] [-D|--debug mask] [-u username]
[-f] [-p pid] [[--] command [arg ...]]
ltrace-c [-e filter|-L] [-l|--library=library_pattern] [-x filter]
[-S] [-o|--output filename] [-f] [-p pid] [[--] command [arg ...]]
ltrace -V|--version
ltrace -h|--help
DESCRIPTIONltrace is a program that simply runs the specified 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 exe‐
cuted by the program.
Its use is very similar to strace(1).
ltrace shows parameters of invoked functions and system calls. To
determine what arguments each function has, it needs external declara‐
tion of function prototypes. Those are stored in files called proto‐
type libraries--see ltrace.conf(5) for details on the syntax of these
files. See the section PROTOTYPE LIBRARY DISCOVERY to learn how ltrace
finds prototype libraries.
OPTIONS-a, --align column
Align return values in a specific column (default column is 5/8
of screen width).
-A maxelts
Maximum number of array elements to print before suppressing the
rest with an ellipsis ("..."). This also limits number of
recursive structure expansions.
-b, --no-signals
Disable printing of signals recieved by the traced process.
-c Count time and calls for each library call and report a summary
on program exit.
-C, --demangle
Decode (demangle) low-level symbol names into user-level names.
Besides removing any initial underscore prefix used by the sys‐
tem, this makes C++ function names readable.
-D, --debug mask
Show debugging output of ltrace itself. mask is a number with
internal meaning that's not really well defined at all. mask of
77 shows all debug messages, which is what you usually need.
-e filter
A qualifying expression which modifies which library calls to
trace. The format of the filter expression is described in the
section FILTER EXPRESSIONS. 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, @MAIN is assumed as a default.
-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.
-F pathlist
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 PROTOTYPE LIBRARY DISCOVERY). If
it refers to a file, that file is imported implicitly to all
loaded prototype libraries.
-h, --help
Show a summary of the options to ltrace and exit.
-i Print the instruction pointer at the time of the library call.
-l, --library library_pattern
Display only calls to functions implemented by libraries that
match library_pattern. Multiple library patters can be speci‐
fied with several instances of this option. Syntax of
library_pattern is described in section FILTER EXPRESSIONS.
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 sim‐
ply dependency ordering. If you want to make sure that symbols
in given library are actually called, use -x @library_pattern
instead.
-L When no -e option is given, don't assume the default action of
@MAIN.
-n, --indent nr
Indent trace output by nr 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.
-o, --output filename
Write the trace output to the file filename rather than to
stderr.
-p pid Attach to the process with the process ID pid and begin tracing.
This option can be used together with passing a command to exe‐
cute. It is possible to attach to several processes by passing
more than one option -p.
-r Print a relative timestamp with each line of the trace. This
records the time difference between the beginning of successive
lines.
-s strsize
Specify the maximum string size to print (the default is 32).
-S Display system calls as well as library calls
-t Prefix each line of the trace with the time of day.
-tt If given twice, the time printed will include the microseconds.
-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.
-T Show the time spent inside each call. This records the time
difference between the beginning and the end of each call.
-u username
Run command with the userid, groupid and supplementary groups of
username. This option is only useful when running as root and
enables the correct execution of setuid and/or setgid binaries.
-w, --where nr
Show backtrace of nr stack frames for each traced function. This
option enabled only if libunwind support was enabled at compile
time.
-x filter
A qualifying expression which modifies which symbol table entry
points to trace. The format of the filter expression is
described in the section FILTER EXPRESSIONS. 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.
-V, --version
Show the version number of ltrace and exit.
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:
{[+-][symbol_pattern][@library_pattern]}
Symbol_pattern is used to match symbol names, library_pattern to match
library SONAMEs. Both are implicitly globs, but can be regular expres‐
sions as well (see below). The glob syntax supports meta-characters *
and ? and character classes, similarly to what basic bash globs sup‐
port. ^ and $ are recognized to mean, respectively, start and end of
given name.
Both symbol_pattern and library_pattern 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 *. (So
malloc is really malloc@* and @libc.* is really *@libc.*.)
In libraries that don't have an explicit SONAME, basename is taken for
SONAME. That holds for main binary as well: /bin/echo has an implicit
SONAME of echo. In addition to that, special library pattern MAIN
always matches symbols in the main binary and never a library with
actual SONAME MAIN (use e.g. ^MAIN or [M]AIN 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 /x/@/y/, you can write /x@y/.
If the library pattern starts with a slash, it is not a SONAME expres‐
sion, but a path expression, and is matched against the library path
name.
The first rule may lack a sign, in which case + is assumed. If, on the
other hand, the first rule has a - sign, it is as if there was another
rule @ 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. Ini‐
tially, the symbol is unmarked. If it matches a + rule, it becomes
marked, if it matches a - rule, it becomes unmarked again. If, after
applying all rules, the symbol is marked, it will be traced.
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 proto‐
type library corresponding to the library where the hit occured. When
a library call (such as those traced by -e and -l) is hit, the proto‐
type 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 par‐
tial 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, X/ltrace for each X 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.
BUGS
It has most of the bugs stated in strace(1).
It only works on Linux and in a small subset of architectures.
If you would like to report a bug, send a message to the mailing list
(ltrace-devel@lists.alioth.debian.org), or use the reportbug(1) program
if you are under the Debian GNU/Linux distribution.
FILES
/etc/ltrace.conf
System configuration file
~/.ltrace.conf
Personal config file, overrides /etc/ltrace.conf
AUTHOR
Juan Cespedes <cespedes@debian.org>
Petr Machata <pmachata@redhat.com>
SEE ALSOltrace.conf(5), strace(1), ptrace(2)
January 2013 LTRACE(1)
