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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | FILTER SPECIFICATIONS | FILTER EXPRESSIONS | PROTOTYPE LIBRARY DISCOVERY | BUGS | FILES | AUTHOR | SEE ALSO | COLOPHON |
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LTRACE(1) User Commands LTRACE(1)
ltrace - A library call tracer
ltrace [-e filter|-L] [-l|--library=library_pattern] [-x filter]
[-S] [-b|--no-signals] [-i] [-w|--where=nr] [-r|-t|-tt|-ttt] [-T]
[[-F|--config] pathlist] [-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
ltrace 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 executed 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
declaration of function prototypes. Those are stored in files
called prototype 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.
-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 received 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 system, this makes C++ function names
readable.
-D, --debug mask
Show debugging output of ltrace itself. mask is a number
describing which debug messages should be displayed. Use
the option -Dh to see what can be used, but note that
currently the only reliable debugmask is 77, which shows
all debug messages.
-e filter
A qualifying expression which modifies which library calls
(i.e. calls done through PLT slots, which are typically
calls from the main binary to a library, or inter-library
calls) to trace. Usage examples and the syntax description
appear below in sections FILTER SPECIFICATIONS and 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, --config 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. This is as if you specified
one -e for every symbol implemented in a library specified
by library_pattern. Multiple library patters can be
specified with several instances of this option. Usage
examples and the syntax description of library_pattern
appear below in sections FILTER SPECIFICATIONS and 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 simply 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. In practice this means that library
calls will not be traced.
-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 execute. 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 elfutils or
libunwind support was enabled at compile time.
-x filter
A qualifying expression which modifies which symbol table
entry points to trace (those are typically calls inside a
library or main binary, though PLT calls, traced by -e,
land on entry points as well). Usage examples and the
syntax description appear below in sections FILTER
SPECIFICATIONS and 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.
Filters are specified with the -l, -e and -x options. In short
they mean:
-x is ´show me what calls these symbols (including local calls)´
-e is ´show me what calls these symbols (inter-library calls
only)´
-l is ´show me what calls into this library´
Suppose I have a library defined with this header tstlib.h:
void func_f_lib(void);
void func_g_lib(void);
and this implementation tstlib.c:
#include "tstlib.h"
void func_f_lib(void)
{
func_g_lib();
}
void func_g_lib(void)
{
}
Suppose I have an executable that uses this library defined like
this tst.c:
#include "tstlib.h"
void func_f_main(void)
{
}
void main(void)
{
func_f_main();
func_f_lib();
}
If linking without -Bsymbolic, the internal func_g_lib() call
uses the PLT like external calls, and thus ltrace says:
$ ltrace -x 'func*' -L ./tst
func_f_main() = <void>
[email protected]( <unfinished ...>
[email protected]() = <void>
<... func_f_lib resumed> ) = <void>
+++ exited (status 163) +++
$ ltrace -e 'func*' ./tst
tst->func_f_lib( <unfinished ...>
tstlib.so->func_g_lib() = <void>
<... func_f_lib resumed> ) = <void>
+++ exited (status 163) +++
$ ltrace -l tstlib.so ./tst
tst->func_f_lib( <unfinished ...>
tstlib.so->func_g_lib() = <void>
<... func_f_lib resumed> ) = <void>
+++ exited (status 163) +++
By contrast, if linking with -Bsymbolic, then the internal
func_g_lib() call bypasses the PLT, and ltrace says:
$ ltrace -x 'func*' -L ./tst
func_f_main() = <void>
[email protected]( <unfinished ...>
[email protected]() = <void>
<... func_f_lib resumed> ) = <void>
+++ exited (status 163) +++
$ ltrace -e 'func*' ./tst
tst->func_f_lib() = <void>
+++ exited (status 163) +++
$ ltrace -l tstlib.so ./tst
tst->func_f_lib() = <void>
+++ exited (status 163) +++
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 [email protected]*
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 expressions as well (see below). The glob syntax
supports meta-characters * and ? and character classes, similarly
to what basic bash globs support. ^ 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
expression, 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.
Initially, 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.
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 occurred. 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, 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.
It has most of the bugs stated in strace(1).
It only works on Linux and in some architectures.
If you would like to report a bug, send a message to the mailing
list ([email protected]), or use the
reportbug(1) program if you are under the Debian GNU/Linux
distribution.
/etc/ltrace.conf
System configuration file
~/.ltrace.conf
Personal config file, overrides /etc/ltrace.conf
Juan Cespedes <[email protected]>
Petr Machata <[email protected]>
ltrace.conf(5), strace(1), ptrace(2)
This page is part of the ltrace (library call tracer) project.
Information about the project can be found at
⟨http://ltrace.org/⟩. If you have a bug report for this manual
page, see ⟨http://ltrace.org/⟩. This page was obtained from the
project's upstream Git repository
⟨https://gitlab.com/cespedes/ltrace.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2023-07-05.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
January 2013 LTRACE(1)
Pages that refer to this page: strace(1), ptrace(2), ltrace.conf(5)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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