ltrace - Online in the Cloud

PROGRAM:

NAME

ltrace - A library call tracer

SYNOPSIS

ltrace [-e filter|-L] [-l|--library=library_pattern] [-x filter] [-S] [-b|--no-signals]
[-i] [-w|--where=nr] [-r|-t|-tt|-ttt] [-T] [-F filename] [-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

DESCRIPTION

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).

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 system, 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 filename
Load an alternate config file. Normally, /etc/ltrace.conf and ~/.ltrace.conf will
be read (the latter only if it exists). Use this option to load the given file or
files instead of those two default files. See ltrace.conf(5) for details on the
syntax of ltrace configuration files.

-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 specified 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 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.

-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 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+[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.

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