truss(1) User Commands truss(1)
truss - trace system calls and signals
truss [-fcaeildDE] [- [tTvx] [!] syscall ,...]
[- [sS] [!] signal ,...] [- [mM] [!] fault ,...]
[- [rw] [!] fd ,...]
[- [uU] [!] lib ,... : [:] [!] func ,...]
[-o outfile] command | -p pid[/lwps]...
The truss utility executes the specified command and produces a trace
of the system calls it performs, the signals it receives, and the
machine faults it incurs. Each line of the trace output reports either
the fault or signal name or the system call name with its arguments and
return value(s). System call arguments are displayed symbolically when
possible using defines from relevant system headers. For any path name
pointer argument, the pointed-to string is displayed. Error returns are
reported using the error code names described in Intro(3). If, in the
case of an error, the kernel reports a missing privilege, a privilege
name as described in privileges(5) is reported in square brackets ([ ])
after the error code name.
Optionally (see the -u option), truss also produce an entry/exit trace
of user-level function calls executed by the traced process, indented
to indicate nesting.
For those options that take a list argument, the name all can be used
as a shorthand to specify all possible members of the list. If the list
begins with a !, the meaning of the option is negated (for example,
exclude rather than trace). Multiple occurrences of the same option can
be specified. For the same name in a list, subsequent options (those to
the right) override previous ones (those to the left).
The following options are supported:
Shows the argument strings that are passed in each exec() system
Counts traced system calls, faults, and signals rather than dis‐
playing the trace line-by-line. A summary report is produced after
the traced command terminates or when truss is interrupted. If -f
is also specified, the counts include all traced system calls,
faults, and signals for child processes.
Includes a time stamp on each line of trace output. The time stamp
appears as a field containing seconds.fraction at the start of the
line. This represents a time in seconds relative to the beginning
of the trace. The first line of the trace output shows the base
time from which the individual time stamps are measured, both as
seconds since the epoch (see time(2)) and as a date string (see
ctime(3C) and date(1)). The times that are reported are the times
that the event in question occurred. For all system calls, the
event is the completion of the system call, not the start of the
Includes a time delta on each line of trace output. The value
appears as a field containing seconds.fraction and represents the
elapsed time for the LWP that incurred the event since the last
reported event incurred by that LWP. Specifically, for system
calls, this is not the time spent within the system call.
Shows the environment strings that are passed in each exec() system
Includes a time delta on each line of trace output. The value
appears as a field containing seconds.fraction and represents the
difference in time elapsed between the beginning and end of a sys‐
In contrast to the -D option, this is the amount of time spent
within the system call.
Follows all children created by fork() or vfork() and includes
their signals, faults, and system calls in the trace output. Nor‐
mally, only the first-level command or process is traced. When -f
is specified, the process-id is included with each line of trace
output to indicate which process executed the system call or
received the signal.
Does not display interruptible sleeping system calls. Certain sys‐
tem calls, such as open() and read() on terminal devices or pipes,
can sleep for indefinite periods and are interruptible. Normally,
truss reports such sleeping system calls if they remain asleep for
more than one second. The system call is reported again a second
time when it completes. The -i option causes such system calls to
be reported only once, when they complete.
Includes the id of the responsible lightweight process (LWP) with
each line of trace output. If -f is also specified, both the
process-id and the LWP-id are included.
Machine faults to trace or exclude. Those faults specified in the
comma-separated list are traced. Faults can be specified by name or
number (see <sys/fault.h>). If the list begins with a !, the speci‐
fied faults are excluded from the trace output. Default is -mall -m
Machine faults that stop the process. The specified faults are
added to the set specified by -m. If one of the specified faults is
incurred, truss leaves the process stopped and abandoned (see the
-T option). Default is -M!all.
File to be used for the trace output. By default, the output goes
to standard error.
Interprets the command arguments to truss as a list of process-ids
for existing processes (see ps(1)) rather than as a command to be
executed. truss takes control of each process and begins tracing it
provided that the userid and groupid of the process match those of
the user or that the user is a privileged user. Users can trace
only selected threads by appending /thread-id to the process-id.
Mutiple threads can be selected using the - and , delimiters. For
example /1,2,7-9 traces threads 1, 2, 7, 8, and 9. Processes can
also be specified by their names in the /proc directory, for exam‐
Shows the full contents of the I/O buffer for each read() on any of
the specified file descriptors. The output is formatted 32 bytes
per line and shows each byte as an ASCII character (preceded by one
blank) or as a 2-character C language escape sequence for control
characters such as horizontal tab (\t) and newline (\n). If ASCII
interpretation is not possible, the byte is shown in 2-character
hexadecimal representation. (The first 12 bytes of the I/O buffer
for each traced print >read() are shown even in the absence of -r.)
Default is -r!all.
Signals to trace or exclude. Those signals specified in the comma-
separated list are traced. The trace output reports the receipt of
each specified signal, even if the signal is being ignored (not
blocked). (Blocked signals are not received until they are
unblocked.) Signals can be specified by name or number (see
<sys/signal.h>). If the list begins with a !, the specified signals
are excluded from the trace output. Default is -sall.
Signals that stop the process. The specified signals are added to
the set specified by -s. If one of the specified signals is
received, truss leaves the process stopped and abandoned (see the
-T option). Default is -S!all.
System calls to trace or exclude. Those system calls specified in
the comma-separated list are traced. If the list begins with a !,
the specified system calls are excluded from the trace output.
Default is -tall.
Specifies system calls that stop the process. The specified system
calls are added to the set specified by -t. If one of the specified
system calls is encountered, truss leaves the process stopped and
abandoned. That is, truss releases the process and exits but leaves
the process in the stopped state at completion of the system call
in question. A debugger or other process inspection tool (see
proc(1)) can then be applied to the stopped process. truss can be
reapplied to the stopped process with the same or different options
to continue tracing. Default is -T!all.
A process left stopped in this manner cannot be restarted by the
application of kill -CONT because it is stopped on an event of
interest via /proc, not by the default action of a stopping signal
(see signal.h(3HEAD)). The prun(1) command described in proc(1) can
be used to set the stopped process running again.
User-level function call tracing. lib,... is a comma-separated list
of dynamic library names, excluding the ``.so.n'' suffix. func,...
is a comma-separated list of function names. In both cases the
names can include name-matching metacharacters *,?, with the same
meanings as those of sh(1) but as applied to the library/function
name spaces, not to files. An empty library or function list
defaults to *, trace all libraries or functions in a library. A
leading ! on either list specifies an exclusion list, names of
libraries or functions not to be traced. Excluding a library
excludes all functions in that library; any function list following
a library exclusion list is ignored.
A single : separating the library list from the function list means
to trace calls into the libraries from outside the libraries, but
omit calls made to functions in a library from other functions in
the same library. A double :: means to trace all calls, regardless
Library patterns do not match either the executable file or the
dynamic linker unless there is an exact match (l* does not match
ld.so.1). To trace functions in either of these objects, the names
must be specified exactly, as in:
truss -u a.out -u ld ...
a.out is the literal name to be used for this purpose; it does not
stand for the name of the executable file. Tracing a.out function
calls implies all calls (default is ::).
Multiple -u options can be specified and they are honored left-to-
right. The id of the thread that performed the function call is
included in the trace output for the call. truss searches the
dynamic symbol table in each library to find function names and
also searches the standard symbol table if it has not been
User-level function calls that stop the process. The specified
functions are added to the set specified by -u. If one of the spec‐
ified functions is called, truss leaves the process stopped and
abandoned (see the -T option).
Verbose. Displays the contents of any structures passed by address
to the specified system calls (if traced by -t). Input values as
well as values returned by the operating system are shown. For any
field used as both input and output, only the output value is
shown. Default is -v!all.
Shows the contents of the I/O buffer for each write() on any of the
specified file descriptors (see the -r option). Default is -w!all.
Displays the arguments to the specified system calls (if traced by
-t) in raw form, usually hexadecimal, rather than symbolically.
This is for unredeemed hackers who must see the raw bits to be
happy. Default is -x!all.
See man pages section 2: System Calls for system call names accepted by
the -t, -T, -v, and -x options. System call numbers are also accepted.
If truss is used to initiate and trace a specified command and if the
-o option is used or if standard error is redirected to a non-terminal
file, then truss runs with hangup, interrupt, and quit signals ignored.
This facilitates tracing of interactive programs that catch interrupt
and quit signals from the terminal.
If the trace output remains directed to the terminal, or if existing
processes are traced (the -p option), then truss responds to hangup,
interrupt, and quit signals by releasing all traced processes and exit‐
ing. This enables the user to terminate excessive trace output and to
release previously-existing processes. Released processes continue nor‐
mally, as though they had never been touched.
Example 1 Tracing a Command
The following example produces a trace of the find(1) command on the
example$ truss find . -print >find.out
Example 2 Tracing Common System Calls
The following example shows only a trace of the open, close, read, and
write system calls:
example$ truss -t open,close,read,write find . -print >find.out
Example 3 Tracing a Shell Script
The following example produces a trace of the spell(1) command on the
example$ truss -f -o truss.out spell document
spell is a shell script, so the -f flag is needed to trace not only the
shell but also the processes created by the shell. (The spell script
runs a pipeline of eight processes.)
Example 4 Abbreviating Output
The following example abreviates output:
example$ truss nroff -mm document >nroff.out
because 97% of the output reports lseek(), read(), and write() system
calls. To abbreviate it:
example$ truss -t !lseek,read,write nroff -mm document >nroff.out
Example 5 Tracing Library Calls From Outside the C Library
The following example traces all user-level calls made to any function
in the C library from outside the C library:
example$ truss -u libc ...
Example 6 Tracing library calls from within the C library
The following example includes calls made to functions in the C library
from within the C library itself:
example$ truss -u libc:: ...
Example 7 Tracing Library Calls Other Than the C Library
The following example traces all user-level calls made to any library
other than the C library:
example$ truss -u '*' -u !libc ...
Example 8 Tracing printf and scanf Function Calls
The following example traces all user-level calls to functions in the
printf and scanf family contained in the C library:
example$ truss -u 'libc:*printf,*scanf' ...
Example 9 Tracing Every User-level Function Call
The following example traces every user-level function call from any‐
where to anywhere:
example$ truss -u a.out -u ld:: -u :: ...
Example 10 Tracing a System Call Verbosely
The following example verbosely traces the system call activity of
process #1, init(1M) (if you are a privileged user):
example# truss -p -v all 1
Interrupting truss returns init to normal operation.
/proc/* Process files
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Availability │SUNWtoo │
date(1), find(1), proc(1), ps(1), sh(1), spell(1), init(1M), Intro(3),
exec(2), fork(2), lseek(2), open(2), read(2), time(2), vfork(2),
write(2), ctime(3C), signal.h(3HEAD), proc(4), attributes(5), privi‐
man pages section 2: System Calls
Some of the system calls described in man pages section 2: System Calls
differ from the actual operating system interfaces. Do not be surprised
by minor deviations of the trace output from the descriptions in that
Every machine fault (except a page fault) results in the posting of a
signal to the LWP that incurred the fault. A report of a received sig‐
nal immediately follows each report of a machine fault (except a page
fault) unless that signal is being blocked.
The operating system enforces certain security restrictions on the
tracing of processes. In particular, any command whose object file
(a.out) cannot be read by a user cannot be traced by that user; set-uid
and set-gid commands can be traced only by a privileged user. Unless it
is run by a privileged user, truss loses control of any process that
performs an exec() of a set-id or unreadable object file; such pro‐
cesses continue normally, though independently of truss, from the point
of the exec().
To avoid collisions with other controlling processes, truss does not
trace a process that it detects is being controlled by another process
via the /proc interface. This allows truss to be applied to
proc(4)-based debuggers as well as to another instance of itself.
The trace output contains tab characters under the assumption that
standard tab stops are set (every eight positions).
The trace output for multiple processes or for a multithreaded process
(one that contains more than one LWP) is not produced in strict time
order. For example, a read() on a pipe can be reported before the cor‐
responding write(). For any one LWP (a traditional process contains
only one), the output is strictly time-ordered.
When tracing more than one process, truss runs as one controlling
process for each process being traced. For the example of the spell
command shown above, spell itself uses 9 process slots, one for the
shell and 8 for the 8-member pipeline, while truss adds another 9 pro‐
cesses, for a total of 18.
Not all possible structures passed in all possible system calls are
displayed under the -v option.
SunOS 5.11 31 Jul 2004 truss(1)