truss -- trace system calls and signals


truss [-flcaein] [-[tvx] [!] syscall . . .]
[-s [!] signal . . .] [-m [!] fault . . .] [-[rw] [!] fd . . .]
[-o outfile] command | -p pid


truss 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 header files; for any pathname pointer argument, the pointed-to string is displayed. Error returns are reported using the error code names described in intro(2).

The following options are recognized. For those options which 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 may be specified. For the same name in a list, subsequent options (those to the right) override previous ones (those to the left).

Interpret the 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. Processes may also be specified by their names in the /proc directory, for example, /proc/1234; this works for remotely-mounted /proc directories as well.

Follow all children created by fork and include their signals, faults, and system calls in the trace output. Normally, 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 show which process executed the system call or received the signal.

Include the id of the responsible lightweight process (LWP) with each line of trace output. Both the process-id and the LWP id are included if -f is also specified.

Count traced system calls, faults, and signals rather than displaying 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.

Show the argument strings which are passed in each exec system call.

Show the environment strings which are passed in each exec system call.

Don't display interruptible sleeping system calls. Certain system 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.

Show Internet services and IP addresses in numeric form. If this option is not specified, truss attempts to display IP addresses as domain names, and service numbers as names. For example, when used to trace a socket system call, this option causes truss to display a socket address as:
sa={16 AF_INET 53}
rather than as:
sa={16 AF_INET nameserver nile.myrivers.COM}

-t [!] syscall,. . .
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.

-v [!] syscall,. . .
Verbose. Display the contents of any structures passed by address to the specified system calls (if traced). 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.

Names of certain system calls should be prefixed by an ``x'' if these calls are versioned. For example, connect(3sock) should be specified by -v xconnect. The output from truss shows the version in use.

-x [!] syscall,. . .
Display the arguments to the specified system calls (if traced) 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.

-s [!] signal,. . .
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) by the process. (Blocked signals are not received until the process releases them.) Signals may 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.

-m [``!''] fault,. . .
Machine faults to trace or exclude. Those machine faults specified in the comma-separated list are traced. Faults may be specified by name or number (see sys/fault.h). If the list begins with a ``!'', the specified faults are excluded from the trace output. Default is -mall -m!fltpage.

-r [!] fd,. . .
Show 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 two-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 two-character hexadecimal representation. (The first 16 bytes of the I/O buffer for each traced read are shown even in the absence of -r.) Default is -r!all.

-w [!] fd,. . .
Show the contents of the I/O buffer for each write on any of the specified file descriptors (see -r). Default is -w!all.

-o outfile
File to be used for the trace output. By default, the output goes to standard error.

See Section 2 manual pages for syscall names accepted by the -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 which 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 exiting. This enables the user to terminate excessive trace output and to release previously-existing processes. Released processes continue normally, as though they had never been touched.


This example produces a trace of the find(1) command on the terminal:
   truss find . -print >find.out

Or, to see only a trace of the open, close, read, and write system calls:

   truss -t open,close,read,write find . -print > find.out

This produces a trace of the spell(1) command on the file truss.out:

   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 concurrent processes.)

A particularly boring example is:

   truss nroff -mm document > nroff.out

because 97% of the output reports lseek, read, and write system calls. To abbreviate it:

   truss -t !lseek,read,write nroff -mm document > nroff.out

This example verbosely traces the activity of process #1, init(1M) (provided you are a privileged user):

   truss -p -v all 1

Interrupting truss returns init to normal operation.


process files


Some of the system calls described in Section 2 manual pages differ from the actual operating system interfaces. Do not be surprised by minor deviations of the trace output from the descriptions in Section 2.

Every machine fault (except a page fault) results in the posting of a signal to the process which incurred the fault. A report of a received signal will immediately follow each report of a machine fault (except a page fault) unless that signal is being blocked by the process.

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 which performs an exec(2) of a set-id or unreadable object file; such processes continue normally, though independently of truss, from the point of the exec.

To avoid collisions with other controlling processes, truss will not trace a process which 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 is not produced in strict time order. For example, a read on a pipe may be reported before the corresponding write. For any one process, the output is strictly time-ordered.

The system may run out of per-user process slots when tracing of children is requested. 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 nine process slots, one for the shell and eight for the eight-member pipeline, while truss adds another nine processes, for a total of 18. This is perilously close to the usual system-imposed limit of 25 processes per user.

truss uses shared memory and semaphores when dealing with more than one process (-f option or -p with more than one ``pid''). It issues a warning message and proceeds when these are needed but not configured in the system. However, the trace output may become garbled in this case and the output of the -c option reports only the top-level command or first ``pid'' and no children are counted.

Not all possible structures passed in all possible system calls are displayed under the -v option.


intro(2), proc(4)
© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004