.\"
.Dd August 31, 2026
.Dt TRUSS 1
.Os
.Sh NAME
.Nm truss
.Nd trace system calls
.Sh SYNOPSIS
.Nm
.Op Fl facedyDHS
.Op Fl o Ar file
.Op Fl s Ar strsize
.Op Fl t Ar expr
.Fl p Ar pid
.Nm
.Op Fl facedDHS
.Op Fl o Ar file
.Op Fl s Ar strsize
.Op Fl t Ar expr
.Ar command Op Ar args
.Nm
.Fl t
.Sh DESCRIPTION
The
.Nm
utility traces the system calls called by the specified process or program.
Output is to the specified output file, or standard error by default.
It does this by stopping and restarting the process being monitored via
.Xr ptrace 2 .
.Pp
By default,
.Nm
enters the capability mode
.Pq see Xr capsicum 4
after opening the output file and creating and
attaching to the target process.
An error to enter the mode is not fatal, but a warning is
issued to the standard error.
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl f
Trace descendants of the original traced process created by
.Xr fork 2 ,
.Xr vfork 2 ,
etc.
To distinguish events between processes,
the process ID
.Pq PID
of the process is included in the output of each event.
.It Fl a
Show the argument strings that are passed in each
.Xr execve 2
system call.
.It Fl c
Do not display individual system calls or signals.
Instead, before exiting, print a summary containing for each system call:
the total system time used,
the number of times the call was invoked,
and the number of times the call returned with an error.
.It Fl e
Show the environment strings that are passed in each
.Xr execve 2
system call.
.It Fl d
Include timestamps in the output showing the time elapsed
since the trace was started.
.It Fl D
Include timestamps in the output showing the time elapsed
since the last recorded event.
.It Fl H
Include the thread ID of in the output of each event.
.It Fl S
Do not display information about signals received by the process.
(Normally,
.Nm
displays signal as well as system call events.)
.It Fl y
Use the
.Sq classic
combination of
.Xr ptrace 2
and
.Xr waitpid 2
system calls to control the traced processes, instead of the
process descriptors and kqueue PROCDESC filter
.Pq see Xr pdfork 2 and Xr kqueue 2.
Do not try to enter capability mode.
.It Fl o Ar file
Print the output to the specified
.Ar file
instead of standard error.
.It Fl s Ar strsize
Display strings using at most
.Ar strsize
characters.
If the buffer is larger,
.Dq Li ...
will be displayed at the end of the string.
The default
.Ar strsize
is 32.
.It Fl t Ar expr
Report only the system calls selected by
.Ar expr ,
a comma-separated list of terms.
Whatever follows
.Fl t
is taken to be the
.Ar expr ,
so one must be given;
the sole exception is the third form shown above,
.Ql truss -t
with nothing after it,
which prints the groups described below and exits.
That is not the same as giving an empty
.Ar expr :
.Ql truss -t \(dq\(dq
is an expression with no terms in it,
and so filters nothing.
.Pp
Each term takes one of three forms:
.Bl -tag -width "@group" -offset indent
.It Ar name
The name of a system call,
as
.Nm
prints it,
for example
.Ql read .
A name may also contain the wildcards
.Ql * ,
.Ql \&?
and
.Ql []
described in
.Xr fnmatch 3 ,
in which case it selects every system call whose name it matches:
.Ql read*
selects
.Xr read 2 ,
.Xr readv 2
and
.Xr readlink 2
alike.
.It Ar number
A system call number in decimal,
for example
.Ql 3 .
The number is the one the ABI of the traced process uses,
so
.Ql 3
selects
.Xr read 2
from a native process but
.Fn close
from a Linux one.
.It Cm @ Ns Ar group
One of the groups of related system calls listed below,
for example
.Ql @file .
.El
.Pp
A term prefixed with
.Ql \&!
excludes the system calls it matches instead of including them.
The
.Ql \&!
applies only to the term it prefixes,
not to the terms that follow it.
.Pp
An expression whose terms are all negated subtracts from the set of
every system call;
any other expression selects from an empty set.
Terms are applied in the order they are given,
and the last one to match a system call decides whether it is reported.
Repeating
.Fl t
appends to the expression rather than replacing it,
so that
.Fl t Ar a Fl t Ar b
and
.Fl t Ar a , Ns Ar b
are equivalent.
An empty term is ignored,
which is why an empty
.Ar expr
filters nothing,
and why a stray comma is not an error.
.Pp
The groups are:
.Bl -tag -width ".Cm @signal" -compact -offset indent
.It Cm @all
every system call
.It Cm @creds
get or set process credentials
.It Cm @desc
operate on a file descriptor
.It Cm @file
operate on a pathname
.It Cm @ipc
System V and POSIX IPC
.It Cm @memory
memory mapping and locking
.It Cm @net
network and socket operations
.It Cm @none
no system call
.It Cm @proc
process and thread lifecycle
.It Cm @read
read data from a descriptor
.It Cm @signal
signal delivery and handling
.It Cm @time
clocks, timers and sleeping
.It Cm @write
write data to a descriptor
.El
.Pp
A group is itself defined by an expression of the same form as
.Ar expr ,
so a group may name other groups and may exclude as well as include:
.Cm @desc
is defined to include
.Cm @read
and
.Cm @write ,
and
.Cm @none
is defined as
.Ql \&!* .
.Pp
A system call is matched both under the name
.Nm
displays for it and under that name with any prefix naming a
compatibility layer or a non-native ABI removed,
so that
.Fl t Cm @file
selects
.Ql compat11.stat
and
.Ql linux_newstat
as well as
.Ql stat .
.Pp
A name or pattern that matches no system call of any ABI that
.Nm
understands is reported with a warning,
since it is almost always a typo,
but it is not an error:
the term is kept and simply never matches.
A number is not checked against any table,
as a process may issue any system call number the kernel can hold and
.Nm
reports the attempt whether or not a system call is implemented behind
it;
only a number too large to be one at all is rejected.
An unknown group is an error.
.It Fl p Ar pid
Follow the process specified by
.Ar pid
instead of a new command.
.It Ar command Op Ar args
Execute
.Ar command
and trace the system calls of it.
(The
.Fl p
and
.Ar command
options are mutually exclusive.)
.El
.Sh EXAMPLES
Follow the system calls used in echoing "hello":
.Dl $ truss /bin/echo hello
.Pp
Do the same, but put the output into a file:
.Dl $ truss -o /tmp/truss.out /bin/echo hello
.Pp
Follow an already-running process:
.Dl $ truss -p 34
.Pp
List the groups that
.Fl t
understands:
.Dl $ truss -t
.Pp
Show only the file and network activity of a command:
.Dl $ truss -t @file,@net fetch https://www.freebsd.org/
.Pp
Show everything a command does except its memory management:
.Dl $ truss -t '!@memory' make buildworld
.Pp
Show the descriptor activity of a running process apart from the
data transfers themselves:
.Dl $ truss -t '@desc,!@read,!@write' -p 34
.Pp
Count the
.Xr readlink 2
and
.Xr readlinkat 2
calls made while starting a program:
.Dl $ truss -c -t 'readlink*' /bin/ls
.Sh SEE ALSO
.Xr dtrace 1 ,
.Xr kdump 1 ,
.Xr ktrace 1 ,
.Xr pdfork 2 ,
.Xr pdtrace 2 ,
.Xr ptrace 2 ,
.Xr utrace 2 ,
.Xr fnmatch 3 ,
.Xr sysdecode 3 ,
.Xr capsicum 4
.Sh HISTORY
The
.Nm
command was written by
.An Sean Eric Fagan
for
.Fx .
It was modeled after
similar commands available for System V Release 4 and SunOS.