strace.1

.\" Copyright (c) 1991, 1992 Paul Kranenburg <pk@cs.few.eur.nl>
.\" Copyright (c) 1993 Branko Lankester <branko@hacktic.nl>
.\" Copyright (c) 1993, 1994, 1995, 1996 Rick Sladkey <jrs@world.std.com>
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.\"
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.\" 3. The name of the author may not be used to endorse or promote products
.\"    derived from this software without specific prior written permission.
.\"
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\"
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.TH STRACE 1 "2010-03-30"
.SH NAME
strace \- trace system calls and signals
.SH SYNOPSIS
.B strace
[
.B \-CdDffhiqrtttTvxxy
]
[
.BI \-a column
]
[
.BI \-e expr
]
\&...
[
.BI \-o file
]
[
.BI \-p pid
]
\&...
[
.BI \-P path
]
\&...
[
.BI \-s strsize
]
[
.BI \-u username
]
[
.BI \-E var=val
]
\&...
[
.BI \-E var
]
\&...
[
.I command
[
.I arg
\&...
]
]
.sp
.B strace
.B \-c
[
.B \-D
]
[
.BI \-e expr
]
\&...
[
.BI \-O overhead
]
[
.BI \-S sortby
]
[
.I command
[
.I arg
\&...
]
]
.SH DESCRIPTION
.IX "strace command" "" "\fLstrace\fR command"
.LP
In the simplest case
.B strace
runs the specified
.I command
until it exits.
It intercepts and records the system calls which are called
by a process and the signals which are received by a process.
The name of each system call, its arguments and its return value
are printed on standard error or to the file specified with the
.B \-o
option.
.LP
.B strace
is a useful diagnostic, instructional, and debugging tool.
System administrators, diagnosticians and trouble-shooters will find
it invaluable for solving problems with
programs for which the source is not readily available since
they do not need to be recompiled in order to trace them.
Students, hackers and the overly-curious will find that
a great deal can be learned about a system and its system calls by
tracing even ordinary programs.  And programmers will find that
since system calls and signals are events that happen at the user/kernel
interface, a close examination of this boundary is very
useful for bug isolation, sanity checking and
attempting to capture race conditions.
.LP
Each line in the trace contains the system call name, followed
by its arguments in parentheses and its return value.
An example from stracing the command ``cat /dev/null'' is:
.CW
open("/dev/null", O_RDONLY) = 3
.CE
Errors (typically a return value of \-1) have the errno symbol
and error string appended.
.CW
open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory)
.CE
Signals are printed as a signal symbol and a signal string.
An excerpt from stracing and interrupting the command ``sleep 666'' is:
.CW
sigsuspend([] <unfinished ...>
--- SIGINT (Interrupt) ---
+++ killed by SIGINT +++
.CE
If a system call is being executed and meanwhile another one is being called
from a different thread/process then
.B strace
will try to preserve the order of those events and mark the ongoing call as
being
.IR unfinished .
When the call returns it will be marked as
.IR resumed .
.CW
[pid 28772] select(4, [3], NULL, NULL, NULL <unfinished ...>
[pid 28779] clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
[pid 28772] <... select resumed> )      = 1 (in [3])
.CE
Interruption of a (restartable) system call by a signal delivery is processed
differently as kernel terminates the system call and also arranges its
immediate reexecution after the signal handler completes.
.CW
read(0, 0x7ffff72cf5cf, 1)              = ? ERESTARTSYS (To be restarted)
--- SIGALRM (Alarm clock) @ 0 (0) ---
rt_sigreturn(0xe)                       = 0
read(0, ""..., 1)                       = 0
.CE
Arguments are printed in symbolic form with a passion.
This example shows the shell performing ``>>xyzzy'' output redirection:
.CW
open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3
.CE
Here the three argument form of open is decoded by breaking down the
flag argument into its three bitwise-OR constituents and printing the
mode value in octal by tradition.  Where traditional or native
usage differs from ANSI or POSIX, the latter forms are preferred.
In some cases,
.B strace
output has proven to be more readable than the source.
.LP
Structure pointers are dereferenced and the members are displayed
as appropriate.  In all cases arguments are formatted in the most C-like
fashion possible.
For example, the essence of the command ``ls \-l /dev/null'' is captured as:
.CW
lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
.CE
Notice how the `struct stat' argument is dereferenced and how each member is
displayed symbolically.  In particular, observe how the st_mode member
is carefully decoded into a bitwise-OR of symbolic and numeric values.
Also notice in this example that the first argument to lstat is an input
to the system call and the second argument is an output.  Since output
arguments are not modified if the system call fails, arguments may not
always be dereferenced.  For example, retrying the ``ls \-l'' example
with a non-existent file produces the following line:
.CW
lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory)
.CE
In this case the porch light is on but nobody is home.
.LP
Character pointers are dereferenced and printed as C strings.
Non-printing characters in strings are normally represented by
ordinary C escape codes.
Only the first
.I strsize
(32 by default) bytes of strings are printed;
longer strings have an ellipsis appended following the closing quote.
Here is a line from ``ls \-l'' where the
.B getpwuid
library routine is reading the password file:
.CW
read(3, "root::0:0:System Administrator:/"..., 1024) = 422
.CE
While structures are annotated using curly braces, simple pointers
and arrays are printed using square brackets with commas separating
elements.  Here is an example from the command ``id'' on a system with
supplementary group ids:
.CW
getgroups(32, [100, 0]) = 2
.CE
On the other hand, bit-sets are also shown using square brackets
but set elements are separated only by a space.  Here is the shell
preparing to execute an external command:
.CW
sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0
.CE
Here the second argument is a bit-set of two signals, SIGCHLD and SIGTTOU.
In some cases the bit-set is so full that printing out the unset
elements is more valuable.  In that case, the bit-set is prefixed by
a tilde like this:
.CW
sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0
.CE
Here the second argument represents the full set of all signals.
.SH OPTIONS
.TP 12
.TP
.B \-c
Count time, calls, and errors for each system call and report a summary on
program exit.  On Linux, this attempts to show system time (CPU time spent
running in the kernel) independent of wall clock time.  If
.B \-c
is used with
.B \-f
or
.B \-F
(below), only aggregate totals for all traced processes are kept.
.TP
.B \-C
Like
.B \-c
but also print regular output while processes are running.
.TP
.B \-D
(Not available on SVR4 and FreeBSD.)
Run tracer process as a detached grandchild, not as parent of the
tracee.  This reduces the visible effect of
.B strace
by keeping the tracee a direct child of the calling process.
.TP
.B \-d
Show some debugging output of
.B strace
itself on the standard error.
.TP
.B \-f
Trace child processes as they are created by currently traced
processes as a result of the
.BR fork (2)
system call.
.IP
On non-Linux platforms the new process is
attached to as soon as its pid is known (through the return value of
.BR fork (2)
in the parent process). This means that such children may run
uncontrolled for a while (especially in the case of a
.BR vfork (2)),
until the parent is scheduled again to complete its
.RB ( v ) fork (2)
call.  On Linux the child is traced from its first instruction with no delay.
If the parent process decides to
.BR wait (2)
for a child that is currently
being traced, it is suspended until an appropriate child process either
terminates or incurs a signal that would cause it to terminate (as
determined from the child's current signal disposition).
.IP
On SunOS 4.x the tracing of
.BR vfork s
is accomplished with some dynamic linking trickery.
.TP
.B \-ff
If the
.B \-o
.I filename
option is in effect, each processes trace is written to
.I filename.pid
where pid is the numeric process id of each process.
This is incompatible with
.BR \-c ,
since no per-process counts are kept.
.TP
.B \-F
This option is now obsolete and it has the same functionality as
.BR \-f .
.TP
.B \-h
Print the help summary.
.TP
.B \-i
Print the instruction pointer at the time of the system call.
.TP
.B \-q
Suppress messages about attaching, detaching etc.  This happens
automatically when output is redirected to a file and the command
is run directly instead of attaching.
.TP
.B \-r
Print a relative timestamp upon entry to each system call.  This
records the time difference between the beginning of successive
system calls.
.TP
.B \-t
Prefix each line of the trace with the time of day.
.TP
.B \-tt
If given twice, the time printed will include the microseconds.
.TP
.B \-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.
.TP
.B \-T
Show the time spent in system calls. This records the time
difference between the beginning and the end of each system call.
.TP
.B \-v
Print unabbreviated versions of environment, stat, termios, etc.
calls.  These structures are very common in calls and so the default
behavior displays a reasonable subset of structure members.  Use
this option to get all of the gory details.
.TP
.B \-V
Print the version number of
.BR strace .
.TP
.B \-x
Print all non-ASCII strings in hexadecimal string format.
.TP
.B \-xx
Print all strings in hexadecimal string format.
.TP
.B \-y
Print paths associated with file descriptor arguments.
.TP
.BI "\-a " column
Align return values in a specific column (default column 40).
.TP
.BI "\-e " expr
A qualifying expression which modifies which events to trace
or how to trace them.  The format of the expression is:
.RS 15
.IP
[\fIqualifier\fB=\fR][\fB!\fR]\fIvalue1\fR[\fB,\fIvalue2\fR]...
.RE
.IP
where
.I qualifier
is one of
.BR trace ,
.BR abbrev ,
.BR verbose ,
.BR raw ,
.BR signal ,
.BR read ,
or
.B write
and
.I value
is a qualifier-dependent symbol or number.  The default
qualifier is
.BR trace .
Using an exclamation mark negates the set of values.  For example,
.BR \-e "\ " open
means literally
.BR \-e "\ " trace = open
which in turn means trace only the
.B open
system call.  By contrast,
.BR \-e "\ " trace "=!" open
means to trace every system call except
.BR open .
In addition, the special values
.B all
and
.B none
have the obvious meanings.
.IP
Note that some shells use the exclamation point for history
expansion even inside quoted arguments.  If so, you must escape
the exclamation point with a backslash.
.TP
\fB\-e\ trace\fR=\fIset\fR
Trace only the specified set of system calls.  The
.B \-c
option is useful for determining which system calls might be useful
to trace.  For example,
.BR trace = open,close,read,write
means to only
trace those four system calls.  Be careful when making inferences
about the user/kernel boundary if only a subset of system calls
are being monitored.  The default is
.BR trace = all .
.TP
.BR "\-e\ trace" = file
Trace all system calls which take a file name as an argument.  You
can think of this as an abbreviation for
.BR "\-e\ trace" = open , stat , chmod , unlink ,...
which is useful to seeing what files the process is referencing.
Furthermore, using the abbreviation will ensure that you don't
accidentally forget to include a call like
.B lstat
in the list.  Betchya woulda forgot that one.
.TP
.BR "\-e\ trace" = process
Trace all system calls which involve process management.  This
is useful for watching the fork, wait, and exec steps of a process.
.TP
.BR "\-e\ trace" = network
Trace all the network related system calls.
.TP
.BR "\-e\ trace" = signal
Trace all signal related system calls.
.TP
.BR "\-e\ trace" = ipc
Trace all IPC related system calls.
.TP
.BR "\-e\ trace" = desc
Trace all file descriptor related system calls.
.TP
\fB\-e\ abbrev\fR=\fIset\fR
Abbreviate the output from printing each member of large structures.
The default is
.BR abbrev = all .
The
.B \-v
option has the effect of
.BR abbrev = none .
.TP
\fB\-e\ verbose\fR=\fIset\fR
Dereference structures for the specified set of system calls.  The
default is
.BR verbose = all .
.TP
\fB\-e\ raw\fR=\fIset\fR
Print raw, undecoded arguments for the specified set of system calls.
This option has the effect of causing all arguments to be printed
in hexadecimal.  This is mostly useful if you don't trust the
decoding or you need to know the actual numeric value of an
argument.
.TP
\fB\-e\ signal\fR=\fIset\fR
Trace only the specified subset of signals.  The default is
.BR signal = all .
For example,
.B signal "=!" SIGIO
(or
.BR signal "=!" io )
causes SIGIO signals not to be traced.
.TP
\fB\-e\ read\fR=\fIset\fR
Perform a full hexadecimal and ASCII dump of all the data read from
file descriptors listed in the specified set.  For example, to see
all input activity on file descriptors
.I 3
and
.I 5
use
\fB\-e\ read\fR=\fI3\fR,\fI5\fR.
Note that this is independent from the normal tracing of the
.BR read (2)
system call which is controlled by the option
.BR -e "\ " trace = read .
.TP
\fB\-e\ write\fR=\fIset\fR
Perform a full hexadecimal and ASCII dump of all the data written to
file descriptors listed in the specified set.  For example, to see
all output activity on file descriptors
.I 3
and
.I 5
use
\fB\-e\ write\fR=\fI3\fR,\fI5\fR.
Note that this is independent from the normal tracing of the
.BR write (2)
system call which is controlled by the option
.BR -e "\ " trace = write .
.TP
.BI "\-o " filename
Write the trace output to the file
.I filename
rather than to stderr.
Use
.I filename.pid
if
.B \-ff
is used.
If the argument begins with `|' or with `!' then the rest of the
argument is treated as a command and all output is piped to it.
This is convenient for piping the debugging output to a program
without affecting the redirections of executed programs.
.TP
.BI "\-O " overhead
Set the overhead for tracing system calls to
.I overhead
microseconds.
This is useful for overriding the default heuristic for guessing
how much time is spent in mere measuring when timing system calls using
the
.B \-c
option.  The accuracy of the heuristic can be gauged by timing a given
program run without tracing (using
.BR time (1))
and comparing the accumulated
system call time to the total produced using
.BR \-c .
.TP
.BI "\-p " pid
Attach to the process with the process
.SM ID
.I pid
and begin tracing.
The trace may be terminated
at any time by a keyboard interrupt signal (\c
.SM CTRL\s0-C).
.B strace
will respond by detaching itself from the traced process(es)
leaving it (them) to continue running.
Multiple
.B \-p
options can be used to attach to up to 32 processes in addition to
.I command
(which is optional if at least one
.B \-p
option is given).
.TP
.BI "\-P " path
Trace only system calls accessing
.I path.
Multiple
.B \-P
options can be used to specify up to 256 paths.
.TP
.BI "\-s " strsize
Specify the maximum string size to print (the default is 32).  Note
that filenames are not considered strings and are always printed in
full.
.TP
.BI "\-S " sortby
Sort the output of the histogram printed by the
.B \-c
option by the specified criterion.  Legal values are
.BR time ,
.BR calls ,
.BR name ,
and
.B nothing
(default is
.BR time ).
.TP
.BI "\-u " username
Run command with the user \s-1ID\s0, group \s-2ID\s0, and
supplementary groups of
.IR username .
This option is only useful when running as root and enables the
correct execution of setuid and/or setgid binaries.
Unless this option is used setuid and setgid programs are executed
without effective privileges.
.TP
\fB\-E\ \fIvar\fR=\fIval\fR
Run command with
.IR var = val
in its list of environment variables.
.TP
.BI "\-E " var
Remove
.IR var
from the inherited list of environment variables before passing it on to
the command.
.SH DIAGNOSTICS
When
.I command
exits,
.B strace
exits with the same exit status.
If
.I command
is terminated by a signal,
.B strace
terminates itself with the same signal, so that
.B strace
can be used as a wrapper process transparent to the invoking parent process.
.LP
When using
.BR \-p ,
the exit status of
.B strace
is zero unless there was an unexpected error in doing the tracing.
.SH "SETUID INSTALLATION"
If
.B strace
is installed setuid to root then the invoking user will be able to
attach to and trace processes owned by any user.
In addition setuid and setgid programs will be executed and traced
with the correct effective privileges.
Since only users trusted with full root privileges should be allowed
to do these things,
it only makes sense to install
.B strace
as setuid to root when the users who can execute it are restricted
to those users who have this trust.
For example, it makes sense to install a special version of
.B strace
with mode `rwsr-xr--', user
.B root
and group
.BR trace ,
where members of the
.B trace
group are trusted users.
If you do use this feature, please remember to install
a non-setuid version of
.B strace
for ordinary lusers to use.
.SH "SEE ALSO"
.BR ltrace (1),
.BR time (1),
.BR ptrace (2),
.BR proc (5)
.SH NOTES
It is a pity that so much tracing clutter is produced by systems
employing shared libraries.
.LP
It is instructive to think about system call inputs and outputs
as data-flow across the user/kernel boundary.  Because user-space
and kernel-space are separate and address-protected, it is
sometimes possible to make deductive inferences about process
behavior using inputs and outputs as propositions.
.LP
In some cases, a system call will differ from the documented behavior
or have a different name.  For example, on System V-derived systems
the true
.BR time (2)
system call does not take an argument and the
.B stat
function is called
.B xstat
and takes an extra leading argument.  These
discrepancies are normal but idiosyncratic characteristics of the
system call interface and are accounted for by C library wrapper
functions.
.LP
On some platforms a process that has a system call trace applied
to it with the
.B \-p
option will receive a
.BR \s-1SIGSTOP\s0 .
This signal may interrupt a system call that is not restartable.
This may have an unpredictable effect on the process
if the process takes no action to restart the system call.
.SH BUGS
Programs that use the
.I setuid
bit do not have
effective user
.SM ID
privileges while being traced.
.LP
A traced process ignores
.SM SIGSTOP
except on SVR4 platforms.
.LP
A traced process which tries to block SIGTRAP will be sent a SIGSTOP
in an attempt to force continuation of tracing.
.LP
A traced process runs slowly.
.LP
Traced processes which are descended from
.I command
may be left running after an interrupt signal (\c
.SM CTRL\s0-C).
.LP
On Linux, exciting as it would be, tracing the init process is forbidden.
.LP
The
.B \-i
option is weakly supported.
.SH HISTORY
.B strace
The original
.B strace
was written by Paul Kranenburg
for SunOS and was inspired by its trace utility.
The SunOS version of
.B strace
was ported to Linux and enhanced
by Branko Lankester, who also wrote the Linux kernel support.
Even though Paul released
.B strace
2.5 in 1992,
Branko's work was based on Paul's
.B strace
1.5 release from 1991.
In 1993, Rick Sladkey merged
.B strace
2.5 for SunOS and the second release of
.B strace
for Linux, added many of the features of
.BR truss (1)
from SVR4, and produced an
.B strace
that worked on both platforms.  In 1994 Rick ported
.B strace
to SVR4 and Solaris and wrote the
automatic configuration support.  In 1995 he ported
.B strace
to Irix
and tired of writing about himself in the third person.
.SH BUGS
The SIGTRAP signal is used internally by the kernel implementation of
system call tracing.  When a traced process receives a SIGTRAP signal not
associated with tracing, strace will not report that signal correctly.
This signal is not normally used by programs, but could be via a hard-coded
break instruction or via
.BR kill (2).
.SH PROBLEMS
Problems with
.B strace
should be reported via the Debian Bug Tracking System,
or to the
.B strace
mailing list at <strace\-devel@lists.sourceforge.net>.