strace - trace system calls and signals
SYNOPSIS
strace [ -dffhiqrtttTvxx ] [ -acolumn ] [ -eexpr ] ... [
-ofile ] [ -ppid ] ... [ -sstrsize ] [ -uusername ] [
command [ arg ... ] ]
strace -c [ -eexpr ] ... [ -Ooverhead ] [ -Ssortby ] [
command [ arg ... ] ]
DESCRIPTION
In the simplest case strace runs the specified 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 -o option.
strace is a useful diagnostic, instructional, and debug
ging tool. System adminstrators, diagnosticians and trou
ble-shooters will find it invaluable for solving problems
with programs for which the source is not readily avail
able 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.
Each line in the trace contains the system call name, fol
lowed by its arguments in parentheses and its return
value. An example from stracing the command ``cat
/dev/null'' is:
open("/dev/null", O_RDONLY) = 3
Errors (typically a return value of -1) have the errno
symbol and error string appended.
open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory)
Signals are printed as a signal symbol and a signal
string. An excerpt from stracing and interrupting the
command ``sleep 666'' is:
sigsuspend([] <unfinished ...>
--- SIGINT (Interrupt) ---
+++ killed by SIGINT +++
Arguments are printed in symbolic form with a passion.
redirection:
open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3
Here the three argument form of open is decoded by break
ing down the flag argument into its three bitwise-OR con
stituents and printing the mode value in octal by tradi
tion. Where traditional or native usage differs from ANSI
or POSIX, the latter forms are preferred. In some cases,
strace output has proven to be more readable than the
source.
Structure pointers are dereferenced and the members are
displayed as appropriate. In all cases arguments are for
matted in the most C-like fashion possible. For example,
the essence of the command ``ls -l /dev/null'' is captured
as:
lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
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-exis
tent file produces the following line:
lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory)
In this case the porch light is on but nobody is home.
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
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
getpwuid library routine is reading the password file:
read(3, "root::0:0:System Administrator:/"..., 1024) = 422
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:
getgroups(32, [100, 0]) = 2
On the other hand, bit-sets are also shown using square
Here is the shell preparing to execute an external com
mand:
sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0
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:
sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0
Here the second argument represents the full set of all
signals.
OPTIONS
-c Count time, calls, and errors for each system
call and report a summary on program exit.
-d Show some debugging output of strace itself on
the standard error.
-f Trace child processes as they are created by
currently traced processes as a result of the
fork(2) system call. The new process is
attached to as soon as its pid is known
(through the return value of fork(2) in the
parent process). This means that such children
may run uncontrolled for a while (especially
in the case of a vfork(2)), until the parent
is scheduled again to complete its (v)fork(2)
call. If the parent process decides to
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).
-ff If the -o filename option is in effect, each
processes trace is written to filename.pid
where pid is the numeric process id of each
process.
-F Attempt to follow vforks. (On SunOS 4.x, this
is accomplished with some dynamic linking
trickery. On Linux, it requires some kernel
functionality not yet in the standard kernel.)
Otherwise, vforks will not be followed even if
-f has been given.
-h Print the help summary.
the system call.
-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.
-r Print a relative timestamp upon entry to each
system call. This records the time difference
between the beginning of successive system
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 in system calls. This
records the time difference between the begin
ning and the end of each system call.
-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.
-V Print the version number of strace.
-x Print all non-ASCII strings in hexadecimal
string format.
-xx Print all strings in hexadecimal string for
mat.
-a column Align return values in a specific column
(default column 40).
-e expr A qualifying expression which modifies which
events to trace or how to trace them. The
format of the expression is:
[qualifier=][!]value1[,value2]...
where qualifier is one of trace, abbrev, ver
bose, raw, signal, read, or write and value is
default qualifier is trace. Using an exclama
tion mark negates the set of values. For
example, -eopen means literally -e trace=open
which in turn means trace only the open system
call. By contrast, -etrace=!open means to
trace every system call except open. In addi
tion, the special values all and none have the
obvious meanings.
Note that some shells use the exclamation
point for history expansion even inside quoted
arguments. If so, you must escape the excla
mation point with a backslash.
-e trace=set
Trace only the specified set of system calls.
The -c option is useful for determining which
system calls might be useful to trace. For
example, trace=open,close,read,write means to
only trace those four system calls. Be care
ful when making inferences about the user/ker
nel boundary if only a subset of system calls
are being monitored. The default is
trace=all.
-e trace=file
Trace all system calls which take a file name
as an argument. You can think of this as an
abbreviation for
-e trace=open,stat,chmod,unlink,... which is
useful to seeing what files the process is
referencing. Furthermore, using the abbrevia
tion will ensure that you don't accidentally
forget to include a call like lstat in the
list. Betchya woulda forgot that one.
-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.
-e trace=network
Trace all the network related system calls.
-e trace=signal
Trace all signal related system calls.
-e trace=ipc
Trace all IPC related system calls.
-e abbrev=set
Abbreviate the output from printing each mem
ber of large structures. The default is
abbrev=none.
-e verbose=set
Dereference structures for the specified set
of system calls. The default is verbose=all.
-e raw=set Print raw, undecoded arguments for the
specifed 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.
-e signal=set
Trace only the specified subset of signals.
The default is signal=all. For example, sig
nal=!SIGIO (or signal=!io) causes SIGIO sig
nals not to be traced.
-e read=set 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 3 and 5 use
-e read=3,5. Note that this is independent
from the normal tracing of the read(2) system
call which is controlled by the option -e
trace=read.
-e write=set
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 3
and 5 use -e write=3,5. Note that this is
independent from the normal tracing of the
write(2) system call which is controlled by
the option -e trace=write.
-o filename Write the trace output to the file filename
rather than to stderr. Use filename.pid if
-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 affect
ing the redirections of executed programs.
-O overhead Set the overhead for tracing system calls to
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 -c option. The
acuracy of the heuristic can be gauged by
(using time(1)) and comparing the accumulated
system call time to the total produced using
-c.
-p pid Attach to the process with the process ID pid
and begin tracing. The trace may be termi
nated at any time by a keyboard interrupt sig
nal (CTRL-C). strace will respond by detach
ing itself from the traced process(es) leaving
it (them) to continue running. Multiple -p
options can be used to attach to up to 32 pro
cesses in addition to command (which is
optional if at least one -p option is given).
-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.
-S sortby Sort the output of the histogram printed by
the -c option by the specified critereon.
Legal values are time, calls, name, and noth
ing (default time).
-u username Run command with the user ID, group ID, 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. Unless this option is used
setuid and setgid programs are executed with
out effective privileges.
SETUID INSTALLATION
If 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 privi
leges. Since only users trusted with full root privileges
should be allowed to do these things, it only makes sense
to install 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 strace with mode `rwsr-xr--', user root and
group trace, where members of the trace group are trusted
users. If you do use this feature, please remember to
install a non-setuid version of strace for ordinary lusers
to use.
SEE ALSO
ptrace(2), proc(4), time(1), trace(1), truss(1)
NOTES
It is a pity that so much tracing clutter is produced by
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 deduc
tive inferences about process behavior using inputs and
outputs as propositions.
In some cases, a system call will differ from the docu
mented behavior or have a different name. For example, on
System V-derived systems the true time(2) system call does
not take an argument and the stat function is called 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.
On some platforms a process that has a system call trace
applied to it with the -p option will receive a SIGSTOP.
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 sys
tem call.
BUGS
Programs that use the setuid bit do not have effective
user ID privileges while being traced.
A traced process ignores SIGSTOP except on SVR4 platforms.
A traced process which tries to block SIGTRAP will be sent
a SIGSTOP in an attempt to force continuation of tracing.
A traced process runs slowly.
Traced processes which are descended from command may be
left running after an interrupt signal (CTRL-C).
On Linux, exciting as it would be, tracing the init pro
cess is forbidden.
The -i option is weakly supported.
HISTORY
strace The original strace was written by Paul Kranenburg
for SunOS and was inspired by its trace utility. The
SunOS version of strace was ported to Linux and enhanced
by Branko Lankester, who also wrote the Linux kernel sup
port. Even though Paul released strace 2.5 in 1992,
Branko's work was based on Paul's strace 1.5 release from
1991. In 1993, Rick Sladkey merged strace 2.5 for SunOS
and the second release of strace for Linux, added many of
the features of truss(1) from SVR4, and produced an strace
to SVR4 and Solaris and wrote the automatic configuration
support. In 1995 he ported strace to Irix and tired of
writing about himself in the third person.
PROBLEMS
Problems with strace should be reported to the current
strace maintainer, Wichert Akkerman, at
<wakkerma@debian.org>.
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