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NAME    [Toc]    [Back]

       strace - trace system calls and signals

SYNOPSIS    [Toc]    [Back]

       strace  [  -dffhiqrtttTvxx  ] [ -acolumn ] [ -eexpr ] ...  [ -ofile ] [
       -ppid ] ...  [ -sstrsize ] [ -uusername ] [ command [ arg ...  ] ]

       strace -c [ -eexpr ] ...  [ -Ooverhead ] [ -Ssortby ] [ command	[  arg
       ...  ] ]

DESCRIPTION    [Toc]    [Back]

       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 debugging tool.  System
 adminstrators, 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.

       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:

       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.	 This  example
       shows the shell peforming ``>>xyzzy'' output redirection:

       open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3

       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, 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 formatted 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-existent 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.  Nonprinting
 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 brackets but
       set elements are separated only by a space.  Here is the shell  preparing
 to execute an external command:

       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    [Toc]    [Back]

       -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.

       -i	   Print  the  instruction  pointer  at the time of 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

       -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

       -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  beginning  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

       -xx	   Print all strings in hexadecimal string format.

       -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:


		   where  qualifier  is  one of trace, abbrev, verbose,
		   raw, signal, read, or write and value  is  a  qualifier-dependent
 symbol or number.  The default qualifier
 is trace.  Using an  exclamation  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
		   addition,  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  exclamation  point	with  a

       -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 careful when making
		   inferences about the user/kernel 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  abbreviation   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 member of
		   large structures.  The default is  abbrev=all.   The
		   -v option has the effect of 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,  signal=!SIGIO
		   (or	signal=!io)  causes  SIGIO  signals  not  to be

       -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
		   affecting the redirections of executed programs.

       -O overhead Set the overhead for tracing system calls  to  over-
		   head  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 timing a given program run without
		   tracing (using time(1)) and	comparing  the	accumulated
  system  call time to the total produced using

       -p pid	   Attach to the process with the process  ID  pid  and
		   begin  tracing.   The trace may be terminated at any
		   time  by  a	keyboard  interrupt  signal   (CTRL-C).
		   strace  will  respond  by  detaching itself from the
		   traced process(es) leaving  it  (them)  to  continue
		   running.   Multiple -p options can be used to attach
		   to up to 32 processes 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 nothing (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 without effective privileges.


       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 privileges.  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  nonsetuid
 version of strace for ordinary lusers to use.

SEE ALSO    [Toc]    [Back]

       ptrace(2), proc(4), time(1), trace(1), truss(1)

NOTES    [Toc]    [Back]

       It is a pity that so much tracing clutter is produced by systems
       employing shared libraries.

       It is instructive to think about system call inputs and	outputs
       as  data-flow  across  the  user/kernel boundary.  Because userspace
 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.

       In some cases, a system call will  differ  from	the  documented
       behavior  or  have  a different name.  For example, on System Vderived
 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 system call.

BUGS    [Toc]    [Back]

       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 process is

       The -i option is weakly supported.

HISTORY    [Toc]    [Back]

       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  support.  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 that worked on both platforms.  In  1994  Rick  ported
       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    [Toc]    [Back]

       Problems  with  strace  should be reported to the current strace
       maintainer, Wichert Akkerman, at <wakkerma@debian.org>.

				   99/06/11			     STRACE(1)
[ Back ]
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