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STRACE(1)							     STRACE(1)



NAME
       strace - trace system calls and signals

SYNOPSIS
       strace  [  -dffhiqrtttTvxx  ] [ -acolumn ] [ -eexpr ] ...  [ -ofile ] [
       -ppid ] ...  [ -sstrsize ] [ -uusername ] [ -Evar=val ] ...  [ -Evar  ]
       ...  [ 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 pro-
       cess 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.  Sys-
       tem  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.

       Each  line  in the trace contains the system call name, followed by its
       arguments in parentheses and its return value.  An example from	strac-
       ing 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 performing ''>>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 mem-
       ber 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, argu-
       ments  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.	  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  ele-
       ments.	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 prepar-
       ing to execute an external command:

       sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0

       Here the second argument is a bit-set of two signals, SIGCHLD and SIGT-
       TOU.   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.   On	 Linux,
		   this	 attempts  to  show system time (CPU time spent
		   running in the kernel)  independent	of  wall  clock
		   time.   If  -c  is  used with -f or -F (below), only
		   aggregate totals for all traced processes are  kept.

       -d	   Show	 some  debugging output of strace itself on the
		   standard error.

       -f	   Trace child processes as they are  created  by  cur-
		   rently  traced  processes as a result of the fork(2)
		   system call.

		   On non-Linux platforms 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.  On Linux the child is traced from  its  first
		   instruction	with  no  delay.  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).

		   On SunOS 4.x the tracing of vforks  is  accomplished
		   with some dynamic linking trickery.

       -ff	   If  the  -o	filename option is in effect, each pro-
		   cesses trace is written to filename.pid where pid is
		   the	numeric	 process  id  of each process.	This is
		   incompatible with -c, since	no  per-process	 counts
		   are kept.

       -F	   This	 option	 is  now  obsolete  and it has the same
		   functionality as -f.

       -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
		   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 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
		   format.

       -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:

			     [qualifier=][!]value1[,value2]...

		   where qualifier is one of  trace,  abbrev,  verbose,
		   raw,	 signal,  read,	 or write and value is a quali-
		   fier-dependent symbol or number.  The default quali-
		   fier	 is  trace.   Using an exclamation mark negates
		   the set of values.  For example, -eopen means liter-
		   ally	 -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
		   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  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 referenc-
		   ing.	   Furthermore,	 using	the  abbreviation  will
		   ensure that you don't accidentally forget to include
		   a  call like lstat in the list.  Betchya woulda for-
		   got that one.

       -e trace=process
		   Trace all system calls which involve process manage-
		   ment.   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 trace=desc
		   Trace all file descriptor 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 sys-
		   tem calls.  The default is verbose=all.

       -e raw=set  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	 decod-
		   ing	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
		   traced.

       -e read=set Perform a full hexadecimal and ASCII dump of all the
		   data read from file descriptors listed in the speci-
		   fied 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 spec-
		   ified  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 pip-
		   ing	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 accuracy of the heuristic
		   can be gauged by timing a given program run	without
		   tracing  (using  time(1))  and comparing the accumu-
		   lated 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 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 consid-
		   ered strings and are always printed in full.

       -S sortby   Sort the output of the histogram printed by	the  -c
		   option by the specified criterion.  Legal values are
		   time, calls, name, and nothing (default time).

       -u username Run command with the user ID, group ID, and	supple-
		   mentary  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.

       -E var=val  Run	command with var=val in its list of environment
		   variables.

       -E var	   Remove var from the inherited  list	of  environment
		   variables before passing it on to the command.

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 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 non-
       setuid version of strace for ordinary lusers to use.

SEE ALSO
       ltrace(1), time(1), ptrace(2), proc(5)

NOTES
       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  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.

       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 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
       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
       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  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.

BUGS
       The SIGTRAP signal is used internally by the kernel  implementa-
       tion  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 kill(2).

PROBLEMS
       Problems	 with  strace  should  be  reported  via the Debian Bug
       Tracking System, or to  the  strace  mailing  list  at  <strace-
       devel@lists.sourceforge.net>.



				  2003-01-21			     STRACE(1)
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