Home Man
Today's Posts

Linux & Unix Commands - Search Man Pages

SuSE 11.3 - man page for strace (suse section 1)

STRACE(1)			     General Commands Manual				STRACE(1)

       strace - trace system calls and signals

       strace  [  -CdffhiqrtttTvxx  ]  [  -acolumn  ] [ -eexpr ] ...  [ -ofile ] [ -ppid ] ...	[
       -sstrsize ] [ -uusername ] [ -Evar=val ] ...  [ -Evar ] ...  [ command [ arg ...  ] ]

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

       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 administrators,
       diagnosticians and trouble-shooters will find it invaluable for solving problems with pro-
       grams  for  which  the source is not readily available since they do not need to be recom-
       piled 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  paren-
       theses 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 +++

       If a system call is being executed and meanwhile another one is being called from  a  dif-
       ferent  thread/process then strace will try to preserve the order of those events and mark
       the ongoing call as being unfinished.  When the call returns it will be marked as resumed.

       [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])

       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.

       read(0, 0x7ffff72cf5cf, 1)	       = ? ERESTARTSYS (To be restarted)
       --- SIGALRM (Alarm clock) @ 0(0) ---
       rt_sigreturn(0xe)		       = 0
       read(0, ""..., 1)		       = 0

       Arguments are printed in symbolic form with a passion.  This example shows the shell  per-
       forming ``>>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 pre-
       ferred.	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.  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 follow-
       ing  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 com-
       mand ``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 sep-
       arated 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.

       -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.

       -C	   Like -c but also print regular output while processes are running.

       -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.

		   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

       -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.  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

       -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 lead-
		   ing 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:


		   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,
		   -e open means literally -e trace=open which in turn means trace only the  open
		   system  call.   By  contrast,  -e trace=!open means to trace every system call
		   except open.  In addition, the special values all and none  have  the  obvious

		   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 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 system calls.  The default is

       -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 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 activ-
		   ity	on  file descriptors 3 and 5 use -e read=3,5.  Note that this is indepen-
		   dent 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  con-
		   trolled by the option -e trace=write.

       -o filename Write  the trace output to the file filename rather than to stderr.	Use file-
		   name.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 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
		   accuracy 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 -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 file-
		   names 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
		   criterion.  Legal values are time, calls, name, and nothing (default is 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 execu-
		   tion  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.

       When command exits, strace exits with the same exit status.  If command is terminated by a
       signal, strace terminates itself with the same signal, so that strace can  be  used  as	a
       wrapper process transparent to the invoking parent process.

       When  using  -p, the exit status of strace is zero unless there was an unexpected error in
       doing the tracing.

       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 exe-
       cuted 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 ver-
       sion of strace for ordinary lusers to use.

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

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

       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-pro-
       tected, 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 inter-
       face 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.

       Programs  that  use  the  setuid  bit do not have effective user ID privileges while being

       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.

       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 plat-
       forms.  In 1994 Rick ported strace to SVR4 and Solaris and wrote the automatic  configura-
       tion  support.  In 1995 he ported strace to Irix and tired of writing about himself in the
       third person.

       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 kill(2).

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

					    2010-03-30					STRACE(1)

All times are GMT -4. The time now is 08:22 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
Show Password