strace



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.

       -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.  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 (espe-
                   cially 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  dis-
                   position).

       -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.

       -F          Attempt  to  follow  vforks.  (On SunOS 4.x, this is
                   accomplished with some  dynamic  linking  trickery.)
                   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
                   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
                   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
                   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 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

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


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.


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)

Man(1) output converted with man2html