Provided by: strace_4.5.15-1.1ubuntu1_i386 bug

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
       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 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 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
       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  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

       -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
                   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.  This  is
                   incompatible  with  -c,  since no per-process counts
                   are kept.

       -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
                   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 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
                   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
                   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
                   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  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 criterion.  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.

       -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.

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)