STRACE(1)                   General Commands Manual                  STRACE(1)



NAME
       strace - trace system calls and signals

SYNOPSIS
       strace  [-CdffhikqrtttTvVxxy]  [-In] [-bexecve] [-eexpr]...  [-acolumn]
       [-ofile]  [-sstrsize]  [-Ppath]...  -ppid...  /  [-D]  [-Evar[=val]]...
       [-uusername] command [args]

       strace  -c[df]  [-In]  [-bexecve]  [-eexpr]...  [-Ooverhead] [-Ssortby]
       -ppid... / [-D] [-Evar[=val]]... [-uusername] command [args]

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.  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 signal symbol and decoded siginfo structure.  An
       excerpt from stracing and interrupting the command "sleep 666" is:

       sigsuspend([] <unfinished ...>
       --- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=...} ---
       +++ killed by SIGINT +++

       If  a  system call is being executed and meanwhile another one is being
       called from a different thread/process then strace will try to preserve
       the  order  of  those  events and mark the ongoing call as being unfin-
       ished.  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 ... ---
       rt_sigreturn(0xe)                       = 0
       read(0, "", 1)                          = 0

       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  third  argument 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  sup-
       plementary 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.

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

       -D          Run tracer process as a detached grandchild, not  as
                   parent  of  the  tracee.   This  reduces the visible
                   effect of strace by  keeping  the  tracee  a  direct
                   child of the calling process.

       -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),
                   vfork(2) and clone(2) system calls.   Note  that  -p
                   PID  -f will attach all threads of process PID if it
                   is multi-threaded, not only thread with thread_id  =
                   PID.

       -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.

       -k          Print  the  execution stack trace of the traced pro-
                   cesses after each system call (experimental).

       -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.

       -qq         If given twice, suppress messages about process exit
                   status.

       -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.

       -w          Summarise  the time difference between the beginning
                   and end of each system call.  The default is to sum-
                   marise the system time.

       -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.

       -y          Print paths associated with  file  descriptor  argu-
                   ments.

       -a column   Align  return  values  in a specific column (default
                   column 40).

       -b syscall  If specified syscall is reached, detach from  traced
                   process.   Currently,  only  execve  syscall is sup-
                   ported.  This option is useful if you want to  trace
                   multi-threaded process and therefore require -f, but
                   don't want to trace its (potentially  very  complex)
                   children.

       -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, -e open means lit-
                   erally -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 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 trace=memory
                   Trace all memory mapping 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.

       -I interruptible
                   When strace can be interrupted by signals  (such  as
                   pressing  ^C).   1: no signals are blocked; 2: fatal
                   signals   are   blocked   while   decoding   syscall
                   (default);  3:  fatal  signals  are  always  blocked
                   (default if '-o FILE PROG'); 4:  fatal  signals  and
                   SIGTSTP  (^Z)  are  always  blocked  (useful to make
                   strace -o FILE PROG not stop on ^Z).

       -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 many processes.  -p "`pidof PROG`" syntax is sup-
                   ported.

       -P path     Trace only system calls accessing path.  Multiple -P
                   options can be used to specify several paths.

       -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 is 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.

DIAGNOSTICS
       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.  Note  that  parent-
       child  relationship (signal stop notifications, getppid() value,
       etc) between traced process and its  parent  are  not  preserved
       unless -D is used.

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

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 is  attached  to  with  the  -p
       option may observe a spurious EINTR return from the current sys-
       tem call that is not restartable.  (Ideally,  all  system  calls
       should  be restarted on strace attach, making the attach invisi-
       ble to the traced  process,  but  a  few  system  calls  aren't.
       Arguably,  every  instance  of  such  behavior is a kernel bug.)
       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 runs slowly.

       Traced processes which are descended from command  may  be  left
       running after an interrupt signal (CTRL-C).

       The -i option is weakly supported.

HISTORY
       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 to  the  strace  mailing
       list at <strace-devel@lists.sourceforge.net>.



                                  2010-03-30                         STRACE(1)
