strace


SYNOPSIS
       strace  [  -CdffhiqrtttTvxx ] [ -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.  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 +++

       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])

       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:

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

       -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
                   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, -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-

       -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.

                   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 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.

       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)

       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
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