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

       sigsuspend([] 
       --- 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 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 
       [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 per‐
       forming ">>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 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  pro‐
       duces 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.

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 processes 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 currently 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 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.

       -k          Print the execution stack trace of the traced processes after each system call
                   (experimental).   This option is available only if strace is built with libun‐
                   wind.

       -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 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.

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

       -yy         Print ip:port pairs associated with socket file descriptors.

       -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 supported.  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 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
                   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 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 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 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.

       -I interruptible
                   When  strace  can be interrupted by signals (such as pressing ^C).  1: no sig‐
                   nals 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 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 many pro‐
                   cesses.  -p "`pidof PROG`" syntax is supported.

       -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 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.

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 rela‐
       tionship (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 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.

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-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.

       Some system calls have different names in different architectures and  personalities.   In
       these  cases,  system  call filtering and printing uses the names that match corresponding
       __NR_* kernel macros of the tracee's architecture and personality.  There are  two  excep‐
       tions  from  this general rule: arm_fadvise64_64(2) ARM syscall and xtensa_fadvise64_64(2)
       Xtensa syscall are filtered and printed as fadvise64_64(2).

       On some platforms a process that is attached to with the -p option may observe a  spurious
       EINTR  return  from the current system call that is not restartable.  (Ideally, all system
       calls should be restarted on strace attach, making the  attach  invisible  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  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.

PROBLEMS
       Problems   with   strace   should   be   reported   to   the   strace   mailing   list  at
       .

                                            2010-03-30                                  STRACE(1)

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