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 adminstrators, 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 peforming ``>>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.

    -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. On
    Linux, it requires some kernel functionality not yet
    in the standard kernel.) 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 quali-
    fier-dependent symbol or number. The default quali-
    fier  is trace.  Using an exclamation mark negates
    the set of values. For example, -eopen means liter-
    ally  -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 specifed 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 acuracy 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 critereon. 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 truetime(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>.