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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | DIAGNOSTICS | SETUID INSTALLATION | NOTES | BUGS | HISTORY | REPORTING BUGS | SEE ALSO | COLOPHON |
STRACE(1) General Commands Manual STRACE(1)
strace - trace system calls and signals
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]
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 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
unfinished. 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
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.
Output format
-a column Align return values in a specific column (default column
40).
-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 libunwind.
-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.
-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.
-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.
-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.
-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 protocol specific information associated with
socket file descriptors.
Statistics
-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 , only aggregate totals for all traced
processes are kept.
-C Like -c but also print regular output while processes are
running.
-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.
-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).
-w Summarise the time difference between the beginning and
end of each system call. The default is to summarise the
system time.
Filtering
-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, write, fault, or inject 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. Question mark before the
syscall qualification allows suppression of error in case
no syscalls matched the qualification provided. 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=/regex
Trace only those system calls that match the regex. You
can use POSIX Extended Regular Expression syntax (see
regex(7)).
-e trace=%file
-e trace=file (deprecated)
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
-e trace=process (deprecated)
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
-e trace=network (deprecated)
Trace all the network related system calls.
-e trace=%signal
-e trace=signal (deprecated)
Trace all signal related system calls.
-e trace=%ipc
-e trace=ipc (deprecated)
Trace all IPC related system calls.
-e trace=%desc
-e trace=desc (deprecated)
Trace all file descriptor related system calls.
-e trace=%memory
-e trace=memory (deprecated)
Trace all memory mapping related system calls.
-e trace=%stat
Trace stat syscall variants.
-e trace=%lstat
Trace lstat syscall variants.
-e trace=%fstat
Trace fstat and fstatat syscall variants.
-e trace=%%stat
Trace syscalls used for requesting file status (stat, lstat,
fstat, fstatat, statx, and their variants).
-e trace=%statfs
Trace statfs, statfs64, statvfs, osf_statfs, and osf_statfs64
system calls. The same effect can be achieved with
-e trace=/^(.*_)?statv?fs regular expression.
-e trace=%fstatfs
Trace fstatfs, fstatfs64, fstatvfs, osf_fstatfs, and
osf_fstatfs64 system calls. The same effect can be achieved
with -e trace=/fstatv?fs regular expression.
-e trace=%%statfs
Trace syscalls related to file system statistics (statfs-like,
fstatfs-like, and ustat). The same effect can be achieved
with -e trace=/statv?fs|fsstat|ustat regular expression.
-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.
-e inject=set[:error=errno|:retval=value][:signal=sig][:when=expr]
Perform syscall tampering for the specified set of syscalls.
At least one of error, retval, or signal options has to be
specified. error and retval are mutually exclusive.
If :error=errno option is specified, a fault is injected into
a syscall invocation: the syscall number is replaced by -1
which corresponds to an invalid syscall, and the error code is
specified using a symbolic errno value like ENOSYS or a
numeric value within 1..4095 range.
If :retval=value option is specified, success injection is
performed: the syscall number is replaced by -1, but a bogus
success value is returned to the callee.
If :signal=sig option is specified with either a symbolic
value like SIGSEGV or a numeric value within 1..SIGRTMAX
range, that signal is delivered on entering every syscall
specified by the set.
If :signal=sig option is specified without :error=errno or
:retval=value options, then only a signal sig is delivered
without a syscall fault injection. Conversely, :error=errno
or :retval=value option without :signal=sig option injects a
fault without delivering a signal.
If both :error=errno or :retval=value and :signal=sig options
are specified, then both a fault or success is injected and a
signal is delivered.
Unless a :when=expr subexpression is specified, an injection
is being made into every invocation of each syscall from the
set.
The format of the subexpression is one of the following:
first
For every syscall from the set, perform an injection for
the syscall invocation number first only.
first+
For every syscall from the set, perform injections for the
syscall invocation number first and all subsequent
invocations.
first+step
For every syscall from the set, perform injections for
syscall invocations number first, first+step,
first+step+step, and so on.
For example, to fail each third and subsequent chdir syscalls
with ENOENT, use -e inject=chdir:error=ENOENT:when=3+.
The valid range for numbers first and step is 1..65535.
An injection expression can contain only one error= or retval=
specification. If an injection expression contains multiple
when= specifications, the last one takes precedence.
Accounting of syscalls that are subject to injection is done
per syscall and per tracee.
Specification of syscall injection can be combined with other
syscall filtering options, for example, -P /dev/urandom -e
inject=file:error=ENOENT.
-e fault=set[:error=errno][:when=expr]
Perform syscall fault injection for the specified set of
syscalls.
This is equivalent to more generic -e inject= expression with
default value of errno option set to ENOSYS.
-P path
Trace only system calls accessing path. Multiple -P options
can be used to specify several paths.
-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.
Tracing
-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.
-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.
-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.
-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).
Startup
-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.
-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 in addition to
command (which is optional if at least one -p option is
given). -p "`pidof PROG`" syntax is supported.
-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.
Miscellaneous
-d Show some debugging output of strace itself on the
standard error.
-F This option is now obsolete and it has the same
functionality as -f.
-h Print the help summary.
-V Print the version number of strace.
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 without a command, the exit status of strace is zero
unless no processes has been attached or there was an unexpected
error in doing the tracing.
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.
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.
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 exceptions 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.
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.
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 with strace should be reported to the strace mailing list at
<strace-devel@lists.sourceforge.net>.
ltrace(1), time(1), ptrace(2), proc(5)
This page is part of the strace (system call tracer) project.
Information about the project can be found at
⟨http://sourceforge.net/projects/strace/⟩. If you have a bug report
for this manual page, send it to strace-devel@lists.sourceforge.net.
This page was obtained from the project's upstream Git repository
⟨git://git.code.sf.net/p/strace/code⟩ on 2017-07-05. If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
2010-03-30 STRACE(1)
Pages that refer to this page: ltrace(1), ptrace(2), proc(5), vdso(7)