strace - trace system calls and signals
[
-ACdffhikqqrtttTvVwxxyyzZ]
[
-I n] [
-b
execve] [
-e expr]...
[
-O overhead]
[
-S sortby] [
-U
columns] [
-a column]
[
-o file] [
-s
strsize] [
-X format] [
-P path]... [
-p pid]...
[
--seccomp-bpf] {
-p pid | [
-DDD] [
-E var[=val]]... [
-u
username] command [
args]
}
-c
[
-dfwzZ] [
-I
n] [
-b execve] [
-e expr]... [
-O
overhead] [
-S sortby]
[
-U columns] [
-P path]... [
-p pid]...
[
--seccomp-bpf] {
-p pid | [
-DDD] [
-E var[=val]]... [
-u
username] 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, {tv_sec=1130322148, tv_nsec=3977000}) = 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 {si_signo=SIGALRM, si_code=SI_KERNEL} ---
rt_sigreturn({mask=[]}) = 0
read(0, "", 1) = 0
Arguments are printed in symbolic form with passion. This example shows the
shell performing ">>xyzzy" output redirection:
open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3
Here, the second and the third argument of
open(2) are decoded by
breaking down the flag argument into its three bitwise-OR constituents and
printing the mode value in octal by tradition. Where the traditional or native
usage differs from ANSI or POSIX, the latter forms are preferred. In some
cases,
strace output is proven to be more readable than the source.
Structure pointers are dereferenced and the members are displayed as
appropriate. In most 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(0x1, 0x3), ...}) = 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(2) 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.
Syscalls unknown to
strace are printed raw, with the unknown system call
number printed in hexadecimal form and prefixed with "syscall_":
syscall_0xbad(0x1, 0x2, 0x3, 0x4, 0x5, 0x6) = -1 ENOSYS (Function not implemented)
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(3) library routine is
reading the password file:
read(3, "root::0:0:System Administrator:/"..., 1024) = 422
While structures are annotated using curly braces, pointers to basic types and
arrays are printed using square brackets with commas separating the elements.
Here is an example from the command
id(1) 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.
-
-e expr
- A qualifying expression which modifies which events to
trace or how to trace them. The format of the expression is:
- [qualifier=][!]value[,value]...
- where qualifier is one of trace (or
t), abbrev (or a), verbose (or v),
raw (or x), signal (or signals or s),
read (or reads or r), write (or writes
or w), fault, inject, status, quiet (or
silent or silence or q), decode-fds (or
decode-fd), decode-pids (or decode-pid), or
kvm, 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 var=val
-
--env=var=val Run
command with var=val in its list of environment
variables.
-
-E var
-
--env=var Remove var from the
inherited list of environment variables before passing it on to the
command.
-
-p pid
-
--attach=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). Multiple process IDs, separated by either comma
(“,”), space (“ ”), tab, or newline character,
can be provided as an argument to a single -p option, so, for
example, -p "$(pidof PROG)" and -p "$(pgrep
PROG)" syntaxes are supported.
-
-u username
-
--user=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.
-
-b syscall
-
--detach-on=syscall If specified
syscall is reached, detach from traced process. Currently, only
execve(2) 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
-
--daemonize --daemonize=grandchild Run
tracer process as a grandchild, not as the parent of the tracee. This
reduces the visible effect of strace by keeping the tracee a direct
child of the calling process.
- -DD
-
--daemonize=pgroup
--daemonize=pgrp Run tracer process as tracee's grandchild
in a separate process group. In addition to reduction of the visible
effect of strace, it also avoids killing of strace with
kill(2) issued to the whole process group.
- -DDD
-
--daemonize=session Run tracer process as
tracee's grandchild in a separate session ("true
daemonisation"). In addition to reduction of the visible effect of
strace, it also avoids killing of strace upon session
termination.
- -f
-
--follow-forks 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.
- --output-separately
- If the --output=filename option is in
effect, each processes trace is written to filename.pid
where pid is the numeric process id of each process.
- -ff
-
--follow-forks --output-separately Combine the
effects of --follow-forks and --output-separately options.
This is incompatible with -c, since no per-process counts are
kept.
- One might want to consider using strace-log-merge(1)
to obtain a combined strace log view.
-
-I interruptible
-
--interruptible=interruptible When
strace can be interrupted by signals (such as pressing
CTRL-C).
-
1, anywhere
- no signals are blocked; 2, waiting fatal
signals are blocked while decoding syscall (default); 3,
never fatal signals are always blocked (default if -o
FILE PROG); 4, never_tstp fatal
signals and SIGTSTP (CTRL-Z) are always blocked (useful to
make strace -o FILE PROG not stop on CTRL-Z, default
if -D).
-
-e trace=syscall_set
-
-e t=syscall_set
--trace=syscall_set Trace only the specified set of system
calls. syscall_set is defined as [
!]value[,value], and value can be one
of the following:
- syscall
- Trace specific syscall, specified by its name (see
syscalls(2) for a reference, but also see NOTES).
-
?value
- Question mark before the syscall qualification allows
suppression of error in case no syscalls matched the qualification
provided.
-
value@64
- Limit the syscall specification described by value
to 64-bit personality.
-
value@32
- Limit the syscall specification described by value
to 32-bit personality.
-
value@x32
- Limit the syscall specification described by value
to x32 personality.
- all
- Trace all system calls.
-
/regex
- Trace only those system calls that match the regex.
You can use POSIX Extended Regular Expression syntax (see
regex(7)).
- %file
-
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(2) in the list.
Betchya woulda forgot that one. The syntax without a preceding percent
sign ("-e trace=file") is deprecated.
- %process
-
process Trace system calls associated with process
lifecycle (creation, exec, termination). The syntax without a preceding
percent sign ("-e trace=process") is
deprecated.
- %net
-
%network network Trace all the network
related system calls. The syntax without a preceding percent sign
("-e trace=network") is deprecated.
- %signal
-
signal Trace all signal related system calls. The
syntax without a preceding percent sign ("-e
trace=signal") is deprecated.
- %ipc
-
ipc Trace all IPC related system calls. The syntax
without a preceding percent sign ("-e trace=ipc")
is deprecated.
- %desc
-
desc Trace all file descriptor related system calls.
The syntax without a preceding percent sign ("-e
trace=desc") is deprecated.
- %memory
-
memory Trace all memory mapping related system
calls. The syntax without a preceding percent sign ("-e
trace=memory") is deprecated.
- %creds
- Trace system calls that read or modify user and group
identifiers or capability sets.
- %stat
- Trace stat syscall variants.
- %lstat
- Trace lstat syscall variants.
- %fstat
- Trace fstat, fstatat, and statx syscall variants.
- %%stat
- Trace syscalls used for requesting file status (stat,
lstat, fstat, fstatat, statx, and their variants).
- %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.
- %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.
- %%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.
- %clock
- Trace system calls that read or modify system clocks.
- %pure
- Trace syscalls that always succeed and have no arguments.
Currently, this list includes arc_gettls(2),
getdtablesize(2), getegid(2), getegid32(2),
geteuid(2), geteuid32(2), getgid(2),
getgid32(2), getpagesize(2), getpgrp(2),
getpid(2), getppid(2), get_thread_area(2) (on
architectures other than x86), gettid(2), get_tls(2),
getuid(2), getuid32(2), getxgid(2),
getxpid(2), getxuid(2), kern_features(2), and
metag_get_tls(2) syscalls.
- 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 signal=set
-
-e signals=set
-e s=set --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 status=set
-
--status=set Print only system calls with the
specified return status. The default is status=all. When
using the status qualifier, because strace waits for system
calls to return before deciding whether they should be printed or not, the
traditional order of events may not be preserved anymore. If two system
calls are executed by concurrent threads, strace will first print
both the entry and exit of the first system call to exit, regardless of
their respective entry time. The entry and exit of the second system call
to exit will be printed afterwards. Here is an example when
select(2) is called, but a different thread calls
clock_gettime(2) before select(2) finishes:
[pid 28779] 1130322148.939977 clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
[pid 28772] 1130322148.438139 select(4, [3], NULL, NULL, NULL) = 1 (in [3])
set can include the following elements:
- successful
- Trace system calls that returned without an error code. The
-z option has the effect of status=successful.
failed Trace system calls that returned with an error code. The
-Z option has the effect of status=failed.
unfinished Trace system calls that did not return. This might
happen, for example, due to an execve call in a neighbour thread.
unavailable Trace system calls that returned but strace failed to
fetch the error status. detached Trace system calls for which
strace detached before the return.
-
-P path
-
--trace-path=path Trace only system
calls accessing path. Multiple -P options can be used to
specify several paths.
- -z
-
--successful-only Print only syscalls that returned
without an error code.
- -Z
-
--failed-only Print only syscalls that returned with
an error code.
-
-a column
-
--columns=column Align return values
in a specific column (default column 40).
-
-e abbrev=syscall_set
-
-e a=syscall_set
--abbrev=syscall_set Abbreviate the output from printing
each member of large structures. The syntax of the syscall_set
specification is the same as in the -e trace option. The default is
abbrev=all. The -v option has the effect of
abbrev=none.
-
-e verbose=syscall_set
-
-e v=syscall_set
--verbose=syscall_set Dereference structures for the
specified set of system calls. The syntax of the syscall_set
specification is the same as in the -e trace option. The default is
verbose=all.
-
-e raw=syscall_set
-
-e x=syscall_set
--raw=syscall_set Print raw, undecoded arguments for the
specified set of system calls. The syntax of the syscall_set
specification is the same as in the -e trace option. 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. See also -X raw option.
-
-e read=set
-
-e reads=set
-e r=set --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
-
-e writes=set
-e w=set --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 quiet=set
-
-e silent=set
-e silence= set -e q=set
--quiet= set --silent=set
--silence=set Suppress various information messages. The
default is quiet=none. set can include the following
elements:
- attach
- Suppress messages about attaching and detaching ("[
Process NNNN attached ]", "[ Process NNNN detached
]"). exit Suppress messages about process exits
("+++ exited with SSS +++"). path-resolution
Suppress messages about resolution of paths provided via the -P
option ("Requested path "..." resolved into
"...""). personality Suppress messages about
process personality changes ("[ Process PID=NNNN runs in PPP mode.
]"). thread-execve superseded Suppress messages
about process being superseded by execve(2) in another thread
("+++ superseded by execve in pid NNNN +++").
-
-e decode-fds=set
-
--decode-fds=set Decode various information
associated with file descriptors. The default is
decode-fds=none. set can include the following
elements:
- path
- Print file paths. Also enables printing of tracee's current
working directory when AT_FDCWD constant is used. socket
Print socket protocol-specific information, dev Print
character/block device numbers. pidfd Print PIDs associated with
pidfd file descriptors.
-
-e decode-pids=set
-
--decode-pids=set Decode various information
associated with process IDs (and also thread IDs, process group IDs, and
session IDs). The default is decode-pids=none. set
can include the following elements:
- comm
- Print command names associated with thread or process IDs.
pidns Print thread, process, process group, and session IDs in
strace's PID namespace if the tracee is in a different PID namespace.
-
-e kvm=vcpu
-
--kvm=vcpu Print the exit reason of kvm vcpu.
Requires Linux kernel version 4.16.0 or higher.
- -i
-
--instruction-pointer Print the instruction pointer
at the time of the system call.
- -n
-
--syscall-number Print the syscall number.
- -k
-
--stack-traces Print the execution stack trace of
the traced processes after each system call.
-
-o filename
-
--output=filename Write the trace
output to the file filename rather than to stderr.
filename.pid form is used if -ff option is supplied.
If the argument begins with '|' or '!', 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. The latter is not compatible with
-ff option currently.
- -A
-
--output-append-mode Open the file provided in the
-o option in append mode.
- -q
-
--quiet
--quiet=attach,personality Suppress messages about
attaching, detaching, and personality changes. This happens automatically
when output is redirected to a file and the command is run directly
instead of attaching.
- -qq
-
--quiet=attach,personality,exit
Suppress messages attaching, detaching, personality changes, and about
process exit status.
- -qqq
-
--quiet=all Suppress all suppressible
messages (please refer to the -e quiet option description for the
full list of suppressible messages).
- -r
-
--relative-timestamps[=precision]
Print a relative timestamp upon entry to each system call. This records
the time difference between the beginning of successive system calls.
precision can be one of s (for seconds), ms
(milliseconds), us (microseconds), or ns (nanoseconds), and
allows setting the precision of time value being printed. Default is
us (microseconds). Note that since -r option uses the
monotonic clock time for measuring time difference and not the wall clock
time, its measurements can differ from the difference in time reported by
the -t option.
-
-s strsize
-
--string-limit=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.
-
--absolute-timestamps[=[[format:]format],[[precision:]precision]]
-
--timestamps[=[[format:]format],[[precision:]precision]]
Prefix each line of the trace with the wall clock time in the
specified format with the specified precision.
format can be one of the following:
- none
- No time stamp is printed. Can be used to override the
previous setting. time Wall clock time (strftime(3) format
string is %T). unix Number of seconds since the epoch
(strftime(3) format string is %s).
-
precision can be one of s (for seconds),
ms (milliseconds), us (microseconds), or ns
(nanoseconds). Default arguments for the option are
format:time,precision:s.
- -t
-
--absolute-timestamps Prefix each line of the trace
with the wall clock time.
- -tt
-
--absolute-timestamps=precision:us If given
twice, the time printed will include the microseconds.
- -ttt
-
--absolute-timestamps=format:unix,precision:us
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
-
--syscall-times[=precision] Show the
time spent in system calls. This records the time difference between the
beginning and the end of each system call. precision can be one of
s (for seconds), ms (milliseconds), us
(microseconds), or ns (nanoseconds), and allows setting the
precision of time value being printed. Default is us
(microseconds).
- -v
-
--no-abbrev 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.
-
--strings-in-hex[=option]
- Control usage of escape sequences with hexadecimal numbers
in the printed strings. Normally (when no --strings-in-hex or
-x option is supplied), escape sequences are used to print
non-printable and non-ASCII characters (that is, characters with a
character code less than 32 or greater than 127), or to disambiguate the
output (so, for quotes and other characters that encase the printed
string, for example, angle brackets, in case of file descriptor path
output); for the former use case, unless it is a white space character
that has a symbolic escape sequence defined in the C standard (that is,
“ \t” for a horizontal tab, “
\n” for a newline, “ \v” for a vertical
tab, “ \f” for a form feed page break, and “
\r” for a carriage return) are printed using escape
sequences with numbers that correspond to their byte values, with octal
number format being the default. option can be one of the
following:
- none
- Hexadecimal numbers are not used in the output at all. When
there is a need to emit an escape sequence, octal numbers are used.
non-ascii-chars Hexadecimal numbers are used instead of octal in
the escape sequences. non-ascii Strings that contain non-ASCII
characters are printed using escape sequences with hexadecimal numbers.
all All strings are printed using escape sequences with hexadecimal
numbers.
- When the option is supplied without an argument, all
is assumed.
- -x
-
--strings-in-hex=non-ascii Print all
non-ASCII strings in hexadecimal string format.
- -xx
-
--strings-in-hex[=all] Print all strings in
hexadecimal string format.
-
-X format
-
--const-print-style=format Set the
format for printing of named constants and flags. Supported format
values are:
- raw
- Raw number output, without decoding. abbrev Output a
named constant or a set of flags instead of the raw number if they are
found. This is the default strace behaviour. verbose Output
both the raw value and the decoded string (as a comment).
- -y
-
--decode-fds --decode-fds=path Print
paths associated with file descriptor arguments and with the
AT_FDCWD constant.
- -yy
-
--decode-fds=all Print all available
information associated with file descriptors: protocol-specific
information associated with socket file descriptors, block/character
device number associated with device file descriptors, and PIDs associated
with pidfd file descriptors.
- --pidns-translation
-
--decode-pids=pidns If strace and tracee are
in different PID namespaces, print PIDs in strace's namespace, too.
- -Y
-
--decode-pids=comm Print command names for
PIDs.
- -c
-
--summary-only Count time, calls, and errors for
each system call and report a summary on program exit, suppressing the
regular output. This attempts to show system time (CPU time spent running
in the kernel) independent of wall clock time. If -c is used with
-f, only aggregate totals for all traced processes are kept.
- -C
-
--summary Like -c but also print regular
output while processes are running.
-
-O overhead
-
--summary-syscall-overhead=overhead
Set the overhead for tracing system calls to overhead. 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.
- The format of overhead specification is described in
section Time specification format description.
-
-S sortby
-
--summary-sort-by=sortby Sort the
output of the histogram printed by the -c option by the specified
criterion. Legal values are time (or time-percent or
time-total or total-time), min-time (or
shortest or time-min), max-time (or longest or
time-max), avg-time (or time-avg), calls (or
count), errors (or error), name (or
syscall or syscall-name), and nothing (or
none); default is time.
-
-U columns
-
--summary-columns=columns Configure a
set (and order) of columns being shown in the call summary. The
columns argument is a comma-separated list with items being one of
the following:
-
time-percent (or time)
- Percentage of cumulative time consumed by a specific system
call. total-time (or time-total) Total system (or wall
clock, if -w option is provided) time consumed by a specific system
call. min-time (or shortest or time-min) Minimum
observed call duration. max-time (or longest or
time-max) Maximum observed call duration. avg-time (or
time-avg) Average call duration. calls (or count)
Call count. errors (or error) Error count. name (or
syscall or syscall-name) Syscall name.
- The default value is
time-percent,total-time,avg-time,calls,errors,name.
If the name field is not supplied explicitly, it is added as the
last column.
- -w
-
--summary-wall-clock Summarise the time difference
between the beginning and end of each system call. The default is to
summarise the system time.
-
-e inject=syscall_set[:error=errno|:retval=value][:signal=sig][:syscall=syscall][:delay_enter=delay][:delay_exit=delay][:poke_enter=@argN=DATAN,@argM=DATAM...][:poke_exit=@argN=DATAN,@argM=DATAM...][:when=expr]
{
-
--inject=syscall_set[:error=errno|:retval=value][:signal=sig][:syscall=syscall][:delay_enter=delay][:delay_exit=delay][:poke_enter=@argN=DATAN,@argM=DATAM...][:poke_exit=@argN=DATAN,@argM=DATAM...][:when=expr]
{ Perform syscall tampering for the specified set of syscalls. The syntax
of the syscall_set specification is the same as in the -e
trace option.
- At least one of error, retval, signal,
delay_enter, delay_exit, poke_enter, or
poke_exit 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 (unless a syscall is specified
with : syscall= option), 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 :delay_enter=delay or
:delay_exit=delay options are specified, delay injection is
performed: the tracee is delayed by time period specified by delay
on entering or exiting the syscall, respectively. The format of
delay specification is described in section Time specification
format description.
- If :poke_enter=@argN=DATAN,@argM=DATAM... or
: poke_exit=@argN=DATAN,@argM=DATAM... options are
specified, tracee's memory at locations, pointed to by system call
arguments argN and argM (going from arg1 to
arg7) is overwritten by data DATAN and DATAM
(specified in hexadecimal format; for example :
poke_enter=@arg1=0000DEAD0000BEEF). : poke_enter
modifies memory on syscall enter, and : poke_exit - on exit.
- If :signal=sig option is specified without :
error=errno, :retval=value or :
delay_{enter,exit}= usecs options, then only a signal
sig is delivered without a syscall fault or delay injection.
Conversely, : error=errno or : retval=value
option without : delay_enter=delay, :
delay_exit=delay or : signal=sig options
injects a fault without delivering a signal or injecting a delay,
etc.
- If : signal=sig option is specified together
with : error=errno or :retval=value, then both
injection of a fault or success and signal delivery are performed.
- if :syscall=syscall option is specified, the
corresponding syscall with no side effects is injected instead of -1.
Currently, only "pure" (see -e trace=%pure
description) syscalls can be specified there.
- 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:
- Number first stands for the first invocation number
in the range, number last stands for the last invocation number in
the range, and step stands for the step between two consecutive
invocations. The following combinations are useful:
- first
- For every syscall from the set, perform an injection
for the syscall invocation number first only.
first..last For every syscall from the set,
perform an injection for the syscall invocation number first and
all subsequent invocations until the invocation number last
(inclusive). first+ For every syscall from the set,
perform injections for the syscall invocation number first and all
subsequent invocations. first..last+ For every
syscall from the set, perform injections for the syscall invocation
number first and all subsequent invocations until the invocation
number last (inclusive). first+step For every
syscall from the set, perform injections for syscall invocations
number first, first+step,
first+step+step, and so on.
first..last +step Same as the previous,
but consider only syscall invocations with numbers up to last
(inclusive).
- 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, and for number last is 1..65534.
- An injection expression can contain only one error=
or retval= specification, and only one signal=
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=syscall_set[:error=errno][:when=expr]
-
--fault=syscall_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.
- -d
-
--debug Show some debugging output of strace
itself on the standard error.
- -F
- This option is deprecated. It is retained for backward
compatibility only and may be removed in future releases. Usage of
multiple instances of -F option is still equivalent to a single
-f, and it is ignored at all if used along with one or more
instances of -f option.
- -h
-
--help Print the help summary.
- --seccomp-bpf
- Try to enable use of seccomp-bpf (see seccomp(2)) to
have ptrace(2)-stops only when system calls that are being traced
occur in the traced processes. This option has no effect unless
-f/--follow-forks is also specified. --seccomp-bpf is
also not applicable to processes attached using -p/--attach
option. An attempt to enable system calls filtering using seccomp-bpf may
fail for various reasons, e.g. there are too many system calls to filter,
the seccomp API is not available, or strace itself is being traced.
In cases when seccomp-bpf filter setup failed, strace proceeds as
usual and stops traced processes on every system call.
-
--tips[=[[id:]id],[[format:]format]]
- Show strace tips, tricks, and tweaks before exit. id
can be a non-negative integer number, which enables printing of specific
tip, trick, or tweak (these ID are not guaranteed to be stable), or
random (the default), in which case a random tip is printed.
format can be one of the following:
- none
- No tip is printed. Can be used to override the previous
setting. compact Print the tip just big enough to contain all the
text. full Print the tip in its full glory.
- Default is id:random,format:compact.
- -V
-
--version Print the version number of strace.
Multiple instances of the option beyond specific threshold tend to
increase Strauss awareness.
Time values can be specified as a decimal floating point number (in a format
accepted by
strtod(3)), optionally followed by one of the following
suffices that specify the unit of time:
s (seconds),
ms
(milliseconds),
us (microseconds), or
ns (nanoseconds). If no
suffix is specified, the value is interpreted as microseconds.
The described format is used for
-O,
-e inject=
delay_enter,
and
-e inject=
delay_exit options.
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(2) 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 regular non-setuid
version of
strace for ordinary users to use.
On some architectures,
strace supports decoding of syscalls for processes
that use different ABI rather than the one
strace uses. Specifically,
in addition to decoding native ABI,
strace can decode the following
ABIs on the following architectures:
Architecture |
ABIs supported |
x86_64 |
i386, x32 [1]; i386 [2] |
AArch64 |
ARM 32-bit EABI |
PowerPC 64-bit [3] |
PowerPC 32-bit |
s390x |
s390 |
SPARC 64-bit |
SPARC 32-bit |
TILE 64-bit |
TILE 32-bit |
- [1]
- When strace is built as an x86_64 application [2]
When strace is built as an x32 application [3] Big endian only
This support is optional and relies on ability to generate and parse structure
definitions during the build time. Please refer to the output of the
strace
-V command in order to figure out what support is available in your
strace build ("non-native" refers to an ABI that differs from
the ABI
strace has):
- m32-mpers
-
strace can trace and properly decode non-native
32-bit binaries. no-m32-mpers strace can trace, but cannot
properly decode non-native 32-bit binaries. mx32-mpers
strace can trace and properly decode non-native 32-on-64-bit
binaries. no-mx32-mpers strace can trace, but cannot
properly decode non-native 32-on-64-bit binaries.
If the output contains neither
m32-mpers nor
no-m32-mpers, then
decoding of non-native 32-bit binaries is not implemented at all or not
applicable.
Likewise, if the output contains neither
mx32-mpers nor
no-mx32-mpers, then decoding of non-native 32-on-64-bit binaries is not
implemented at all or not applicable.
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, the
faccessat(2) system call does not have
flags argument, and the
setrlimit(2) library function uses
prlimit64(2) system call on modern (2.6.38+) kernels. 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 x32, syscalls that are intended to be used by 64-bit processes and not x32
ones (for example,
readv(2), that has syscall number 19 on x86_64, with
its x32 counterpart has syscall number 515), but called with
__X32_SYSCALL_BIT flag being set, are designated with
#64
suffix.
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.
As
strace executes the specified
command directly and does not
employ a shell for that, scripts without shebang that usually run just fine
when invoked by shell fail to execute with
ENOEXEC error. It is
advisable to manually supply a shell as a
command with the script as
its argument.
Programs that use the
setuid bit do not have effective user
ID privileges while being traced.
A traced process runs slowly (but check out the
--seccomp-bpf option).
Traced processes which are descended from
command may be left running
after an interrupt signal (
CTRL-C).
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 became
tired of writing about himself in the third person.
Beginning with 1996,
strace was maintained by Wichert Akkerman. During
his tenure,
strace development migrated to CVS; ports to FreeBSD and
many architectures on Linux (including ARM, IA-64, MIPS, PA-RISC, PowerPC,
s390, SPARC) were introduced. In 2002, the burden of
strace
maintainership was transferred to Roland McGrath. Since then,
strace
gained support for several new Linux architectures (AMD64, s390x, SuperH),
bi-architecture support for some of them, and received numerous additions and
improvements in syscalls decoders on Linux;
strace development migrated
to
Git during that period. Since 2009,
strace is actively
maintained by Dmitry Levin.
strace gained support for AArch64, ARC,
AVR32, Blackfin, Meta, Nios II, OpenRISC 1000, RISC-V, Tile/TileGx, Xtensa
architectures since that time. In 2012, unmaintained and apparently broken
support for non-Linux operating systems was removed. Also, in 2012
strace gained support for path tracing and file descriptor path
decoding. In 2014, support for stack traces printing was added. In 2016,
syscall fault injection was implemented.
For the additional information, please refer to the
NEWS file and
strace repository commit log.
Problems with
strace should be reported to the
strace
mailing list
strace-log-merge(1),
ltrace(1),
perf-trace(1),
trace-cmd(1),
time(1),
ptrace(2),
syscall(2),
proc(5),
signal(7)
strace Home Page
The complete list of
strace contributors can be found in the
CREDITS file.