STRACE(1) General Commands Manual STRACE(1)
NAME
strace - trace system calls and signals
SYNOPSIS
strace [-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] }
strace -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] }
DESCRIPTION
In the simplest case strace runs the specified command until it exits.
It intercepts and records the system calls which are called by a
process and the signals which are received by a process. The name of
each system call, its arguments and its return value are printed on
standard error or to the file specified with the -o option.
strace is a useful diagnostic, instructional, and debugging tool. Sys-
tem administrators, diagnosticians and trouble-shooters will find it
invaluable for solving problems with programs for which the source is
not readily available since they do not need to be recompiled in order
to trace them. Students, hackers and the overly-curious will find that
a great deal can be learned about a system and its system calls by
tracing even ordinary programs. And programmers will find that since
system calls and signals are events that happen at the user/kernel in-
terface, 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 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 unfin-
ished. When the call returns it will be marked as resumed.
[pid 28772] select(4, [3], NULL, NULL, NULL <unfinished ...>
[pid 28779] clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
[pid 28772] <... select resumed> ) = 1 (in [3])
Interruption of a (restartable) system call by a signal delivery is
processed differently as kernel terminates the system call and also ar-
ranges 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 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 tradi-
tional 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 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(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) li-
brary 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(1) on a system with sup-
plementary 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
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
General
-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), ver-
bose (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 de-
code-fd), decode-pids (or decode-pid), or kvm, and value is
a qualifier-dependent symbol or number. The default quali-
fier 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 sys-
tem 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.
Startup
-E var=val
--env=var=val
Run command with var=val in its list of environment vari-
ables.
-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 key-
board 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 sup-
ported.
-u username
--user=username
Run command with the user ID, group ID, and supplementary
groups of username. This option is only useful when run-
ning 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 privi-
leges.
Tracing
-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 ef-
fect 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 pro-
cesses 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-sepa-
rately options. This is incompatible with -c, since no
per-process counts are kept.
One might want to consider using strace-log-merge(1) to ob-
tain 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 al-
ways blocked (useful to make strace -o FILE
PROG not stop on CTRL-Z, default if -D).
Filtering
-e trace=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
(but see NOTES).
?value Question mark before the syscall qualification
allows suppression of error in case no
syscalls matched the qualification provided.
/regex Trace only those system calls that match the
regex. You can use POSIX Extended Regular Ex-
pression syntax (see regex(7)).
syscall@64 Trace syscall only for the 64-bit personality.
syscall@32 Trace syscall only for the 32-bit personality.
syscall@x32 Trace syscall only for the 32-on-64-bit per-
sonality.
%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,un-
link,... which is useful to seeing what files
the process is referencing. Furthermore, us-
ing 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 preced-
ing percent sign ("-e trace=file") is depre-
cated.
%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 syn-
tax 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 vari-
ants.
%%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 ef-
fect can be achieved with
-e trace=/^(.*_)?statv?fs regular expression.
%fstatfs Trace fstatfs, fstatfs64, fstatvfs, osf_fs-
tatfs, and osf_fstatfs64 system calls. The
same effect can be achieved with -e trace=/fs-
tatv?fs regular expression.
%%statfs Trace syscalls related to file system statis-
tics (statfs-like, fstatfs-like, and ustat).
The same effect can be achieved with
-e trace=/statv?fs|fsstat|ustat regular ex-
pression.
%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), get-
gid(2), getgid32(2), getpagesize(2), getp-
grp(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
--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 quali-
fier, because strace waits for system calls to return be-
fore 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 sta-
tus=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 op-
tions 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.
Output format
-a column
--columns=column
Align return values in a specific column (default column
40).
-e abbrev=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 ab-
brev=all. The -v option has the effect of abbrev=none.
-e verbose=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 ver-
bose=all.
-e raw=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 hexadeci-
mal. This is mostly useful if you don't trust the decoding
or you need to know the actual numeric value of an argu-
ment. See also -X raw option.
-e read=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
--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
--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 de-
taching ("[ 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 ("Re-
quested path "..." resolved into "..."").
personality Suppress messages about process personal-
ity changes ("[ Process PID=NNNN runs in
PPP mode. ]").
thread-execve
superseded Suppress messages about process being su-
perseded 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 descrip-
tors. 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 af-
ter 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 sup-
plied. 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 personal-
ity changes. This happens automatically when output is
redirected to a file and the command is run directly in-
stead 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 mea-
suring time difference and not the wall clock time, its
measurements can differ from the difference in time re-
ported 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 (strf-
time(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 microsec-
onds.
-ttt
--absolute-timestamps=format:unix,precision:us
If given thrice, the time printed will include the mi-
croseconds 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 (mil-
liseconds), us (microseconds), or ns (nanoseconds), and al-
lows 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 se-
quences with hexadecimal numbers.
When the option is supplied without an argument, all is as-
sumed.
-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 de-
scriptors: protocol-specific information associated with
socket file descriptors, block/character device number as-
sociated 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.
Statistics
-c
--summary-only
Count time, calls, and errors for each system call and re-
port 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 tim-
ing system calls using the -c option. The accuracy of the
heuristic can be gauged by timing a given program run with-
out 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 sec-
tion 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 er-
ror), 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 cumula-
tive 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,to-
tal-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 sys-
tem time.
Tampering
-e inject=syscall_set[:error=errno|:retval=value][:sig-
nal=sig][:syscall=syscall][:delay_enter=delay][:delay_exit=de-
lay][:poke_en-
ter=@argN=DATAN,@argM=DATAM...][:poke_exit=@argN=DATAN,@argM=DATAM...][:when=expr]
--inject=syscall_set[:error=errno|:retval=value][:sig-
nal=sig][:syscall=syscall][:delay_en-
ter=delay][:delay_exit=delay][:poke_en-
ter=@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, de-
lay_exit, poke_enter, or poke_exit options has to be speci-
fied. 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 bo-
gus 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 de-
layed by time period specified by delay on entering or ex-
iting the syscall, respectively. The format of delay spec-
ification is described in section Time specification format
description.
If :poke_enter=@argN=DATAN,@argM=DATAM... or
:poke_exit=@argN=DATAN,@argM=DATAM... options are speci-
fied, 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 hexadeci-
mal format; for example :poke_en-
ter=@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=er-
rno 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 injec-
tion is being made into every invocation of each syscall
from the set.
The format of the subexpression is:
first[..last][+[step]]
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 consec-
utive 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 invoca-
tions until the invocation number last
(inclusive).
first+ For every syscall from the set, perform
injections for the syscall invocation
number first and all subsequent invoca-
tions.
first..last+ For every syscall from the set, perform
injections for the syscall invocation
number first and all subsequent invoca-
tions 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:er-
ror=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 ret-
val= specification, and only one signal= specification. If
an injection expression contains multiple when= specifica-
tions, 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/uran-
dom -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.
Miscellaneous
-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 equiva-
lent 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. --sec-
comp-bpf is also not applicable to processes attached using
-p/--attach option. An attempt to enable system calls fil-
tering 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.
-V
--version Print the version number of strace.
Time specification format description
Time values can be specified as a decimal floating point number (in a
format accepted by strtod(3)), optionally followed by one of the fol-
lowing suffices that specify the unit of time: s (seconds), ms (mil-
liseconds), 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 in-
ject=delay_exit options.
DIAGNOSTICS
When command exits, strace exits with the same exit status. If command
is terminated by a signal, strace terminates itself with the same sig-
nal, 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 un-
less no processes has been attached or there was an unexpected error in
doing the tracing.
SETUID INSTALLATION
If strace is installed setuid to root then the invoking user will be
able to attach to and trace processes owned by any user. In addition
setuid and setgid programs will be executed and traced with the correct
effective privileges. Since only users trusted with full root privi-
leges should be allowed to do these things, it only makes sense to in-
stall 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 regu-
lar non-setuid version of strace for ordinary users to use.
MULTIPLE PERSONALITIES SUPPORT
On some architectures, strace supports decoding of syscalls for pro-
cesses that use different ABI rather than the one strace uses. Specif-
ically, in addition to decoding native ABI, strace can decode the fol-
lowing 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 out-
put 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 dif-
fers 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 decod-
ing of non-native 32-bit binaries is not implemented at all or not ap-
plicable.
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.
NOTES
It is a pity that so much tracing clutter is produced by systems em-
ploying 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 ker-
nel-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 dis-
crepancies 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 gen-
eral 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 argu-
ment.
BUGS
Programs that use the setuid bit do not have effective user ID privi-
leges 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).
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 pro-
duced an strace that worked on both platforms. In 1994 Rick ported
strace to SVR4 and Solaris and wrote the automatic configuration sup-
port. In 1995 he ported strace to Irix and tired of writing about him-
self 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, Pow-
erPC, 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, Su-
perH), bi-architecture support for some of them, and received numerous
additions and improvements in syscalls decoders on Linux; strace devel-
opment migrated to git during that period. Since 2009, strace is ac-
tively maintained by Dmitry Levin. strace gained support for AArch64,
ARC, AVR32, Blackfin, Meta, Nios II, OpenRISC 1000, RISC-V, Tile/Ti-
leGx, 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 descrip-
tor 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.
REPORTING BUGS
Problems with strace should be reported to the strace mailing list
⟨mailto:strace-devel@lists.strace.io⟩.
SEE ALSO
strace-log-merge(1), ltrace(1), perf-trace(1), trace-cmd(1), time(1),
ptrace(2), proc(5)
strace Home Page ⟨https://strace.io/⟩
AUTHORS
The complete list of strace contributors can be found in the CREDITS
file.
strace 5.16 2022-01-04 STRACE(1)
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