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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | DIAGNOSTICS | SETUID INSTALLATION | MULTIPLE PERSONALITIES SUPPORT | NOTES | BUGS | HISTORY | REPORTING BUGS | SEE ALSO | AUTHORS | COLOPHON |
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STRACE(1) General Commands Manual STRACE(1)
strace - trace system calls and signals
strace [-ACdffhikkqqrtttTvVwxxyyYzZ] [-a column] [-b execve]
[-e expr]... [-I n] [-o file] [-O overhead] [-p pid]...
[-P path]... [-s strsize] [-S sortby] [-U columns]
[-X format] [--seccomp-bpf]
[--stack-trace-frame-limit=limit] [--syscall-limit=limit]
[--secontext[=format]] [--tips[=format]] { -p pid | [-DDD]
[-E var[=val]]... [-u username] command [args] }
strace -c [-dfwzZ] [-b execve] [-e expr]... [-I n] [-O overhead]
[-p pid]... [-P path]... [-S sortby] [-U columns]
[--seccomp-bpf] [--syscall-limit=limit] [--tips[=format]] {
-p pid | [-DDD] [-E var[=val]]... [-u username] command
[args] }
strace --tips[=format]
In its simplest use case, strace runs the specified command until
it exits. It intercepts and records the system calls made by a
process and the signals a process receives. The name of each
system call, its arguments, and its return value are printed to
standard error or to the file specified with the -o option.
strace is a useful diagnostic, instructional, and debugging tool.
System administrators, diagnosticians, and troubleshooters will
find it invaluable for solving problems with programs for which
source code is not readily available, as recompilation is not
required for tracing. Students, hackers, and the overly-curious
will discover that a great deal can be learned about a system and
its system calls by tracing even ordinary programs. Programmers
will find that since system calls and signals occur 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 tracing the command "cat /dev/null" is:
open("/dev/null", O_RDONLY) = 3
Errors, typically indicated by a return value of -1, have the
errno symbol and error string appended.
open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory)
Signals are printed as a signal symbol and a decoded siginfo
structure. An excerpt from tracing 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 while another is called from a
different thread or process, strace will attempt to preserve the
order of these events and mark the ongoing call as 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])
The interruption of a (restartable) system call by a signal
delivery is handled differently, as the kernel terminates the
system call and arranges for its immediate re-execution 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 third arguments of open(2) are decoded by
breaking down the flag argument into its three bitwise-OR
constituents and printing the mode value in octal, following
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 code itself.
Structure pointers are dereferenced, and their 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, note that in this example, 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. The
pointer's value is displayed because the structure it points to
was not populated due to the error.
System calls unknown to strace are printed in a raw format, with
the hexadecimal system call number 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
standard 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 it is more
valuable to print the unset elements. 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.
General
-e expr
Modifies which events to trace or how to trace them by
specifying a qualifying expression. The format of the
expression is:
[qualifier=][!]value[,value]...
where qualifier is one of trace (or t), trace-fds (or
trace-fd or fd or fds), 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), secontext, 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 is equivalent to
-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 may be used to trace every event or no
events, respectively.
Note that some shells use the exclamation mark for history
expansion even inside quoted arguments. In that case, the
exclamation mark must be escaped with a backslash.
Startup
-E var=val
--env=var=val
Runs the command with the environment variable var=val set
for execution.
-E var
--env=var
Removes var from the inherited environment variables before
executing the command.
-p pid
--attach=pid
Attaches 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 processes, leaving them to
continue running.
Multiple -p options can be used to attach to several
processes in addition to the command, which is optional if
at least one -p option is given.
A single -p option can accept multiple process IDs
separated by a comma (“,”), space (“ ”), tab, or newline.
Consequently, syntaxes like -p "$(pidof PROG)" and -p
"$(pgrep PROG)" are supported.
-u username
--user=username
Runs command with the user ID, group ID, and supplementary
groups of username. This option is only useful when
running as root, as it enables the correct execution of
setuid and/or setgid binaries. Unless this option is used,
setuid and setgid programs are executed without their
effective privileges.
-u UID:GID
--user=UID:GID
Alternative syntax where the program is started with
exactly the given user and group IDs, and an empty list of
supplementary groups. In this case, user and group name
lookups are not performed.
--argv0=name
Sets the executed command's argv[0] to name. This is
useful for tracing multi-call executables that interpret
argv[0], such as busybox or kmod.
Tracing
-b syscall
--detach-on=syscall
Detaches from the traced process if the specified system
call is reached. Currently, only execve keyword is
supported, which includes execve(2) and execveat(2) system
calls. This option is useful for tracing a multi-threaded
process with -f without also tracing its (potentially very
complex) child processes.
-D
--daemonize
--daemonize=grandchild
Runs the tracer process as a grandchild of the tracee, not
as its parent. This reduces the visible effect of strace
by keeping the tracee a direct child of the calling
process.
-DD
--daemonize=pgroup
--daemonize=pgrp
Runs tracer process as tracee's grandchild in a separate
process group. In addition to reducing the visible effect
of strace, this also prevents strace from being terminated
by a kill(2) signal sent to the entire process group.
-DDD
--daemonize=session
Runs the tracer process as the tracee's grandchild in a
separate session (known as "true daemonisation"). In
addition to reduction of the visible effect of strace, this
also prevents strace from being terminated upon session
termination.
-f
--follow-forks
Traces 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 if process PID is multi-
threaded, using -f -p PID attaches to all of its threads,
not just the one with thread_id = PID.
--output-separately
If the --output=filename option is in effect, the trace for
each process is written to a separate filename.pid file,
where pid is the process ID.
-ff
--follow-forks --output-separately
Combines the effects of --follow-forks and
--output-separately options. This is incompatible with -c,
since no per-process counts are kept.
Use strace-log-merge(1) to get a combined view of the log
files.
-I interruptible
--interruptible=interruptible
Controls 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 system call
(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).
--syscall-limit=limit
Detaches all tracees after limit system calls have been
captured. System calls filtered out via --trace,
--trace-path or --status options are not considered when
keeping track of the number of system calls that are
captured.
--kill-on-exit
Applies the PTRACE_O_EXITKILL ptrace option to all tracees,
which sends a SIGKILL signal to a tracee if the tracer
exits. This prevents tracees from being left running after
the tracer exits, as they will not be detached on cleanup.
--kill-on-exit is not compatible with -p/--attach options.
Filtering
-e trace=syscall_set
-e t=syscall_set
--trace=syscall_set
Traces only the specified set of system calls. syscall_set
is defined as [!]value[,value], and value can be one of the
following:
syscall
Traces specific system call, specified by its name
(see syscalls(2) for a reference, but also see
NOTES).
?value A question mark preceding the qualification
suppresses errors if no matching system calls are
found.
value@64
Limits the system call specification described by
value to the 64-bit personality.
value@32
Limits the system call specification described by
value to the 32-bit personality.
value@x32
Limits the system call specification described by
value to the x32 personality.
all Traces all system calls.
/regex Traces only those system calls that match the regex.
You can use POSIX Extended Regular Expression syntax
(see regex(7)).
%file
file Traces all system calls that take a file name as an
argument. You can think of this as an abbreviation
for --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 newfstatat(2) in the list. The
syntax without a preceding percent sign
("--trace=file") is deprecated.
%process
process
Traces system calls associated with process
lifecycle (creation, exec, termination). The syntax
without a preceding percent sign ("--trace=process")
is deprecated.
%net
%network
network
Traces all the network related system calls. The
syntax without a preceding percent sign
("--trace=network") is deprecated.
%signal
signal Traces all signal related system calls. The syntax
without a preceding percent sign ("--trace=signal")
is deprecated.
%ipc
ipc Traces all IPC related system calls. The syntax
without a preceding percent sign ("--trace=ipc") is
deprecated.
%desc
desc Traces all file descriptor related system calls.
The syntax without a preceding percent sign
("--trace=desc") is deprecated.
%memory
memory Traces all memory mapping related system calls. The
syntax without a preceding percent sign
("--trace=memory") is deprecated.
%creds Traces system calls that read or modify user and
group identifiers or capability sets.
%stat Traces stat system call variants.
%lstat Traces lstat system call variants.
%fstat Traces fstat, fstatat, and statx system call
variants.
%%stat Traces system calls used for requesting file status
(stat, lstat, fstat, fstatat, statx, and their
variants).
%statfs
Traces statfs, statfs64, statvfs, osf_statfs, and
osf_statfs64 system calls. The same effect can be
achieved with --trace=/^(.*_)?statv?fs regular
expression.
%fstatfs
Traces fstatfs, fstatfs64, fstatvfs, osf_fstatfs,
and osf_fstatfs64 system calls. The same effect can
be achieved with --trace=/fstatv?fs regular
expression.
%%statfs
Traces system calls related to file system
statistics (statfs-like, fstatfs-like, and ustat).
The same effect can be achieved with
--trace=/statv?fs|fsstat|ustat regular expression.
%clock Traces system calls that read or modify system
clocks.
%pure Traces system calls 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) 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-fd=set
-e trace-fds=set
-e fd=set
-e fds=set
--trace-fds=set
Traces only the system calls that operate on the specified
subset of (non-negative) file descriptors. Note that usage
of this option also filters out all the system calls that
do not operate on file descriptors at all.
This filter is combined with the --trace-path filter; a
system call is traced if it matches either of them.
-e signal=set
-e signals=set
-e s=set
--signal=set
Traces 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
Prints only system calls with the specified return status.
The default is --status=all. When using the status
qualifier, the chronological order of events may not be
preserved. This is because strace must wait for a system
call to complete before deciding whether to print it. 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
Traces system calls that returned without an error
code. The -z option has the effect of
--status=successful.
failed Traces system calls that returned with an error
code. The -Z option has the effect of
--status=failed.
unfinished
Traces system calls that did not return. This might
happen, for example, due to an execve call in a
different thread from the same thread group.
unavailable
Traces system calls that returned but strace failed
to fetch the error status.
detached
Traces system calls for which strace detached before
the return.
-P path
--trace-path=path
Traces only system calls accessing path. Multiple -P
options can be used to specify several paths. This filter
is combined with the --trace-fds filter; a system call is
traced if it matches either option.
-z
--successful-only
Prints only system calls that returned without an error
code.
-Z
--failed-only
Prints only system calls that returned with an error code.
Output format
-a column
--columns=column
Aligns return values in a specific column (default column
40).
-e abbrev=syscall_set
-e a=syscall_set
--abbrev=syscall_set
Abbreviates the output from printing each member of large
structures. The syntax of the syscall_set specification is
the same as in the --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
Dereferences structures for the specified set of system
calls. The syntax of the syscall_set specification is the
same as in the --trace option. The default is
--verbose=all.
-e raw=syscall_set
-e x=syscall_set
--raw=syscall_set
Prints raw, undecoded arguments for the specified set of
system calls. The syntax of the syscall_set specification
is the same as in the --trace option. This option has the
effect of causing all arguments to be printed in
hexadecimal. This option is useful if the decoding is not
trusted, or if the actual numeric value of an argument is
needed. See also -X raw option.
-e read=set
-e reads=set
-e r=set
--read=set
Performs 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 --read=3,5. Note that this is independent from
the normal tracing of the read(2) system call that is
controlled by the option --trace=read.
-e write=set
-e writes=set
-e w=set
--write=set
Performs 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 --write=3,5. Note that this is independent
from the normal tracing of the write(2) system call that is
controlled by the option --trace=write.
-e quiet=set
-e silent=set
-e silence=set
-e q=set
--quiet=set
--silent=set
--silence=set
Suppresses various information messages. The default is
--quiet=none. set can include the following elements:
attach Suppresses 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
Decodes various information associated with file
descriptors. The default is --decode-fds=none. set can
include the following elements:
path Prints file paths. Also enables printing of
tracee's current working directory when AT_FDCWD
constant is used.
socket Prints socket protocol-specific information.
dev Prints character/block device numbers.
eventfd Prints eventfd object details associated with
eventfd file descriptors.
pidfd Prints PIDs associated with pidfd file
descriptors.
signalfd Prints signal masks associated with signalfd file
descriptors.
-e decode-pids=set
--decode-pids=set
Decodes 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 Prints command names associated with thread or
process IDs.
pidns Prints 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
Prints the exit reason of kvm vcpu. Requires Linux kernel
version 4.16.0 or higher.
-e namespace=new
--namespace=new
Prints the new namespaces entered by the tracee. The
following system calls are supported: clone(2), clone3(2),
setns(2), and unshare(2).
-i
--instruction-pointer
Prints the instruction pointer at the time of the system
call.
-n
--syscall-number
Prints the system call number.
-N
--arg-names
Prints the system call argument names.
-k
--stack-trace[=symbol]
Prints the execution stack trace of the traced processes
after each system call.
-kk
--stack-trace=source
Prints the execution stack trace and source code
information of the traced processes after each system call.
This option expects the target program is compiled with
appropriate debug options: "-g" (gcc), or "-g
-gdwarf-aranges" (clang).
--stack-trace-frame-limit=limit
Prints no more than this amount of stack trace frames when
backtracing a system call (the default is 256). Use this
option with the --stack-trace (or -k) option.
-o filename
--output=filename
Writes 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. Piping output to a command is not
currently compatible with the -ff option.
-A
--output-append-mode
Opens the file provided in the -o option in append mode.
-q
--quiet
--quiet=attach,personality
Suppresses 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
Suppresses messages about attaching, detaching, personality
changes, and process exit status.
-qqq
--quiet=all
Suppresses all suppressible messages (please refer to the
--quiet option description for the full list of
suppressible messages).
-r
--relative-timestamps[=precision]
Prints 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 because the -r option uses the monotonic clock, its
measurements may differ from the time differences reported
by the -t option, which uses the wall clock.
-s strsize
--string-limit=strsize
Specifies 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]]
Prefixes 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
Prefixes each line of the trace with the wall clock time.
-tt
--absolute-timestamps=precision:us
Prints the wall clock time with microsecond precision.
-ttt
--absolute-timestamps=format:unix,precision:us
Prints the wall clock time as seconds since the epoch, with
microsecond precision.
-T
--syscall-times[=precision]
Shows 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
Prints unabbreviated versions of environment, stat,
termios, etc. calls. These structures are very common, so
the default behavior is to display a reasonable subset of
their members. Use this option to see all members in full
detail.
--strings-in-hex[=option]
Controls the use of hexadecimal escape sequences when
printing strings. This option alters the default escaping
behavior.
Normally (when neither this option nor -x is used), strace
introduces escape sequences in two situations: to represent
non-printable and non-ASCII characters (i.e., those with
character codes less than 32 or greater than 127), or to
disambiguate output, for example, by escaping the quotation
marks that enclose a string or the angle brackets used in
file descriptor paths. When a character must be escaped,
strace prioritizes symbolic C-standard sequences if one
exists: “\t” (tab), “\n” (newline), “\v” (vertical tab),
“\f” (form feed), and “\r” (carriage return). For all
other characters that require escaping, strace defaults to
using an octal representation of the character's byte
value. This option allows you to override this default
behavior and use hexadecimal escapes instead of octal ones.
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
Prints all non-ASCII strings in hexadecimal string format.
-xx
--strings-in-hex[=all]
Prints all strings in hexadecimal string format.
-X format
--const-print-style=format
Sets the format for printing of named constants and flags.
Supported format values are:
raw Raw number output, without decoding.
abbrev Outputs a named constant or a set of flags instead
of the raw number if they are found. This is the
default strace behaviour.
verbose
Outputs both the raw value and the decoded string
(as a comment).
-y
--decode-fds
--decode-fds=path
Prints paths associated with file descriptor arguments and
with the AT_FDCWD constant.
-yy
--decode-fds=all
Prints 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
Prints command names for PIDs.
--secontext[=format]
-e secontext=format
When SELinux is available and is not disabled, prints in
square brackets SELinux contexts of processes, files, and
descriptors. The format argument is a comma-separated list
of items being one of the following:
full Prints the full context (user, role, type
level and category).
mismatch Also prints the context recorded by the
SELinux database in case the current
context differs. The latter is printed
after two exclamation marks (!!).
The default value for --secontext is !full,mismatch that
prints only the type instead of full context and doesn't
check for context mismatches.
--always-show-pid
Shows PID prefix also for the process started by strace.
Implied when -f and -o are both specified.
Statistics
-c
--summary-only
Counts time, calls, and errors for each system call and
report a summary on program exit, suppressing the regular
output. This shows system time (CPU time spent in the
kernel), which is 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 prints the regular output while processes
are running.
-O overhead
--summary-syscall-overhead=overhead
Sets the overhead for tracing system calls to overhead.
This is useful for overriding the default heuristic, which
estimates the time spent in the measurement process itself
when timing system calls with 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
Sorts the output of the histogram printed by the -c option
by the specified criterion. Valid 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
Configures the set and order of columns shown in the call
summary. The columns argument is a comma-separated list
containing one or more of the following values:
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)
System call 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
Summarizes the wall clock time for each system call,
measured from its beginning to its end. The default is to
summarize the system time.
Tampering
--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]
Performs system call tampering for the specified set of
system calls.
The syntax of the syscall_set specification is the same as
in the --trace option.
At least one of error, retval, signal, delay_enter,
delay_exit, poke_enter, or poke_exit action options must be
specified. error and retval are mutually exclusive.
If the error=errno option is specified, a fault is injected
into the system call. This is achieved by replacing the
system call number with -1 (representing an invalid system
call) and setting the error code to the specified errno.
This behavior of replacing the syscall number with -1 can
be overridden using the syscall= option. The errno can be
a symbolic name like ENOSYS or a numeric value in the range
1..4095.
If the retval=value option is specified, a success value is
injected. The system call number is replaced as with the
error= option, but instead of an error, the specified
success value is returned to the caller process.
If the 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 system call specified by the syscall_set.
If the 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 system call, respectively. The format of delay
specification is described in section Time specification
format description.
If the 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). The poke_enter option
modifies memory on system call enter, while poke_exit does
so on system call exit.
The injection actions are independent. For example,
specifying only signal= delivers a signal without altering
the system call's outcome or delaying it. Similarly,
specifying only error= injects a system call fault without
adding a signal or delay.
If the 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 the syscall=syscall option is specified, the given
syscall is injected instead of the default -1. The
specified syscall must have no side effects; currently,
only system calls from the %pure set are supported.
Unless the when=expr subexpression is specified, an
injection is being made into every invocation of each
system call from the syscall_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
consecutive invocations. The following combinations are
useful:
first Injects into invocation number first only for each
system call in the syscall_set.
first..last
Injects into invocations from first through last
(inclusive) for each system call in the syscall_set.
first+ Injects into every invocation, starting with number
first, for each system call in the syscall_set.
first+step
Injects into invocations number first, first+step,
first+step+step, and so on, for each system call in
the syscall_set.
first..last+step
Same as the previous, but consider only invocations
with numbers up to last (inclusive).
For example, to fail each third and subsequent chdir system
calls with ENOENT, use --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 at most one fault or
return value specification (i.e., either error= or retval=)
and at most one signal= specification. If an injection
expression contains multiple when= specifications, the last
one takes precedence.
Accounting of system calls that are subject to injection is
done per system call and per tracee.
Specification of system call injection can be combined with
other system call filtering options, for example, -P
/dev/urandom --inject=file:error=ENOENT.
-e inject=args
This is equivalent to --inject=args.
--fault=syscall_set[:error=errno][:when=expr]
Performs system call fault injection for the specified set
of system calls.
This is a shortcut for the more general --inject= option,
using a default errno of ENOSYS.
-e fault=args
This is equivalent to --fault=args.
Miscellaneous
-d
--debug
Shows 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.
Using multiple -F options is equivalent to a single -f.
This option is ignored entirely if used in conjunction with
one or more -f options.
-h
--help Prints the help summary.
--seccomp-bpf
Attempts to use seccomp-bpf (see seccomp(2)) to cause the
kernel to stop the tracee only for the system calls that
are being traced.
This option has no effect unless -f/--follow-forks is also
specified. --seccomp-bpf is not compatible with
--syscall-limit and -b/--detach-on options. It 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. If the seccomp-bpf
filter setup fails, strace proceeds as usual, stopping
traced processes on every system call.
When --seccomp-bpf is activated and -p/--attach option is
not used, --kill-on-exit option is activated as well.
Note that in cases when the tracee has another seccomp
filter that returns an action value with a precedence
greater than SECCOMP_RET_TRACE, strace --seccomp-bpf will
not be notified. That is, if another seccomp filter, for
example, disables the system call or kills the tracee, then
strace --seccomp-bpf will not be aware of that system call
invocation at all.
--tips[=[[id:]id],[[format:]format]]
Shows strace tips, tricks, and tweaks before exit. The id
can be a non-negative integer to print a specific tip
(note: these IDs are not guaranteed to be stable). It can
also be 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 Prints the tip just big enough to contain all the
text.
full Prints the tip in its full glory.
Default is id:random,format:compact.
-V
--version
Prints the version number of strace and the list of enabled
optional features. Multiple instances of this option
beyond specific threshold tend to increase der Strauss
awareness.
Time specification format description
Time values are specified as a decimal floating point number (in a
format accepted by strtod(3)), optionally followed by a suffix to
indicate the unit of time: s (seconds), ms (milliseconds), us
(microseconds), or ns (nanoseconds). If no suffix is specified,
the value defaults to microseconds.
The described format is used for -O, --inject=delay_enter, and
--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 the parent-
child relationship (signal stop notifications, the getppid(2)
value, etc) between the traced process and its parent is not
preserved unless -D is used.
When using -p without a command, the exit status of strace is zero
unless no processes have been attached or an unexpected error
occurred during 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 these capabilities
should only be granted to users with full root privileges,
installing strace as setuid to root is only appropriate when its
use is restricted to such trusted users. For example, a special
version of strace could be installed with mode 'rwsr-x---', user
root, and group trace. In this configuration, only trusted users
who are members of the trace group could execute it. If you use
this feature, remember to also install a regular, non-setuid
version of strace for ordinary users.
On some architectures, strace can decode system calls for
processes that use a different Application Binary Interface (ABI)
from 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 depends on the ability to generate
and parse structure definitions at build time. Refer to the
output of the strace -V command to determine which ABIs are
supported by your strace build. In this context, "non-native"
refers to an ABI that differs from the one strace is using:
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, it
means that support for decoding non-native 32-bit binaries is not
applicable to the architecture.
Likewise, if the output contains neither mx32-mpers nor
no-mx32-mpers, it means that support for decoding non-native
32-on-64-bit binaries is not applicable to the architecture.
Systems that use shared libraries often produce a large amount of
tracing output when loading them.
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 underlying
faccessat(2) system call does not have a flags argument, and the
setrlimit(2) library function is implemented using prlimit64(2)
system call on modern (2.6.38+) kernels. These discrepancies are
normal characteristics of the system call interface and are
handled 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
system call and xtensa_fadvise64_64(2) Xtensa system call are
filtered and printed as fadvise64_64(2).
On the x32 ABI, some system calls are intended for 64-bit
processes but can be invoked from x32 by setting the
__X32_SYSCALL_BIT flag. When this occurs, strace designates these
calls with a #64 suffix. An example is readv(2), which is syscall
number 19 on x86_64, whereas its distinct x32 counterpart is
syscall number 515.
On some platforms, a process attached with the -p option may
receive a spurious EINTR error from a non-restartable system call.
This can have an unpredictable effect on the process if it does
not attempt to restart the call. 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.
Since strace executes the specified command directly without a
shell, scripts that lack a shebang line (e.g., #!/bin/sh) will
fail with an ENOEXEC error, even if a shell could run them
correctly. 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 more slowly than a non-traced one. The
performance impact can be mitigated by using the --seccomp-bpf
option.
When tracing a command, its descendant processes may be left
running after strace is terminated by an interrupt signal (such as
CTRL-C). This can be prevented by using the --kill-on-exit
option, or by using --seccomp-bpf option in a way that implies
--kill-on-exit.
A traced process can use the CLONE_UNTRACED flag with the clone
system call to create a child process that is not traced by
strace. This breaks a guarantee of the --seccomp-bpf option, as
this untraced child may be left with an active seccomp filter
after strace terminates.
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 took on the project. He merged strace 2.5
for SunOS with the second release of strace for Linux, added many
features from SVR4's truss(1), and produced a version of 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, responsibility for strace maintenance 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 system calls decoders on Linux; strace development
migrated to Git during that period.
Since 2009, strace has been actively maintained by Dmitry Levin.
During this period, strace has gained support for the AArch64,
ARC, AVR32, Blackfin, C-SKY, LoongArch, Meta, Nios II, OpenRISC
1000, RISC-V, Tile/TileGx, and Xtensa architectures. 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 trace printing was added. In 2016, system call tampering
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
⟨mailto:[email protected]⟩.
strace-log-merge(1), ltrace(1), perf-trace(1), trace-cmd(1),
time(1), ptrace(2), seccomp(2), syscall(2), proc(5), signal(7)
strace Home Page ⟨https://strace.io/⟩
The complete list of strace contributors can be found in the
CREDITS file.
This page is part of the strace (system call tracer) project.
Information about the project can be found at ⟨http://strace.io/⟩.
If you have a bug report for this manual page, send it to
[email protected]. This page was obtained from
the project's upstream Git repository
⟨https://github.com/strace/strace.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-06.) 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
[email protected]
strace 6.16.0.1.ca9fa 2025-08-05 STRACE(1)
Pages that refer to this page: ltrace(1), strace-log-merge(1), ptrace(2), seccomp(2), proc_pid_maps(5), capabilities(7), mount_namespaces(7), vdso(7), ovs-ctl(8), systemd-sysext(8)
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