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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 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.
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), 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 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.
Startup
-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.
-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
Set argv[0] of the command being executed to name. Useful
for tracing multi-call executables which interpret
argv[0], such as busybox or kmod.
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
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).
--syscall-limit=limit
Detach all tracees when limit number of syscalls have been
captured. Syscalls filtered out via --trace, --trace-path
or --status options are not considered when keeping track
of the number of syscalls that are captured.
--kill-on-exit
Apply PTRACE_O_EXITKILL ptrace option to all tracee
processes (which sends a SIGKILL signal to the tracee if
the tracer exits) and do not detach them on cleanup so
they will not be left running after the tracer exit.
--kill-on-exit is not compatible with -p/--attach options.
Filtering
-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 trace-fd=set
-e trace-fds=set
-e fd=set
-e fds=set
--trace-fds=set
Trace only the syscalls that operate on the specified
subset of (non-negative) file descriptors. Note that
usage of this option also filters out all the syscalls
that do not operate on file descriptors at all. Applies
in (inclusive) disjunction with the --trace-path option.
-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. Applies in
(inclusive) disjunction with the --trace-fds option.
-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
-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.
signalfd Print signal masks associated with signalfd 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-trace[=symbol]
Print the execution stack trace of the traced processes
after each system call.
-kk
--stack-trace=source
Print 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
Print 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
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.
--secontext[=format]
-e secontext=format
When SELinux is available and is not disabled, print 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 Print the full context (user, role, type
level and category).
mismatch Also print 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 which
prints only the type instead of full context and doesn't
check for context mismatches.
--always-show-pid
Show PID prefix also for the process started by strace.
Implied when -f and -o are both specified.
Statistics
-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.
Tampering
-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:
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 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.
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
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 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. In cases when seccomp-
bpf filter setup failed, strace proceeds as usual and
stops 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 syscall or kills the tracee, then
strace --seccomp-bpf will not be aware of that syscall
invocation at all.
--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 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 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).
Unless --kill-on-exit option is used (or --seccomp-bpf option is
used in a way that implies --kill-on-exit), traced processes
which are descended from command may be left running after an
interrupt signal (CTRL-C).
By using CLONE_UNTRACED flag of clone system call a tracee can
break the guarantee that --seccomp-bpf will not leave any
processes with a seccomp program installed for syscall filtering
purposes.
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
trace 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 ⟨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 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-04.) 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 6.9.0.16.2a4c4 2024-06-04 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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