Provided by: strace_5.16-0ubuntu3_amd64 
NAME
strace - trace system calls and signals
SYNOPSIS
strace [-ACdffhikqqrtttTvVwxxyyzZ] [-I n] [-b execve] [-e expr]... [-O overhead]
[-S sortby] [-U columns] [-a column] [-o file] [-s strsize] [-X format]
[-P path]... [-p pid]... [--seccomp-bpf] { -p pid | [-DDD] [-E var[=val]]...
[-u username] command [args] }
strace -c [-dfwzZ] [-I n] [-b execve] [-e expr]... [-O overhead] [-S sortby] [-U columns]
[-P path]... [-p pid]... [--seccomp-bpf] { -p pid | [-DDD] [-E var[=val]]...
[-u username] command [args] }
DESCRIPTION
In the simplest case strace runs the specified command until it exits. It intercepts and
records the system calls which are called by a process and the signals which are received
by a process. The name of each system call, its arguments and its return value are
printed on standard error or to the file specified with the -o option.
strace is a useful diagnostic, instructional, and debugging tool. System administrators,
diagnosticians and trouble-shooters will find it invaluable for solving problems with
programs for which the source is not readily available since they do not need to be
recompiled in order to trace them. Students, hackers and the overly-curious will find
that a great deal can be learned about a system and its system calls by tracing even
ordinary programs. And programmers will find that since system calls and signals are
events that happen at the user/kernel interface, a close examination of this boundary is
very useful for bug isolation, sanity checking and attempting to capture race conditions.
Each line in the trace contains the system call name, followed by its arguments in
parentheses and its return value. An example from stracing the command "cat /dev/null"
is:
open("/dev/null", O_RDONLY) = 3
Errors (typically a return value of -1) have the errno symbol and error string appended.
open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory)
Signals are printed as signal symbol and decoded siginfo structure. An excerpt from
stracing and interrupting the command "sleep 666" is:
sigsuspend([] <unfinished ...>
--- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=...} ---
+++ killed by SIGINT +++
If a system call is being executed and meanwhile another one is being called from a
different thread/process then strace will try to preserve the order of those events and
mark the ongoing call as being unfinished. When the call returns it will be marked as
resumed.
[pid 28772] select(4, [3], NULL, NULL, NULL <unfinished ...>
[pid 28779] clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
[pid 28772] <... select resumed> ) = 1 (in [3])
Interruption of a (restartable) system call by a signal delivery is processed differently
as kernel terminates the system call and also arranges its immediate reexecution after the
signal handler completes.
read(0, 0x7ffff72cf5cf, 1) = ? ERESTARTSYS (To be restarted)
--- SIGALRM ... ---
rt_sigreturn(0xe) = 0
read(0, "", 1) = 0
Arguments are printed in symbolic form with 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, simple pointers and arrays are printed
using square brackets with commas separating 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.
OPTIONS
General
-e expr A qualifying expression which modifies which events to trace or how to trace
them. The format of the expression is:
[qualifier=][!]value[,value]...
where qualifier is one of trace (or t), abbrev (or a), verbose (or v), raw (or
x), signal (or signals or s), read (or reads or r), write (or writes or w),
fault, inject, status, quiet (or silent or silence or q), decode-fds (or
decode-fd), decode-pids (or decode-pid), or kvm, and value is a qualifier-
dependent symbol or number. The default qualifier is trace. Using an
exclamation mark negates the set of values. For example, -e open means
literally -e trace=open which in turn means trace only the open system call.
By contrast, -e trace=!open means to trace every system call except open. In
addition, the special values all and none have the obvious meanings.
Note that some shells use the exclamation point for history expansion even
inside quoted arguments. If so, you must escape the exclamation point with a
backslash.
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.
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).
Filtering
-e trace=syscall_set
--trace=syscall_set
Trace only the specified set of system calls. syscall_set is defined as
[!]value[,value], and value can be one of the following:
syscall Trace specific syscall, specified by its name (but see NOTES).
?value Question mark before the syscall qualification allows suppression
of error in case no syscalls matched the qualification provided.
/regex Trace only those system calls that match the regex. You can use
POSIX Extended Regular Expression syntax (see regex(7)).
syscall@64 Trace syscall only for the 64-bit personality.
syscall@32 Trace syscall only for the 32-bit personality.
syscall@x32 Trace syscall only for the 32-on-64-bit personality.
%file
file Trace all system calls which take a file name as an argument.
You can think of this as an abbreviation for
-e trace=open,stat,chmod,unlink,... which is useful to seeing
what files the process is referencing. Furthermore, using the
abbreviation will ensure that you don't accidentally forget to
include a call like lstat(2) in the list. Betchya woulda forgot
that one. The syntax without a preceding percent sign ("-e
trace=file") is deprecated.
%process
process Trace system calls associated with process lifecycle (creation,
exec, termination). The syntax without a preceding percent sign
("-e trace=process") is deprecated.
%net
%network
network Trace all the network related system calls. The syntax without a
preceding percent sign ("-e trace=network") is deprecated.
%signal
signal Trace all signal related system calls. The syntax without a
preceding percent sign ("-e trace=signal") is deprecated.
%ipc
ipc Trace all IPC related system calls. The syntax without a
preceding percent sign ("-e trace=ipc") is deprecated.
%desc
desc Trace all file descriptor related system calls. The syntax
without a preceding percent sign ("-e trace=desc") is deprecated.
%memory
memory Trace all memory mapping related system calls. The syntax
without a preceding percent sign ("-e trace=memory") is
deprecated.
%creds Trace system calls that read or modify user and group identifiers
or capability sets.
%stat Trace stat syscall variants.
%lstat Trace lstat syscall variants.
%fstat Trace fstat, fstatat, and statx syscall variants.
%%stat Trace syscalls used for requesting file status (stat, lstat,
fstat, fstatat, statx, and their variants).
%statfs Trace statfs, statfs64, statvfs, osf_statfs, and osf_statfs64
system calls. The same effect can be achieved with
-e trace=/^(.*_)?statv?fs regular expression.
%fstatfs Trace fstatfs, fstatfs64, fstatvfs, osf_fstatfs, and
osf_fstatfs64 system calls. The same effect can be achieved with
-e trace=/fstatv?fs regular expression.
%%statfs Trace syscalls related to file system statistics (statfs-like,
fstatfs-like, and ustat). The same effect can be achieved with
-e trace=/statv?fs|fsstat|ustat regular expression.
%clock Trace system calls that read or modify system clocks.
%pure Trace syscalls that always succeed and have no arguments.
Currently, this list includes arc_gettls(2), getdtablesize(2),
getegid(2), getegid32(2), geteuid(2), geteuid32(2), getgid(2),
getgid32(2), getpagesize(2), getpgrp(2), getpid(2), getppid(2),
get_thread_area(2) (on architectures other than x86), gettid(2),
get_tls(2), getuid(2), getuid32(2), getxgid(2), getxpid(2),
getxuid(2), kern_features(2), and metag_get_tls(2) syscalls.
The -c option is useful for determining which system calls might be useful to
trace. For example, trace=open,close,read,write means to only trace those
four system calls. Be careful when making inferences about the user/kernel
boundary if only a subset of system calls are being monitored. The default is
trace=all.
-e signal=set
--signal=set
Trace only the specified subset of signals. The default is signal=all. For
example, signal=!SIGIO (or signal=!io) causes SIGIO signals not to be traced.
-e status=set
--status=set
Print only system calls with the specified return status. The default is
status=all. When using the status qualifier, because strace waits for system
calls to return before deciding whether they should be printed or not, the
traditional order of events may not be preserved anymore. If two system calls
are executed by concurrent threads, strace will first print both the entry and
exit of the first system call to exit, regardless of their respective entry
time. The entry and exit of the second system call to exit will be printed
afterwards. Here is an example when select(2) is called, but a different
thread calls clock_gettime(2) before select(2) finishes:
[pid 28779] 1130322148.939977 clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0
[pid 28772] 1130322148.438139 select(4, [3], NULL, NULL, NULL) = 1 (in [3])
set can include the following elements:
successful Trace system calls that returned without an error code. The -z
option has the effect of status=successful.
failed Trace system calls that returned with an error code. The -Z
option has the effect of status=failed.
unfinished Trace system calls that did not return. This might happen, for
example, due to an execve call in a neighbour thread.
unavailable Trace system calls that returned but strace failed to fetch the
error status.
detached Trace system calls for which strace detached before the return.
-P path
--trace-path=path
Trace only system calls accessing path. Multiple -P options can be used to
specify several paths.
-z
--successful-only
Print only syscalls that returned without an error code.
-Z
--failed-only
Print only syscalls that returned with an error code.
Output format
-a column
--columns=column
Align return values in a specific column (default column 40).
-e abbrev=syscall_set
--abbrev=syscall_set
Abbreviate the output from printing each member of large structures. The
syntax of the syscall_set specification is the same as in the -e trace option.
The default is abbrev=all. The -v option has the effect of abbrev=none.
-e verbose=syscall_set
--verbose=syscall_set
Dereference structures for the specified set of system calls. The syntax of
the syscall_set specification is the same as in the -e trace option. The
default is verbose=all.
-e raw=syscall_set
--raw=syscall_set
Print raw, undecoded arguments for the specified set of system calls. The
syntax of the syscall_set specification is the same as in the -e trace option.
This option has the effect of causing all arguments to be printed in
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
--read=set Perform a full hexadecimal and ASCII dump of all the data read from file
descriptors listed in the specified set. For example, to see all input
activity on file descriptors 3 and 5 use -e read=3,5. Note that this is
independent from the normal tracing of the read(2) system call which is
controlled by the option -e trace=read.
-e write=set
--write=set Perform a full hexadecimal and ASCII dump of all the data written to file
descriptors listed in the specified set. For example, to see all output
activity on file descriptors 3 and 5 use -e write=3,5. Note that this is
independent from the normal tracing of the write(2) system call which is
controlled by the option -e trace=write.
-e quiet=set
--quiet=set
--silent=set
--silence=set
Suppress various information messages. The default is quiet=none. set can
include the following elements:
attach Suppress messages about attaching and detaching ("[ Process
NNNN attached ]", "[ Process NNNN detached ]").
exit Suppress messages about process exits ("+++ exited with SSS
+++").
path-resolution Suppress messages about resolution of paths provided via the
-P option ("Requested path "..." resolved into "..."").
personality Suppress messages about process personality changes ("[
Process PID=NNNN runs in PPP mode. ]").
thread-execve
superseded Suppress messages about process being superseded by execve(2)
in another thread ("+++ superseded by execve in pid NNNN
+++").
-e decode-fds=set
--decode-fds=set
Decode various information associated with file descriptors. The default is
decode-fds=none. set can include the following elements:
path Print file paths. Also enables printing of tracee's current working
directory when AT_FDCWD constant is used.
socket Print socket protocol-specific information,
dev Print character/block device numbers.
pidfd Print PIDs associated with pidfd file descriptors.
-e decode-pids=set
--decode-pids=set
Decode various information associated with process IDs (and also thread IDs,
process group IDs, and session IDs). The default is decode-pids=none. set
can include the following elements:
comm Print command names associated with thread or process IDs.
pidns Print thread, process, process group, and session IDs in strace's PID
namespace if the tracee is in a different PID namespace.
-e kvm=vcpu
--kvm=vcpu Print the exit reason of kvm vcpu. Requires Linux kernel version 4.16.0 or
higher.
-i
--instruction-pointer
Print the instruction pointer at the time of the system call.
-n
--syscall-number
Print the syscall number.
-k
--stack-traces
Print the execution stack trace of the traced processes after each system
call.
-o filename
--output=filename
Write the trace output to the file filename rather than to stderr.
filename.pid form is used if -ff option is supplied. If the argument begins
with '|' or '!', the rest of the argument is treated as a command and all
output is piped to it. This is convenient for piping the debugging output to
a program without affecting the redirections of executed programs. The latter
is not compatible with -ff option currently.
-A
--output-append-mode
Open the file provided in the -o option in append mode.
-q
--quiet
--quiet=attach,personality
Suppress messages about attaching, detaching, and personality changes. This
happens automatically when output is redirected to a file and the command is
run directly instead of attaching.
-qq
--quiet=attach,personality,exit
Suppress messages attaching, detaching, personality changes, and about process
exit status.
-qqq
--quiet=all Suppress all suppressible messages (please refer to the -e quiet option
description for the full list of suppressible messages).
-r
--relative-timestamps[=precision]
Print a relative timestamp upon entry to each system call. This records the
time difference between the beginning of successive system calls. precision
can be one of s (for seconds), ms (milliseconds), us (microseconds), or ns
(nanoseconds), and allows setting the precision of time value being printed.
Default is us (microseconds). Note that since -r option uses the monotonic
clock time for measuring time difference and not the wall clock time, its
measurements can differ from the difference in time reported by the -t option.
-s strsize
--string-limit=strsize
Specify the maximum string size to print (the default is 32). Note that
filenames are not considered strings and are always printed in full.
--absolute-timestamps[=[[format:]format],[[precision:]precision]]
--timestamps[=[[format:]format],[[precision:]precision]]
Prefix each line of the trace with the wall clock time in the specified format
with the specified precision. format can be one of the following:
none No time stamp is printed. Can be used to override the previous
setting.
time Wall clock time (strftime(3) format string is %T).
unix Number of seconds since the epoch (strftime(3) format string is
%s).
precision can be one of s (for seconds), ms (milliseconds), us (microseconds),
or ns (nanoseconds). Default arguments for the option are
format:time,precision:s.
-t
--absolute-timestamps
Prefix each line of the trace with the wall clock time.
-tt
--absolute-timestamps=precision:us
If given twice, the time printed will include the microseconds.
-ttt
--absolute-timestamps=format:unix,precision:us
If given thrice, the time printed will include the microseconds and the
leading portion will be printed as the number of seconds since the epoch.
-T
--syscall-times[=precision]
Show the time spent in system calls. This records the time difference between
the beginning and the end of each system call. precision can be one of s (for
seconds), ms (milliseconds), us (microseconds), or ns (nanoseconds), and
allows setting the precision of time value being printed. Default is us
(microseconds).
-v
--no-abbrev Print unabbreviated versions of environment, stat, termios, etc. calls.
These structures are very common in calls and so the default behavior displays
a reasonable subset of structure members. Use this option to get all of the
gory details.
--strings-in-hex[=option]
Control usage of escape sequences with hexadecimal numbers in the printed
strings. Normally (when no --strings-in-hex or -x option is supplied), escape
sequences are used to print non-printable and non-ASCII characters (that is,
characters with a character code less than 32 or greater than 127), or to
disambiguate the output (so, for quotes and other characters that encase the
printed string, for example, angle brackets, in case of file descriptor path
output); for the former use case, unless it is a white space character that
has a symbolic escape sequence defined in the C standard (that is, “\t” for a
horizontal tab, “\n” for a newline, “\v” for a vertical tab, “\f” for a form
feed page break, and “\r” for a carriage return) are printed using escape
sequences with numbers that correspond to their byte values, with octal number
format being the default. option can be one of the following:
none Hexadecimal numbers are not used in the output at all. When
there is a need to emit an escape sequence, octal numbers are
used.
non-ascii-chars Hexadecimal numbers are used instead of octal in the escape
sequences.
non-ascii Strings that contain non-ASCII characters are printed using
escape sequences with hexadecimal numbers.
all All strings are printed using escape sequences with
hexadecimal numbers.
When the option is supplied without an argument, all is assumed.
-x
--strings-in-hex=non-ascii
Print all non-ASCII strings in hexadecimal string format.
-xx
--strings-in-hex[=all]
Print all strings in hexadecimal string format.
-X format
--const-print-style=format
Set the format for printing of named constants and flags. Supported format
values are:
raw Raw number output, without decoding.
abbrev Output a named constant or a set of flags instead of the raw number
if they are found. This is the default strace behaviour.
verbose Output both the raw value and the decoded string (as a comment).
-y
--decode-fds
--decode-fds=path
Print paths associated with file descriptor arguments and with the AT_FDCWD
constant.
-yy
--decode-fds=all
Print all available information associated with file descriptors: protocol-
specific information associated with socket file descriptors, block/character
device number associated with device file descriptors, and PIDs associated
with pidfd file descriptors.
--pidns-translation
--decode-pids=pidns
If strace and tracee are in different PID namespaces, print PIDs in strace's
namespace, too.
-Y
--decode-pids=comm
Print command names for PIDs.
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 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.
-V
--version Print the version number of strace.
Time specification format description
Time values can be specified as a decimal floating point number (in a format accepted by
strtod(3)), optionally followed by one of the 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.
DIAGNOSTICS
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.
SETUID INSTALLATION
If strace is installed setuid to root then the invoking user will be able to attach to and
trace processes owned by any user. In addition setuid and setgid programs will be
executed and traced with the correct effective privileges. Since only users trusted with
full root 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.
MULTIPLE PERSONALITIES SUPPORT
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.
NOTES
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.
BUGS
Programs that use the setuid bit do not have effective user ID privileges while being
traced.
A traced process runs slowly (but check out the --seccomp-bpf option).
Traced processes which are descended from command may be left running after an interrupt
signal (CTRL-C).
HISTORY
The original strace was written by Paul Kranenburg for SunOS and was inspired by its trace
utility. The SunOS version of strace was ported to Linux and enhanced by Branko
Lankester, who also wrote the Linux kernel support. Even though Paul released strace 2.5
in 1992, Branko's work was based on Paul's strace 1.5 release from 1991. In 1993, Rick
Sladkey merged strace 2.5 for SunOS and the second release of strace for Linux, added many
of the features of truss(1) from SVR4, and produced an strace that worked on both
platforms. In 1994 Rick ported strace to SVR4 and Solaris and wrote the automatic
configuration support. In 1995 he ported strace to Irix and tired of writing about
himself in the third person.
Beginning with 1996, strace was maintained by Wichert Akkerman. During his tenure, strace
development migrated to CVS; ports to FreeBSD and many architectures on Linux (including
ARM, IA-64, MIPS, PA-RISC, PowerPC, s390, SPARC) were introduced. In 2002, the burden of
strace maintainership was transferred to Roland McGrath. Since then, strace gained
support for several new Linux architectures (AMD64, s390x, SuperH), bi-architecture
support for some of them, and received numerous additions and improvements in syscalls
decoders on Linux; strace development migrated to git during that period. Since 2009,
strace is actively maintained by Dmitry Levin. strace gained support for AArch64, ARC,
AVR32, Blackfin, Meta, Nios II, OpenRISC 1000, RISC-V, Tile/TileGx, Xtensa architectures
since that time. In 2012, unmaintained and apparently broken support for non-Linux
operating systems was removed. Also, in 2012 strace gained support for path tracing and
file descriptor path decoding. In 2014, support for stack traces printing was added. In
2016, syscall fault injection was implemented.
For the additional information, please refer to the NEWS file and strace repository commit
log.
REPORTING BUGS
Problems with strace should be reported to the strace mailing list
⟨mailto:strace-devel@lists.strace.io⟩.
SEE ALSO
strace-log-merge(1), ltrace(1), perf-trace(1), trace-cmd(1), time(1), ptrace(2), proc(5)
strace Home Page ⟨https://strace.io/⟩
AUTHORS
The complete list of strace contributors can be found in the CREDITS file.