Ubuntu Manpage: strace - trace system calls and signals

NAME

       strace - trace system calls and signals

SYNOPSIS

       strace [-ACdffhikqqrtttTvVwxxyyzZ] [-I n] [-b execve] [-e expr]... [-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] [-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 third argument of open(2) is 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, abbrev, verbose, raw, signal, read, write, fault, inject,
status, 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). -p "`pidof PROG`" syntax is 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 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 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 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 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.

-ff If the -o filename option is in effect, each processes trace is written to filename.pid where
pid is the numeric process id of each process. 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
When strace can be interrupted by signals (such as pressing CTRL-C).

1 no signals are blocked;
2 fatal signals are blocked while decoding syscall (default);
3 fatal signals are always blocked (default if -o FILE PROG);
4 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 all system calls which involve process management. This is useful for
watching the fork, wait, and exec steps of a process. 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 and fstatat 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.

%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 Print only syscalls that returned without an error code.

-Z 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 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.

-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 Suppress messages about attaching, detaching etc. This happens automatically when output is
redirected to a file and the command is run directly instead of attaching.

-qq If given twice, suppress messages about process exit status.

-r Print a relative timestamp upon entry to each system call. This records the time difference
between the beginning of successive system calls. 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.

-t Prefix each line of the trace with the wall clock time.

-tt If given twice, the time printed will include the microseconds.

-ttt 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 Show the time spent in system calls. This records the time difference between the beginning
and the end of each system call.

-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.

-x Print all non-ASCII strings in hexadecimal string format.

-xx 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 Print paths associated with file descriptor arguments.

-yy Print protocol specific information associated with socket file descriptors, and
block/character device number associated with device file descriptors.

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 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_total or total_time), calls (or count), errors (or error), name (or
syscall or syscall_name), and nothing (or none); default is time.

-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][:when=expr]
--inject=syscall_set[:error=errno|:retval=value][:signal=sig][:syscall=syscall][:delay_enter=delay][:delay_exit=delay][: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, or delay_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 :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 both :error=errno or :retval=value and :signal=sig options are specified, then both a
fault or success is injected and a signal is delivered.

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 one of the following:

first For every syscall from the set, perform an injection for the syscall invocation
number first only.
first+ For every syscall from the set, perform injections for the syscall invocation
number first and all subsequent invocations.
first+step For every syscall from the set, perform injections for syscall invocations number
first, first+step, first+step+step, and so on.

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.

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
Enable (experimental) usage of seccomp-bpf (see seccomp(2)) to have ptrace(2)-stops only when
system calls that are being traced occur in the traced processes. Implies the -f option. An
attempt to rely on seccomp-bpf to filter system calls 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. --seccomp-bpf is also ineffective on processes attached using -p. 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.

       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, OpenSISC 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⟩.

AUTHORS

       The complete list of strace contributors can be found in the CREDITS file.