linux神器 strace解析

除了人格以外，人最大的损失，莫过于失掉自信心了。

前言

strace可以说是神器一般的存在了，对于研究代码调用，内核级调用、系统级调用有非常重要的作用。打算了一周了，只有原文，一直没有梳理，拖延症犯了，今天加班把这个神器的官方翻译梳理一下。

linux 7 的 man的官方文档链接如下：http://man7.org/linux/man-pages/index.html，本来以为是linux内建的工具，结果找了半天，没有，哈哈。也就是说它是需要额外安装的。

安装 strace

安装

yum -y install strace

验证

[root@node01 ~]# strace
strace: must have PROG [ARGS] or -p PID
Try 'strace -h' for more information.

安装成功，也可以使用man命令来查看其文档了。

strace功能概述

一言以蔽之，它是一个用来追踪系统调用和信号的工具。

案例说明

man文档中对其描述如下：

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.

在最简单的情况下，strace运行指定的命令，直到它退出。它拦截并记录由进程调用的系统调用和由进程接收的信号。每个系统调用的名称、参数及其返回值都打印在标准错误上，或者打印到使用-o选项指定的文件中。
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.

strace是一个有用的诊断、指导和调试工具。系统管理员、诊断人员和故障排除人员将发现它对于解决程序的问题非常有价值，而这些程序的源代码并不容易获得，因为不需要重新编译来跟踪它们。
学生、黑客和过分好奇的人会发现，通过跟踪一个系统及其系统调用，甚至是普通的程序，都可以了解到很多东西。程序员会发现，由于系统调用和信号是发生在用户/内核接口上的事件，所以应该仔细检查一下。
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:
跟踪中的每一行都包含系统调用名，后面是括号中的参数和返回值。例如，命令“cat /dev/null”为:
open("/dev/null", O_RDONLY) = 3
也就是说， open是系统调用名，"/dev/null", O_RDONLY是传入的参数，返回值是3， 查看open的man手册(http://man7.org/linux/man-pages/man2/open.2.html)，在RETURN VALUES描述中，说道返回 一个文件描述符或者是错误码 -1
Errors (typically a return value of -1) have the errno symbol and error string appended.

一个文件不存在的一个栗子：

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:

信号以信号符号的形式输出，并解码siginfo结构。以下是“sleep 666”命令的一段摘录:

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.
如果正在执行一个系统调用，同时从另一个线程/进程调用另一个系统调用，那么strace将尝试保留这些事件的顺序，并将正在进行的调用标记为未完成。当调用返回时，它将被标记为已恢复。
[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.
当内核终止系统调用并在信号处理程序完成后安排它的立即重新执行时，信号传递对(可重新启动的)系统调用的中断以不同的方式进行处理。

read(0, 0x7ffff72cf5cf, 1) = ? ERESTARTSYS (To be restarted)
--- SIGALRM ... ---
rt_sigreturn(0xe) = 0
read(0, "", 1) = 0

Arguments are printed in symbolic form with a passion. This example shows the shell performing ">>xyzzy" output redirection:
参数是以象征性的形式印出来的。这个例子显示了执行“>>xyzzy”输出重定向的shell:
open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3

Here the third argument of open is decoded by breaking down the flag argument into its three bitwise-OR constituents and printing the mode value in octal by tradition. Where
traditional or native usage differs from ANSI or POSIX, the latter forms are preferred. In some cases, strace output has proven to be more readable than the source.
在这里，open的第三个参数通过将flag参数分解成它的三个bit - or组成部分并按传统打印八进制的模式值来解码。当传统的或本地的用法与ANSI或POSIX不同时，最好使用后者。在某些情况下，strace输出被证明比源代码更具可读性。
Structure pointers are dereferenced and the members are displayed as appropriate. In all 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:
结构指针被取消引用，成员被适当地显示。在所有情况下，参数的格式都尽可能类似于c。例如，“ls -l /dev/null”命令被捕获为:
lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 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 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:
请注意“struct stat”参数是如何取消引用的，以及每个成员是如何象征性地显示的。
特别要注意的是，st_mode成员是如何被仔细地解码成位或符号和数字值的。
在本例中还要注意，lstat的第一个参数是系统调用的输入，第二个参数是输出。
因为如果系统调用失败，输出参数不会被修改，所以参数可能不会总是被取消引用。
例如，用一个不存在的文件重试“ls -l”的例子，结果如下:
lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory)

In this case the porch light is on but nobody is home.
在这种情况下，门廊的灯是亮的，但没有人在家。（哈哈，文档说的很含蓄）
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.
字符指针被取消引用并打印为C字符串。字符串中的非打印字符通常由普通的C转义码表示。只有第一个strsize
(默认为32)打印字符串字节;较长的字符串在结束引用后附加省略号。
Here is a line from "ls -l" where the getpwuid 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.
结构使用花括号进行注释，而简单指针和数组使用方括号和逗号分隔元素进行打印。
Here is an example from the
command "id" 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.
另一方面，位集也使用方括号显示，但集元素仅用空格分隔。
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:
这里的第二个参数是两个信号的位集，SIGCHLD和SIGTTOU。在某些情况下，位集是如此的满，以至于打印出未设置的元素更有价值。在这种情况下，位集前面加一个波浪号，就像这样
sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0

Here the second argument represents the full set of all signals.
这里，第二个参数表示所有信号的完整集合。

用法概览

在安装成功验证阶段，打印出来的日志信息已经告诉我们如何查看了，如下：

[root@node01 ~]# strace -h
usage: strace [-CdffhiqrtttTvVwxxy] [-I n] [-e expr]...
              [-a column] [-o file] [-s strsize] [-P path]...
              -p pid... / [-D] [-E var=val]... [-u username] PROG [ARGS]
   or: strace -c[dfw] [-I n] [-e expr]... [-O overhead] [-S sortby]
              -p pid... / [-D] [-E var=val]... [-u username] PROG [ARGS]

Output format:
  -a column      alignment COLUMN for printing syscall results (default )
  -i             print instruction pointer at time of syscall
  -o file        send trace output to FILE instead of stderr
  -q             suppress messages about attaching, detaching, etc.
  -r             print relative timestamp
  -s strsize     limit length of print strings to STRSIZE chars (default )
  -t             print absolute timestamp
  -tt            print absolute timestamp with usecs
  -T             print time spent in each syscall
  -x             print non-ascii strings in hex
  -xx            print all strings in hex
  -y             print paths associated with file descriptor arguments
  -yy            print protocol specific information associated with socket file descriptors

Statistics:
  -c             count time, calls, and errors for each syscall and report summary
  -C             like -c but also print regular output
  -O overhead    set overhead for tracing syscalls to OVERHEAD usecs
  -S sortby      sort syscall counts by: time, calls, name, nothing (default time)
  -w             summarise syscall latency (default is system time)

Filtering:
  -e expr        a qualifying expression: option=[!]all or option=[!]val1[,val2]...
     options:    trace, abbrev, verbose, raw, signal, read, write
  -P path        trace accesses to path

Tracing:
  -b execve      detach on execve syscall
  -D             run tracer process as a detached grandchild, not as parent
  -f             follow forks
  -ff            follow forks with output into separate files
  -I interruptible
     :          no signals are blocked
     :          fatal signals are blocked while decoding syscall (default)
     :          fatal signals are always blocked (default if '-o FILE PROG')
     :          fatal signals and SIGTSTP (^Z) are always blocked
                 (useful to make 'strace -o FILE PROG' not stop on ^Z)

Startup:
  -E var         remove var from the environment for command
  -E var=val     put var=val in the environment for command
  -p pid         trace process with process id PID, may be repeated
  -u username    run command as username handling setuid and/or setgid

Miscellaneous:
  -d             enable debug output to stderr
  -v             verbose mode: print unabbreviated argv, stat, termios, etc. args
  -h             print help message
  -V             print version

大致可以分为五部分，分别为：输出格式、统计数据类型、如何定义追踪、启动项设定、以及其他的参数。

详细说明

将 strace 帮助信息打印到 输出文件中。如下：

[root@hadoop ~]# man strace | col -b >> aa.txt

man strace 中摘录出来的比较详细的说明。

OPTIONS
       -c       Count time, calls, and errors for each system call and report a summary on program exit.  On Linux, this attempts to show system time (CPU time spent  running
           in the kernel) independent of wall clock time.  If -c is used with -f or -F (below), only aggregate totals for all traced processes are kept.

       -C       Like -c but also print regular output while processes are running.

       -D       Run    tracer    process as a detached grandchild, not as parent of the tracee.    This reduces the visible effect of strace by keeping the tracee a direct child of
           the calling process.

       -d       Show some debugging output of strace itself on the standard error.

       -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 incom‐
           patible with -c, since no per-process counts are kept.

       -F       This option is now obsolete and it has the same functionality as -f.

       -h       Print the help summary.

       -i       Print the instruction pointer at the time of the system call.

       -k       Print the execution stack trace of the traced processes after each system call (experimental).  This option is available only if strace is built  with  libun‐
           wind.

       -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.

       -t       Prefix each line of the trace with the time of day.

       -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.

       -w       Summarise the time difference between the beginning and end of each system call.  The default is to summarise the system time.

       -v       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.

       -V       Print the version number of strace.

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

       -xx       Print all strings in hexadecimal string format.

       -y       Print paths associated with file descriptor arguments.

       -yy       Print protocol specific information associated with socket file descriptors.

       -a column   Align return values in a specific column (default column 40).

       -b syscall  If  specified syscall is reached, detach from traced process.  Currently, only execve 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.

       -e expr       A qualifying expression which modifies which events to trace or how to trace them.  The format of the expression is:

                 [qualifier=][!]value1[,value2]...

           where qualifier is one of trace, abbrev, verbose, raw, signal, read, or write 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 sys‐
           tem call.  By contrast, -e trace=!open means to trace every system call except open.     In addition, the special values all and none have the obvious meanings.

           Note that some shells use the exclamation point for history expansion even inside quoted arguments.    If so, you must escape the exclamation point with a back‐
           slash.

       -e trace=set
           Trace  only    the  specified    set  of     system     calls.      The  -c  option  is  useful  for determining which system calls might be useful to trace.  For example,
           trace=open,close,read,write means to only trace those four system calls.  Be careful when making inferences about the user/kernel boundary if only a subset of
           system calls are being monitored.  The default is trace=all.

       -e trace=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 use‐
           ful 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 in the list.  Betchya woulda forgot that one.

       -e trace=process
           Trace all system calls which involve process management.  This is useful for watching the fork, wait, and exec steps of a process.

       -e trace=network
           Trace all the network related system calls.

       -e trace=signal
           Trace all signal related system calls.

       -e trace=ipc
           Trace all IPC related system calls.

       -e trace=desc
           Trace all file descriptor related system calls.

       -e trace=memory
           Trace all memory mapping related system calls.

       -e abbrev=set
           Abbreviate the output from printing each member of large structures.     The default is abbrev=all.  The -v option has the effect of abbrev=none.

       -e verbose=set
           Dereference structures for the specified set of system calls.  The default is verbose=all.

       -e raw=set  Print raw, undecoded arguments for the specified set of system calls.  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.

       -e 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 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
           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.

       -I interruptible
           When strace can be interrupted by signals (such as pressing ^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 (^Z) are always blocked (useful to make strace -o FILE PROG not
           stop on ^Z).

       -o filename Write the trace output to the file filename rather than to stderr.  Use filename.pid if -ff is used.     If the argument begins with '|' or  with  '!'    then  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.

       -O overhead Set the overhead for tracing system calls to overhead microseconds.    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.

       -p 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 pro‐
           cesses in addition to command (which is optional if at least one -p option is given).  -p "`pidof PROG`" syntax is supported.

       -P path       Trace only system calls accessing path.  Multiple -P options can be used to specify several paths.

       -s 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.

       -S sortby   Sort the output of the histogram printed by the -c option by the specified criterion.  Legal values are time, calls, name, and nothing (default is time).

       -u 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  execu‐
           tion of setuid and/or setgid binaries.  Unless this option is used setuid and setgid programs are executed without effective privileges.

       -E var=val  Run command with var=val in its list of environment variables.

       -E var       Remove var from the inherited list of environment variables before passing it on to the command.

案例

后续补

总结

strace是开发运维问题定位追踪的神器，需要加以练习才能熟练掌握。strace仿佛是一扇窗，给了应用开发从上层了解系统内核以及系统调用的机会。