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。
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 sign 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strace is a useful diagnostic, instructional, and debugging tool. System administrators, diagnosticians and trouble-shooters will find
it invaluable for solving problem 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 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 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 /de 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(/linux/man-pages/man2/open.2.html)，在R 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 event 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，[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 reexecutio 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 traditio 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 ex 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 d 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 o 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-ex 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 firs (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 valua 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]...
- / [-D] [-E var=val]... [-u username] PROG [ARGS]
or: strace -c[dfw] [-I n] [-e expr]... [-O overhead] [-S sortby]
- / [-D] [-E var=val]... [-u username] PROG [ARGS]
Output format:
-
a column alignment COLUMN for printing syscall results (default 40)
-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 32)
-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:
linux字符串转数组-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
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)
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
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 ch 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 - 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 ins 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 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 i
s supported. This option is useful if you want to trace mu 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 sy 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 meaning
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 b 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 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 hexadecim 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 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 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
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 tim 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 tra (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-
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 th 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。
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