什么是 strace
strace是Linux环境下的一款程序调试工具，用来监察一个应用程序所使用的系统调用。
Strace是一个简单的跟踪系统调用执行的工具。在其最简单的形式中，它可以从开始到结束跟踪二进制的执行，并在进程的生命周期中输出一行具有系统调用名称，每个系统调用的参数和返回值的文本行。
strace有两种运行模式
一：通过它启动要跟踪的进程，在原本的命令前加上strace即可：strace ls -lh /var/log/cron
二：跟踪已经在运行的进程，在不中断进程执行的情况下，查看它在干嘛。 给strace传递个-p pid 选项即可：strace -p 28397
starce 常用的选项
-c 统计每一系统调用的所执行的时间,次数和出错的次数等.
-d 输出strace关于标准错误的调试信息.
-f 跟踪由fork调用所产生的子进程.
-ff 如果提供-o filename,则所有进程的跟踪结果输出到相应的filename.pid中,pid是各进程的进程号.
-F 尝试跟踪vfork调用.在-f时,vfork不被跟踪.
-h 输出简要的帮助信息.
-i 输出系统调用的入口指针.
-q 禁止输出关于脱离的消息.
-r 打印出相对时间关于,,每一个系统调用.
-t 在输出中的每一行前加上时间信息.
-tt 在输出中的每一行前加上时间信息,微秒级.
-ttt 微秒级输出,以秒了表示时间.
-T 显示每一调用所耗的时间.
-v 输出所有的系统调用.一些调用关于环境变量,状态,输入输出等调用由于使用频繁,默认不输出.
-V 输出strace的版本信息.
-x 以十六进制形式输出非标准字符串
-xx 所有字符串以十六进制形式输出.
-a column 设置返回值的输出位置.默认 为40.
-e expr 指定一个表达式,用来控制如何跟踪.格式如下:
[qualifier=]
[!]value1[,value2]…
qualifier只能是 trace,abbrev,verbose,raw,signal,read,write其中之一.value是用来限定的符号或数字.默认的 qualifier是 trace.感叹号是否定符号.例如:
-eopen等价于 -e trace=open,表示只跟踪open调用.而-etrace!=open表示跟踪除了open以外的其他调用.有两个特殊的符号 all 和 none.
注意有些shell使用!来执行历史记录里的命令,所以要使用\.
-e trace=set
只跟踪指定的系统 调用.例如:-e trace=open,close,rean,write表示只跟踪这四个系统调用.默认的为set=all.
-e trace=file
只跟踪有关文件操作的系统调用.
-e trace=process
只跟踪有关进程控制的系统调用.
-e trace=network
跟踪与网络有关的所有系统调用.
-e strace=signal
跟踪所有与系统信号有关的 系统调用
-e trace=ipc
跟踪所有与进程通讯有关的系统调用
-e abbrev=set
设定 strace输出的系统调用的结果集.-v 等与 abbrev=none.默认为abbrev=all.
-e raw=set
将指 定的系统调用的参数以十六进制显示.
-e signal=set
指定跟踪的系统信号.默认为all.如 signal=!SIGIO(或者signal=!io),表示不跟踪SIGIO信号.
-e read=set
输出从指定文件中读出 的数据.例如:
-e read=3,5
-e write=set
输出写入到指定文件中的数据.
-o filename
将strace的输出写入文件filename
-p pid
跟踪指定的进程pid.
-s strsize
指定输出的字符串的最大长度.默认为32.文件名一直全部输出.
-u username
以username 的UID和GID执行被跟踪的命令
strace 输出一部分简单的解释
execve(“./main”, [“./main”], [/* 43 vars */]) = 0
对于命令行下执行的程序，execve(或exec系列调用中的某一个)均为strace输出系统调用中的第一个。
strace首先调用fork或clone函数新建一个子进程，然后在子进程中调用exec载入需要执行的程序(这里为./main)
brk(0) = 0x9ac4000
以0作为参数调用brk，返回值为内存管理的起始地址(若在子进程中调用malloc，则从0x9ac4000地址开始分配空间)
access(“/etc/ld.so.nohwcap”, F_OK) = -1 ENOENT (No such file or directory)
调用access函数检验/etc/ld.so.nohwcap是否存在
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7739000
使用mmap2函数进行匿名内存映射，以此来获取8192bytes内存空间，该空间起始地址为0xb7739000
open(“/etc/ld.so.cache”, O_RDONLY) = 3
调用open函数尝试打开/etc/ld.so.cache文件，返回文件描述符为3
fstat64(3, {st_mode=S_IFREG|0644, st_size=80682, …}) = 0
fstat64函数获取/etc/ld.so.cache文件信息
mmap2(NULL, 80682, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7725000
调用mmap2函数将/etc/ld.so.cache文件映射至内存
close(3) = 0
close关闭文件描述符为3指向的/etc/ld.so.cache文件
open(“/lib/i386-linux-gnu/libc.so.6”, O_RDONLY) = 3
read(3, “\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\220o\1\0004\0\0\0″…, 512) = 512
调用read，从/lib/i386-linux-gnu/libc.so.6该libc库文件中读取512bytes，即读取ELF头信息
mprotect(0x6c7000, 4096, PROT_NONE) = 0
使用mprotect函数对0x6c7000起始的4096bytes空间进行保护(PROT_NONE表示不能访问，PROT_READ表示可以读取)
文章来源：www.rakbuluo.com/p78
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