比如正常时制和夏时制之间的转换。由内核处理或要求人为干预。U N I X则不同，它仅仅提供一
条系统调用，该系统调用返回国际标准时间1 9 7 0年1月1日零点以来所经过的秒数。对该值的
不论什么解释。比如将其变换成人们可读的。使用本地时区的时间和日期，都留给用户进程执行。
在标准C库中，提供了若干例程以处理大多数情况。这些库函数处理各种细节，比如各种夏时
制算法。
应用程序能够调用系统调用或者库函数。而非常多库函数则会调用系统调用。
这在图1 - 2中
显示。
系统调用和库函数之间的还有一个区别是：系统调用通常提供一种最小界面。而库函数通常
提供比較复杂的功能。
我们从s b r k系统调用和m a l l o c库函数之间的区别中能够看
到这一点。在以后当比較不带缓存的I / O函数（见第3章）以及标准I / O函数（见第5章）
时，还将看到这样的区别。
进程控制系统调用（ fork, exec和w a i t）通常由用户的应用程序直接调用（请回顾
程序1 - 5中的基本s h e l l）。
可是为了简化某些常见的情况，U N I X系统也提供了一些库
函数；比如s y s t e m和p o p e n。8 . 1 2节将说明s y s t e m函数的一种实现，它使用主要的进
程控制系统调用。1 0 . 1 8节还将强化这一实例以正确地处理信号。
为使读者了解大多数程序猿应用的U N I X系统界面，我们不得不既说明系统调用，仅仅介绍某些库函数。
比如若仅仅说明s b r k系统调用，那么就会忽略非常多应用程序使用的m a l l o c库函数。
本书除了必需要区分两者时，都将使用术语函数（ f u n c t i o n）来指代系统调用和库函数两者。
CPU硬件决定了这些（这就是为什么它被称作"保护模式"）。系统调用是这些规则的一个例外。
其原理是进程先用适当的值填充寄存器，然后调用一个特殊的指令。这个指令会跳到一个事先定义的内核中的一个位置（当然，这个位置是用户进程可读可是不可写的）。在Intel CPU中，这个由中断0x80实现。
硬件知道一旦你跳到这个位置，你就不是在限制模式下执行的用户。而是作为操作系统的内核--所以你就能够为所欲为。
进程能够跳转到的内核位置叫做sysem_call。这个过程检查系统调用号。这个号码告诉内核进程请求哪种服务。然后。它查看系统调用表(sys_call_table)找到所调用的内核函数入口地址。
接着。就调用函数，等返回后。做一些系统检查，最后返回到进程（或到其它进程，假设这个进程时间用尽）。假设你希望读这段代码，它在<内核源代码文件夹>/kernel/entry.S，Entry(system_call)的下一行。
为防止和正常的返回值混淆，系统调用并不直接返回错误码，而是将错误码放入一个名为errno的全局变量中。
假设一个系统调用失败，你能够读出errno的值来确定问题所在。
errno不同数值所代表的错误消息定义在errno.h中。你也能够通过命令"man 3 errno"来察看它们。
须要注意的是，errno的值仅仅在函数错误发生时设置。假设函数不错误发生，errno的值就无定义，并不会被置为0。另外。在处理errno前最好先把它的值存入还有一个变量，由于在错误处理过程中。即使像printf()这种函数出错时也会改变errno的值。
<span style="font-family:Microsoft YaHei;font-size:12px;">yum install strace ltrace</span>
man strace
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 a signal symbol and a signal string. An excerpt from stracing and interrupting the com-
mand ‘‘sleep 666’’ is:
sigsuspend([] <unfinished ...>
--- SIGINT (Interrupt) ---
+++ 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 (Alarm clock) @ 0 (0) ---
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:
open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3
Here the three argument form 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.
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:
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 par-
ticular, 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 fol-
lowing line:
lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory)
In this case the porch light is on but nobody is home.
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 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’’ 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.
<span style="font-family:Microsoft YaHei;font-size:12px;">[root@localhost ~]# strace ls
execve("/bin/ls", ["ls"], [/* 41 vars */]) = 0
brk(0) = 0xe67000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1176dfa000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=38574, ...}) = 0
mmap(NULL, 38574, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f1176df0000
close(3) = 0
open("/lib64/libselinux.so.1", O_RDONLY) = 3
read(3, "177ELF211 3 > 1 PX`312= "..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=124624, ...}) = 0
mmap(0x3dca600000, 2221912, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3dca600000
mprotect(0x3dca61d000, 2093056, PROT_NONE) = 0
mmap(0x3dca81c000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c000) = 0x3dca81c000
mmap(0x3dca81e000, 1880, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x3dca81e000
close(3) = 0
open("/lib64/librt.so.1", O_RDONLY) = 3</span>因为输出内容比較长。我就不所有拷贝下来了，我们能够看看一个简单的ls命令，究竟做了哪些系统调用<span style="font-family:Microsoft YaHei;font-size:12px;">strace -e 'select' -p 18846
Process 18846 attached - interrupt to quit
select(0, NULL, NULL, NULL, {0, 165000}) = 0 (Timeout)
select(0, NULL, NULL, NULL, {1, 0}) = 0 (Timeout)
select(0, NULL, NULL, NULL, {1, 0}) = 0 (Timeout)
select(0, NULL, NULL, NULL, {1, 0}) = 0 (Timeout)
select(0, NULL, NULL, NULL, {1, 0} <unfinished ...>
Process 18846 detached</span>上面的样例是对18846进程的select调用的显示，这个功能能够非常方便的定位调用的方法<span style="font-family:Microsoft YaHei;font-size:12px;">[root@localhost ~]# strace -o /tmp/a.txt -c hostname localhost [root@localhost ~]# cat /tmp/a.txt % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- -nan 0.000000 0 5 read -nan 0.000000 0 1 write -nan 0.000000 0 6 open -nan 0.000000 0 6 close -nan 0.000000 0 7 fstat -nan 0.000000 0 15 mmap -nan 0.000000 0 7 mprotect -nan 0.000000 0 2 munmap -nan 0.000000 0 3 brk -nan 0.000000 0 1 1 access -nan 0.000000 0 1 execve -nan 0.000000 0 1 uname -nan 0.000000 0 1 statfs -nan 0.000000 0 1 arch_prctl ------ ----------- ----------- --------- --------- ---------------- 100.00 0.000000 57 1 total</span>-o选项是把输出到文件，-c选项是统计所有系统调用的耗时和次数，这也是很有用的功能
<span style="font-family:Microsoft YaHei;font-size:12px;">DESCRIPTION
ltrace is a program that simply runs the specified command until it exits. It intercepts and records the
dynamic library calls which are called by the executed process and the signals which are received by that pro-
cess. It can also intercept and print the system calls executed by the program.
Its use is very similar to strace(1).</span>它的man很短。直接说和strace很相似
<span style="font-family:Microsoft YaHei;font-size:12px;">[root@localhost ~]# ltrace ls
(0, 0, 0, 0x7fc84b52bac0, 88) = 0x3dc8c21160
__libc_start_main(0x408480, 1, 0x7fff73282908, 0x412110, 0x412100 <unfinished ...>
strrchr("ls", '/') = NULL
setlocale(6, "") = "zh_CN.UTF-8"
bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale"
textdomain("coreutils") = "coreutils"
__cxa_atexit(0x40bb20, 0, 0, 0x736c6974756572, 0x3dc958fee8) = 0
isatty(1) = 1
getenv("QUOTING_STYLE") = NULL
getenv("LS_BLOCK_SIZE") = NULL
getenv("BLOCK_SIZE") = NULL
getenv("BLOCKSIZE") = NULL
getenv("POSIXLY_CORRECT") = NULL
getenv("BLOCK_SIZE") = NULL
getenv("COLUMNS") = NULL
ioctl(1, 21523, 0x7fff732827d0) = 0
getenv("TABSIZE") = NULL
getopt_long(1, 0x7fff73282908, "abcdfghiklmnopqrstuvw:xABCDFGHI:"..., 0x619040, 0x7fff732827e8) = -1
__errno_location() = 0x7fc84b5296a0
malloc(56) = 0x104d050
memcpy(0x104d050, "", 56) = 0x104d050</span>和strace一样先拿ls来測试，能够看出非常多库函数的调用如strrchr、setlocale、bindtextdomain等，详细含义能够利用网络搜索