一直有这么个想法，可以通过http像nfs那样快速共享文件。用apache吧，不至于动用那个大家伙，用C写一个吧，太麻烦。最后决定用Perl写，可无意间发现了Python有个更简单的模块——SimpleHTTPServer，于是就用Python写，几行就搞定了。

代码见下：

[python]

!/usr/bin/env python

import sys
import string
import SimpleHTTPServer
import SocketServer

if len(sys.argv) != 3:
sys.exit(1)

addr = sys.argv[1]
port = string.atoi(sys.argv[2])

handler = SimpleHTTPServer.SimpleHTTPRequestHandler
httpd = SocketServer.TCPServer((addr, port), handler)
print “HTTP server is at: “, addr, port
httpd.serve_forever()

[/python]

man page只能查Linux系统调用，C标准函数和其它一些glibc函数，不能查C++标准库函数。怎么才能像man那样查询C++的函数呢？我写了下面这么个Perl脚本来完成。

用法：

$ ./mancpp cmath::sin
$ ./mancpp cmath::cos
$ ./mancpp vector::size
$ ./mancpp algorithm::count_if

代码：

[perl]

!/usr/bin/perl

use strict;
use warnings;
use LWP::Simple;

die “Please provide one argument.n” unless @ARGV==1;
die “Use class::function format.n” unless $ARGV[0] =~ /S::S/;

my ($class, $func) = split(/::/, $ARGV[0]);
my $file = “/tmp/$class.$func.html”;

if ( ! -f “$file”) {
my $url;

if ($class =~ /^c/) {
    $url = "http://www.cplusplus.com/reference/clibrary/$class"."/".$func.".html";
} elsif ($class eq "filebuf" || $class eq "fstream"
    || $class eq "ifstream" || $class eq "ios"
    || $class eq "iostream" || $class eq "ios_base"
    || $class eq "istream" || $class eq "istringstream"
    || $class eq "ofstream" || $class eq "ostream"
    || $class eq "ostringstream" || $class eq "streambuf"
    || $class eq "stringbuf" || $class eq "stringstream"){
    $url = "http://www.cplusplus.com/reference/iostream/$class"."/".$func.".html";
} elsif($class eq "vector" || $class eq "bitset" || $class eq "deque"
    || $class eq "list" || $class eq "stack" || $class eq "map"
    || $class eq "queue" || $class eq "set" || $class eq "multiset"
    || $class eq "priority_queue" || $class eq "multimap") {
    $url = "http://www.cplusplus.com/reference/stl/$class"."/".$func.".html";
} elsif ($class eq "functional" || $class eq "iterator"
    || $class eq "memory" || $class eq "utility") {
    $url = "http://www.cplusplus.com/reference/misc/$class"."/".$func.".html";
} elsif ($class eq "algorithm") {
    $url = "http://www.cplusplus.com/reference/$class"."/".$func.".html";
} elsif ($class eq "string") {
    $url = "http://www.cplusplus.com/reference/string/$class"."/".$func.".html";
}

print "Retrieving $url", "...n";
LWP::Simple::getstore($url, "$file");

}

open my $fd, “$file” or die “No such class or function!n”;
my $found = 0;
my @ret;

while(){
if (m//) {
$found = 1;
}
if ($found) {
s/]*>//g;
s//g;
s/</</g;
s/Parameters/n==Parameters==/;
s/Example/n==Example==/;
s/See also/n==See also==/;
push @ret, $_;
}
if (m/SuOptions()/) {
$found = 0;
pop @ret;
}
}

open my $hd, "|less" or die "Can't open pipe!n";

foreach (@ret){
print $hd $_;
}

close $hd;

close $fd;
exit 0;

[/perl]

$ cat abs.cpp

include <cstdlib>

include <iostream>

using namespace std;

int main()
{
cout<<abs(-19.50)<<endl;
return 0;
}

编译你会得到如下错误：

abs.cpp: In function ‘int main()’:
abs.cpp:7: error: call of overloaded ‘abs(double)’ is ambiguous
/usr/include/stdlib.h:699: note: candidates are: int abs(int)
/usr/lib/gcc/i386-redhat-linux/4.3.0/../../../../include/c++/4.3.0/cstdlib:175: note: long long int __gnu_cxx::abs(long long int)
/usr/lib/gcc/i386-redhat-linux/4.3.0/../../../../include/c++/4.3.0/cstdlib:144: note: long int std::abs(long int)

为啥呢？因为abs()在<cmath>中，而不是C++标准参考手册中提到的<cstdlib>！

谁错了捏？其实谁都没错。这里解释到：

The Standard C++ library is supposed to
overload abs the way you expect, but it’s common for implementations
to omit the overloads, or put them in the wrong place (<cmath> instead
of <math.h>).
这个真的让人很无语，一边是出错，另一边是明知道出错也这么做。。。冏。。。

我没写错标题，我确实是想说如何编译Python代码。:-)

见下：

$ cat foo.py

print “hello”

$ ls
foo.py
$ python -mpy_compile foo.py
$ ls
foo.py foo.pyc
$ python foo.pyc
hello
或者另一种方式：

$ ls dummy/
foo.py
$ python -mcompileall dummy/
Listing dummy/ …
Compiling dummy/foo.py …
$ ls dummy/
foo.py foo.pyc

参考：

1. http://pyfaq.infogami.com/how-do-i-create-a-pyc-file
2. http://effbot.org/zone/python-compile.htm

gcc 4.3之前，-Wconversion是这样的：

Warn if a prototype causes a type conversion that is different from what would
happen to the same argument in the absence of a prototype.

Also, warn if a negative integer constant expression is implicitly converted to
an unsigned type.
到了4.3就变了，发布日志上解释到：
The -Wconversion option has been modified. Its purpose now is to warn for
implicit conversions that may alter a value. This new behavior is available for
both C and C++. Warnings about conversions between signed and unsigned integers
can be disabled by using -Wno-sign-conversion. In C++, they are disabled by
default unless -Wsign-conversion is explicitly requested. The old behavior of
-Wconversion, that is, warn for prototypes causing a type conversion that is
different from what would happen to the same argument in the absence of a
prototype, has been moved to a new option -Wtraditional-conversion, which is
only available for C

这个改变其实挺大的，下面通过这个程序就可以展示这个问题：

[c]
void foo(short i);

void foo(short i)
{
i++; //dummy
}

int main(void)
{
int a = 1;
short b =2;
b = a;
foo(a);
return 0;
}
[/c]

$ gcc -Wall -Wtraditional-conversion -o conv conv.c
conv.c: In function ‘main’:
conv.c:14: warning: passing argument 1 of ‘foo’ with different width due to prototype
$ gcc -Wall -o conv conv.c
$ gcc -Wall -Wconversion -o conv conv.c
conv.c: In function ‘main’:
conv.c:13: warning: conversion to ‘short int’ from ‘int’ may alter its value
conv.c:14: warning: conversion to ‘short int’ from ‘int’ may alter its value

pthread的API一直让我感到头疼，种类多而且名字又长，今天下决心把它们理清楚。

仅从名字上来看，pthread的API可以分为这么几类：

pthread_XXX:
此类API一般是对thread本身进行管理的。共包括如下API：

pthread_atfork()
pthread_create()
pthread_exit()
pthread_cancel()
pthread_join()
pthread_once()
pthread_self()
pthread_equal()
pthread_kill()
pthread_detach()
pthread_yeild()
pthread_sigmask()
pthread_key_create()
pthread_key_delete()
pthread_cleanup_push()
pthread_cleanup_pop()
pthread_testcancel()

它还可以分出两个子类，包括pthread_setWWW和pthread_getWWW，其中有：

pthread_getconcurrency()
pthread_getcpuclockid()
pthread_getschedparam()
pthread_getspecific()

pthread_setcancelstate()
pthread_setconcurrency()
pthread_setschedparam()
pthread_setschedprio()
pthread_setspecific()

pthread_attr_YYY:
YYY一般包括：init, destory, setZZZ, getZZZ。此类API是对thread本身的属性进行管理的。共包括如下API：

pthread_attr_destroy()
pthread_attr_getinheritsched()
pthread_attr_getschedparam()
pthread_attr_getschedpolicy()
pthread_attr_getscope()
pthread_attr_getstackaddr()
pthread_attr_getstack()
pthread_attr_init()
pthread_attr_setdetachstate()
pthread_attr_setguardsize()
pthread_attr_setinheritsched()
pthread_attr_setschedparam()
pthread_attr_setschedpolicy()
pthread_attr_setscope()
pthread_attr_setstackaddr()
pthread_attr_setstack()
pthread_attr_setstacksize()

pthread_MMM_XXX:
XXX一般是上面和那个XXX集合类似的操作，但MMM一般是thread的一个工具，比如：mutex，cond等。此类API是对thread的MMM工具进行操作。共包括如下API：

mutex类：
pthread_mutex_init()
pthread_mutex_destroy()
pthread_mutex_lock()
pthread_mutex_unlock()
pthread_mutex_trylock()
pthread_mutex_setprioceiling()
pthread_mutex_getprioceiling()

cond类：
pthread_cond_init()
pthread_cond_destroy()
pthread_cond_signal()
pthread_cond_broadcast()
pthread_cond_wait()
pthread_cond_timedwait()

rwlock类：
pthread_rwlock_destroy()
pthread_rwlock_init()
pthread_rwlock_rdlock()
pthread_rwlock_timedrdlock()
pthread_rwlock_timedwrlock()
pthread_rwlock_tryrdlock()
pthread_rwlock_trywrlock()
pthread_rwlock_unlock()
pthread_rwlock_wrlock()

spin类：
pthread_spin_destroy()
pthread_spin_init()
pthread_spin_lock()
pthread_spin_trylock()
pthread_spin_unlock()

barrier类：
pthread_barrier_destroy()
pthread_barrier_init()
pthread_barrier_wait()

pthread_MMMattr_YYY:
MMM和YYY同上（spin除外），此类API是对MMM工具的属性进行操作。和上面有着密切的关系。共包括如下API：

mutex类：
pthread_mutexattr_destroy()
pthread_mutexattr_getprioceiling()
pthread_mutexattr_getprotocol()
pthread_mutexattr_getpshared()
pthread_mutexattr_gettype()
pthread_mutexattr_init()
pthread_mutexattr_setprioceiling()
pthread_mutexattr_setprotocol()
pthread_mutexattr_setpshared()
pthread_mutexattr_settype()

cond类：
pthread_condattr_destroy()
pthread_condattr_getclock()
pthread_condattr_getpshared()
pthread_condattr_init()
pthread_condattr_setclock()
pthread_condattr_setpshared()

rwlock类：
pthread_rwlockattr_destroy()
pthread_rwlockattr_getpshared()
pthread_rwlockattr_init()
pthread_rwlockattr_setpshared()

barrier类：
pthread_barrierattr_destroy()
pthread_barrierattr_getpshared()
pthread_barrierattr_init()
pthread_barrierattr_setpshared()

这还没完，还有N多的新类型，比如pthread_t，pthread_attr_t，pthread_once_t，以及随之而来的宏。。。我在这就不总结了。

最后，介绍pthread的书籍有：

“PThreads Primer”. Lewis, Bill and Daniel J. Berg. California: Prentice Hall.
“Pthreads Programming”. B. Nichols et al. O’Reilly and Associates.
“Programming With POSIX Threads”. D. Butenhof. Addison Wesley
“Programming With Threads”. S. Kleiman et al. Prentice Hall

网上有个查询的网站，可惜什么结果都查不出来！靠！我实在看不下去了，动手写一个python程序来搞定，不过仍有局限性，那就是只能查询.iso.org子树。。。啥也不说了，上代码！

[python]

!/usr/bin/env python

import os,sys
import string
import re
import urllib2

if name == ‘main‘:

if len(sys.argv) != 2:
    sys.stderr.write("Please provide one OID number or string to lookup.n");
    sys.exit(1)

flag = 0
found = False

r = re.compile('^[0-9\.]+$')
if r.match(sys.argv[1]):
    r = re.compile('^1\.3')
    if r.match(sys.argv[1]):
        flag = 1
    else:
        sys.stderr.write("Please provide the full OID number under .iso.org!n")
        sys.exit(1)

try:
    req = urllib2.Request('http://www.kix.in/plan9/mirror/sources/contrib/gabidiaz/root/lib/ndb/snmp')
    resp = urllib2.urlopen(req)
    oid = resp.readline()
    name = resp.readline()
    while oid and name:
        if flag == 1:
            if oid.find(sys.argv[1]) != -1:
                print name
                found = True
                break
        else:
            n = name.lower().find(sys.argv[1].lower())
            if n != -1:
                print oid
                found = True
                if n+len(sys.argv[1]) < len(name)-1:
                    print name
        oid = resp.readline()
        name = resp.readline()
    if not found:
        print "Not found!"
    sys.exit(0)
except IOError:
    sys.stderr.write("Probably you don't have Internet.n")
    sys.exit(1)

[/python]

可参考ASN.1关于octet string的介绍。

[bash]

!/bin/bash

i=1;
while(($i<=${#1}))
do
printf "%d" "'$(expr substr $1 $i 1)"
if (($i != ${#1}))
then
echo -n '.'
fi
i=$((i+1))
done
echo
exit 0
[/bash]

上面使用了一个鲜为人知的小技巧，而且不仔细看也不太容易察觉。

此脚本可以这么用（假设此脚本被存为to_string.sh）：

$ snmpwalk -v2c -c test 192.168.90.72 .1.3.6.1.4.1.8072.1.3.2.4.1.2.${#STRING}.$(./to_string.sh $STRING).1

在 arch/x86/include/asm/uaccess.h 中有这么一段代码：

define __range_not_ok(addr, size)

({
unsigned long flag, roksum;
__chk_user_ptr(addr);
asm(“add %3,%1 ; sbb %0,%0 ; cmp %1,%4 ; sbb $0,%0”
: “=&r” (flag), “=r” (roksum)
: “1” (addr), “g” ((long)(size)),
“rm” (current_thread_info()->addr_limit.seg));
flag;
})

这段汇编写得很有技巧性，充分利用了sbb指令和carry flag，值得你仔细体会一番。还有一个关于sbb的技巧：

sbb eax,eax
sbb eax,0FFFFFFFFh

C似乎没多少疑问，不过当有负数时还是需要特别注意的，因为标准要求是要向0对齐的。所以-5/2会是-2而不是-3。

6.5.5

When integers are divided, the result of the / operator is the algebraic quotient
with any fractional part discarded [Footnote].

…

Footnote: This is often called “truncation toward zero”
在C89的时候，这个问题还是交由编译器决定的。C99加强了这一点。C为什么会选择这一点？其实原因很简单，因为这么做节省时间：直接舍去小数部分。

Python在这一点上和C不同，因为它还保留了一个int()，这个和C在取整方向上是一致的：Python中的整数除法向负无穷对齐；而int()才是向0对齐的。见PEP238：

Note that classic division (and floor division) round towards negative infinity,
while int() rounds towards zero, giving different answers for negative numbers.
不要被迷惑了，在Python里int(-5/2)依旧是-3而不是-2，你知道为什么。:-) 在PEP 238中，Python引入了一个新的运算符：//，它始终会向下截取除法的结果（数学上的取整，英文谓之floor），看下面的例子：

>>> -5//2
-3
>>> -5.1/2.0
-2.5499999999999998
>>> -5.1//2.0
-3.0

Perl更不同，它只能通过int()来进行取整，因为普通除法在Perl里得到的结果是浮点数。《Programming Perl》中提到：

You should not use this function for generic rounding, because it truncates towards 0 and because machine representations of floating-point numbers can sometimes produce counterintuitive results.
所以这个也是向0对齐的。

再看ruby，这个似乎很简单，一律是向负无穷对齐的：

$ ruby -e ‘puts -5/2; puts -5.div(2); puts Kernel.Integer(-5/2);’

-3
-3
-3

Bash和C一致：

$ echo $[5/2] && echo $[-5/2]
2
-2

Java似乎也和C一致，不过我未验证。:-)

这里还有一个问题值得思考：为什么这些编程语言要把整数除法和浮点数除法区别对待？对于C，这个问题很简单，因为很久很久以前C并没有浮点数，只有整数，而其它语言似乎是因为受到了这一影响，当然Perl例外。不过，在很多情况下整数除法已经够用，无须打扰浮点数。