http://blog.chinaunix.net/uid-12567959-id-161022.html

Name
uuid_generate, uuid_generate_random, uuid_generate_time - create a newunique UUID value

Synopsis
#include <uuid/uuid.h>


void uuid_generate(uuid_t out);

void uuid_generate_random(uuid_t out);

void uuid_generate_time(uuid_t out);


Description
The uuid_generate function creates a new universally uniqueidentifier (UUID). The uuid will be generated based on high-quality randomnessfrom
/dev/urandom, ifavailable. If it is not available, then 
uuid_generate
will use an alternative algorithm which uses the current time, the localethernet MAC address (if available), and random data generated using apseudo-random generator.



The uuid_generate_random function forces the use of the all-randomUUID format, even if a high-quality random number generator (i.e.,
/dev/urandom)is not available, in which case a pseudo-random generator will be subsituted.Note that the use of a pseudo-random generator may compromise the uniqueness ofUUID's generated in this fashion.



The uuid_generate_time function forces the use of the alternativealgorithm which uses the current time and the local ethernet MAC address (ifavailable). This algorithm used to be the default one used to generate UUID,but because
 of the use of the ethernet MAC address, it can leak informationabout when and where the UUID was generated. This can cause privacy problems insome applications, so the
uuid_generate function only uses thisalgorithm if a high-quality source of randomness is not available.



The UUID is 16 bytes (128 bits) long, which gives approximately 3.4x10^38unique values (there are approximately 10^80 elemntary particles in theuniverse according to Carl Sagan's
Cosmos). The new UUID can reasonablybe considered unique among all UUIDs created on the local system, and amongUUIDs created on other systems in the past and in the future.

Return Value
The newly created UUID is returned in the memory location pointed to by
out.
Conforming To
OSF DCE 1.1 
Author
Theodore Y. Ts'o 
Availability
http://e2fsprogs.sourceforge.net/
See Also
uuid(3),
uuidgen(1),uuid_clear(3),
uuid_compare(3),
uuid_copy(3),
uuid_is_null(3),
uuid_parse(3),
uuid_time(3),
uuid_unparse(3)

Referenced By
libuuid(3) 

A UUID is designed as a number that is globally unique in space and time. Two calls to UUID() are
expected to generate two different values, even if these calls are performed on two separate computers
that are not connected to each other.
uuid是世界上唯一的，就算两个没有任何关系的计算机uuid也是不同的
但是文档上却没有说到他是不是有序的，网上都说uuid是无序，我想一个场景试一下：
场景解释：
建一个表，里面有两个字段 uuid和时间。
每一秒向这个表里插入一条数据，分别是 uuid()和now()
插入一段时间之后，如果按id排序和按t_date排序得到的结果是一样的，那么应该可以证明uuid是有序的，如果排序结果不一样，那么是无序的。
我这里两个进程同时插入，并且跑了两个小时
模拟过程：
在mysql 5.6上测试
mysql> create table song( id char(36), t_date datetime);
Query OK, 0 rows affected (0.00 sec)
cat insert.sh
#!/bin/bash
while true; do
mysql -h10.13.52.100 -P3306 -uroot -ptest  -e "insert into test.song select uuid(),now();"
sleep 1
done
nohup sh insert.sh &
nohup sh insert.sh &
两个一起插入
运行大概两个小时，停掉insert.sh后：
mysql -h10.13.52.100 -P3306 -uroot -ptest  -e "select * from test.song order by t_date;" > a.log
mysql -h10.13.52.100 -P3306 -uroot -ptest  -e "select * from test.song order by id;" > b.log
diff a.log b.log  -y > c.log
[root@10-13-59-213 ~]# vim c.log
id      t_date                                                  id      t_date
b6af9995-50d2-11e6-9d61-5254005ae15d    2016-07-23 20:41:01   | 0019655a-50d5-11e6-9d61-5254005ae15d    2016-07-23 20:57:24
cc161c32-50d2-11e6-9d61-5254005ae15d    2016-07-23 20:41:37   | 001968e2-50d5-11e6-9d61-5254005ae15d    2016-07-23 20:57:24
ccafd852-50d2-11e6-9d61-5254005ae15d    2016-07-23 20:41:38   <
cd495dc1-50d2-11e6-9d61-5254005ae15d    2016-07-23 20:41:39   <
从c.log中可以看到a.log和b.log的不同。 那么 uuid()是无序的
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MySQL's UUID function returns a UUIDv1 GUID. I'm looking for an easy way to generate random GUIDs (i.e. UUIDv4) in SQL.
解决方案
I've spent quite some time looking for a solution and came up with the following
mysql function that generates a random UUID (i.e. UUIDv4) using standard MySQL
functions. I'm answering my own question to share that in the hope that it'll be
useful.
-- Change delimiter so that the function body doesn't end the function declaration
DELIMITER //
CREATE FUNCTION uuid_v4()
RETURNS CHAR(36)
BEGIN
-- Generate 8 2-byte strings that we will combine into a UUIDv4
SET @h1 = LPAD(HEX(FLOOR(RAND() * 0xffff)), 4, '0');
SET @h2 = LPAD(HEX(FLOOR(RAND() * 0xffff)), 4, '0');
SET @h3 = LPAD(HEX(FLOOR(RAND() * 0xffff)), 4, '0');
SET @h6 = LPAD(HEX(FLOOR(RAND() * 0xffff)), 4, '0');
SET @h7 = LPAD(HEX(FLOOR(RAND() * 0xffff)), 4, '0');
SET @h8 = LPAD(HEX(FLOOR(RAND() * 0xffff)), 4, '0');
-- 4th section will start with a 4 indicating the version
SET @h4 = CONCAT('4', LPAD(HEX(FLOOR(RAND() * 0x0fff)), 3, '0'));
-- 5th section first half-byte can only be 8, 9 A or B
SET @h5 = CONCAT(HEX(FLOOR(RAND() * 4 + 8)),
LPAD(HEX(FLOOR(RAND() * 0x0fff)), 3, '0'));
-- Build the complete UUID
RETURN LOWER(CONCAT(
@h1, @h2, '-', @h3, '-', @h4, '-', @h5, '-', @h6, @h7, @h8
));
END
//
-- Switch back the delimiter
DELIMITER ;
Note: The pseudo-random number generation used (MySQL's RAND) is not
cryptographically secure and thus has some bias which can increase the collision
risk.

写这个程序之前需要知道UUID,UUID是全球唯一标识码，所以保证了数字的唯一性。
安装uuid工具链：sudo apt-get install uuid-dev
uuid提供的API：
void uuid_generate(uuid_tout);
void uuid_generate_random(uuid_tout);
void uuid_generate_time(uuid_tout);
int uuid_generate_time_safe(uuid_tout);

#include<stdio.h>
#include<uuid/uuid.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
void uuid_conver(char* source,char* desc){
int i,j;
for(i = 0,j= 0;i < 36;i++,j++){
if(desc[i]=='-'){
i++;
}
source[j] = toupper(desc[i]);
}
source[j] = '\0';
}
int main(int argc, const char *argv[])
{
uuid_t uu;
char buf[100];
uuid_generate_random(uu);
uuid_unparse(uu,buf);
printf("%s\n",buf);
char buff[40];
uuid_conver(buff,buf);
printf("%s\n",buff);
return 0;
}
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