文章目录
- 查看pthread_create()函数文档
- · Demo1 单线程（创建线程pthread_t 、创建线程run方法pthread_create）
- · Demo2 双线程（一个打印1，一个打印2）
- pthread_create()函数第四个参数，用于传递第三个参数中函数的参数使用（参数你可以传递任何类型，到时转换成相应类型的指针即可）
- · Demo3 随机5000个数字，线程一加前2500个，线程2加后2500个，然后两个结果求和
- · Demo4 两个线程同时提取数组元素相加（数组元素为1~5000）
- 假共享（伪共享）（False Sharing）：在多核cpu中，因为多线程不同核缓存的不一致，需要同步导致的时间延误？
thread线程和process进程：前者共享内存，后者不共享
查看pthread_create()函数文档
man pthread_create
PTHREAD_CREATE(3) Linux Programmer's Manual PTHREAD_CREATE(3)
NAME
pthread_create - create a new thread
SYNOPSIS
#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine) (void *), void *arg);
Compile and link with -pthread.
DESCRIPTION
The pthread_create() function starts a new thread in the calling process. The new thread starts execution by invoking start_routine(); arg is passed as the sole argument of start_routine().
The new thread terminates in one of the following ways:
* It calls pthread_exit(3), specifying an exit status value that is available to another thread in the same process that calls pthread_join(3).
* It returns from start_routine(). This is equivalent to calling pthread_exit(3) with the value supplied in the return statement.
* It is canceled (see pthread_cancel(3)).
* Any of the threads in the process calls exit(3), or the main thread performs a return from main(). This causes the termination of all threads in the process.
The attr argument points to a pthread_attr_t structure whose contents are used at thread creation time to determine attributes for the new thread; this structure is initialized using
pthread_attr_init(3) and related functions. If attr is NULL, then the thread is created with default attributes.
Before returning, a successful call to pthread_create() stores the ID of the new thread in the buffer pointed to by thread; this identifier is used to refer to the thread in subsequent calls
to other pthreads functions.
The new thread inherits a copy of the creating thread's signal mask (pthread_sigmask(3)). The set of pending signals for the new thread is empty (sigpending(2)). The new thread does not
inherit the creating thread's alternate signal stack (sigaltstack(2)).
The new thread inherits the calling thread's floating-point environment (fenv(3)).
The initial value of the new thread's CPU-time clock is 0 (see pthread_getcpuclockid(3)).
Linux-specific details
The new thread inherits copies of the calling thread's capability sets (see capabilities(7)) and CPU affinity mask (see sched_setaffinity(2)).
RETURN VALUE
On success, pthread_create() returns 0; on error, it returns an error number, and the contents of *thread are undefined.
ERRORS
EAGAIN Insufficient resources to create another thread.
EAGAIN A system-imposed limit on the number of threads was encountered. There are a number of limits that may trigger this error: the RLIMIT_NPROC soft resource limit (set via setrlimit(2)),
which limits the number of processes and threads for a real user ID, was reached; the kernel's system-wide limit on the number of processes and threads, /proc/sys/kernel/threads-max,
was reached (see proc(5)); or the maximum number of PIDs, /proc/sys/kernel/pid_max, was reached (see proc(5)).
EINVAL Invalid settings in attr.
EPERM No permission to set the scheduling policy and parameters specified in attr.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├─────────────────┼───────────────┼─────────┤
│pthread_create() │ Thread safety │ MT-Safe │
└─────────────────┴───────────────┴─────────┘
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
NOTES
See pthread_self(3) for further information on the thread ID returned in *thread by pthread_create(). Unless real-time scheduling policies are being employed, after a call to pthread_cre‐
ate(), it is indeterminate which thread—the caller or the new thread—will next execute.
A thread may either be joinable or detached. If a thread is joinable, then another thread can call pthread_join(3) to wait for the thread to terminate and fetch its exit status. Only when a
terminated joinable thread has been joined are the last of its resources released back to the system. When a detached thread terminates, its resources are automatically released back to the
system: it is not possible to join with the thread in order to obtain its exit status. Making a thread detached is useful for some types of daemon threads whose exit status the application
does not need to care about. By default, a new thread is created in a joinable state, unless attr was set to create the thread in a detached state (using pthread_attr_setdetachstate(3)).
On Linux/x86-32, the default stack size for a new thread is 2 megabytes. Under the NPTL threading implementation, if the RLIMIT_STACK soft resource limit at the time the program started has
any value other than "unlimited", then it determines the default stack size of new threads. Using pthread_attr_setstacksize(3), the stack size attribute can be explicitly set in the attr
argument used to create a thread, in order to obtain a stack size other than the default.
BUGS
In the obsolete LinuxThreads implementation, each of the threads in a process has a different process ID. This is in violation of the POSIX threads specification, and is the source of many
other nonconformances to the standard; see pthreads(7).
EXAMPLE
The program below demonstrates the use of pthread_create(), as well as a number of other functions in the pthreads API.
In the following run, on a system providing the NPTL threading implementation, the stack size defaults to the value given by the "stack size" resource limit:
$ ulimit -s
8192 # The stack size limit is 8 MB (0x800000 bytes)
$ ./a.out hola salut servus
Thread 1: top of stack near 0xb7dd03b8; argv_string=hola
Thread 2: top of stack near 0xb75cf3b8; argv_string=salut
Thread 3: top of stack near 0xb6dce3b8; argv_string=servus
Joined with thread 1; returned value was HOLA
Joined with thread 2; returned value was SALUT
Joined with thread 3; returned value was SERVUS
In the next run, the program explicitly sets a stack size of 1MB (using pthread_attr_setstacksize(3)) for the created threads:
$ ./a.out -s 0x100000 hola salut servus
Thread 1: top of stack near 0xb7d723b8; argv_string=hola
Thread 2: top of stack near 0xb7c713b8; argv_string=salut
Thread 3: top of stack near 0xb7b703b8; argv_string=servus
Joined with thread 1; returned value was HOLA
Joined with thread 2; returned value was SALUT
Joined with thread 3; returned value was SERVUS
Program source
#include <pthread.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <ctype.h>
#define handle_error_en(en, msg) \
do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
struct thread_info { /* Used as argument to thread_start() */
pthread_t thread_id; /* ID returned by pthread_create() */
int thread_num; /* Application-defined thread # */
char *argv_string; /* From command-line argument */
};
/* Thread start function: display address near top of our stack,
and return upper-cased copy of argv_string */
static void *
thread_start(void *arg)
{
struct thread_info *tinfo = arg;
char *uargv, *p;
printf("Thread %d: top of stack near %p; argv_string=%s\n",
tinfo->thread_num, &p, tinfo->argv_string);
uargv = strdup(tinfo->argv_string);
if (uargv == NULL)
handle_error("strdup");
for (p = uargv; *p != '\0'; p++)
*p = toupper(*p);
return uargv;
}
int
main(int argc, char *argv[])
{
int s, tnum, opt, num_threads;
struct thread_info *tinfo;
pthread_attr_t attr;
int stack_size;
void *res;
/* The "-s" option specifies a stack size for our threads */
stack_size = -1;
while ((opt = getopt(argc, argv, "s:")) != -1) {
switch (opt) {
case 's':
stack_size = strtoul(optarg, NULL, 0);
break;
default:
fprintf(stderr, "Usage: %s [-s stack-size] arg...\n",
argv[0]);
exit(EXIT_FAILURE);
}
}
num_threads = argc - optind;
/* Initialize thread creation attributes */
s = pthread_attr_init(&attr);
if (s != 0)
handle_error_en(s, "pthread_attr_init");
if (stack_size > 0) {
s = pthread_attr_setstacksize(&attr, stack_size);
if (s != 0)
handle_error_en(s, "pthread_attr_setstacksize");
}
/* Allocate memory for pthread_create() arguments */
tinfo = calloc(num_threads, sizeof(struct thread_info));
if (tinfo == NULL)
handle_error("calloc");
/* Create one thread for each command-line argument */
for (tnum = 0; tnum < num_threads; tnum++) {
tinfo[tnum].thread_num = tnum + 1;
tinfo[tnum].argv_string = argv[optind + tnum];
/* The pthread_create() call stores the thread ID into
corresponding element of tinfo[] */
s = pthread_create(&tinfo[tnum].thread_id, &attr,
&thread_start, &tinfo[tnum]);
if (s != 0)
handle_error_en(s, "pthread_create");
}
/* Destroy the thread attributes object, since it is no
longer needed */
s = pthread_attr_destroy(&attr);
if (s != 0)
handle_error_en(s, "pthread_attr_destroy");
/* Now join with each thread, and display its returned value */
for (tnum = 0; tnum < num_threads; tnum++) {
s = pthread_join(tinfo[tnum].thread_id, &res);
if (s != 0)
handle_error_en(s, "pthread_join");
printf("Joined with thread %d; returned value was %s\n",
tinfo[tnum].thread_num, (char *) res);
free(res); /* Free memory allocated by thread */
}
free(tinfo);
exit(EXIT_SUCCESS);
}
SEE ALSO
getrlimit(2), pthread_attr_init(3), pthread_cancel(3), pthread_detach(3), pthread_equal(3), pthread_exit(3), pthread_getattr_np(3), pthread_join(3), pthread_self(3), pthreads(7)
COLOPHON
This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2015-07-23 PTHREAD_CREATE(3)
· Demo1 单线程（创建线程pthread_t 、创建线程run方法pthread_create）
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void* myfunc(void* args){
printf("Hello World\n");
return NULL;
}
int main(){
pthread_t th;
pthread_create(&th, NULL, myfunc, NULL); //第一个参数是线程th的地址，第三个参数是指针函数的名字（是不是函数的指针不知道）
pthread_join(th, NULL); //等待线程th结束
return 0;
}
编译运行：
[yg@ubuntu ~/arnold_test/20211013_pthread_test]5$ gcc pthread_test1.c -lpthread
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$ ./a.out
Hello World
· Demo2 双线程（一个打印1，一个打印2）
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void* myfunc1(void* args){
for(int i = 1; i<1000000; i++){
printf("%d", 1);
}
return NULL;
}
void* myfunc2(void* args){
for(int i = 1; i<1000000; i++){
printf("%d", 2);
}
return NULL;
}
int main(){
pthread_t th1;
pthread_t th2;
pthread_create(&th1, NULL, myfunc1, NULL); //第一个参数是线程th的地址，第三个参数是指针函数的名字（是不是函数的指针不知道）
pthread_create(&th2, NULL, myfunc2, NULL);
pthread_join(th1, NULL); //等待线程th结束，注意这里第一个参数不用地址
pthread_join(th2, NULL);
return 0;
}
编译运行：
[yg@ubuntu ~/arnold_test/20211013_pthread_test]5$ gcc pthread_test1.c -lpthread
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$ ./a.out
运行结果：可以看到，是交替运行的
pthread_create()函数第四个参数，用于传递第三个参数中函数的参数使用（参数你可以传递任何类型，到时转换成相应类型的指针即可）
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void* myfunc(void* args){
char* name = (char*)args;
for(int i = 1; i<10000; i++){
printf("%s：%d\t", name, i);
}
return NULL;
}
int main(){
pthread_t th1;
pthread_t th2;
pthread_create(&th1, NULL, myfunc, "th1"); //第一个参数是线程th的地址，第三个参数是指针函数的名字（是不是函数的指针不知道）
pthread_create(&th2, NULL, myfunc, "th2");
pthread_join(th1, NULL); //等待线程th结束，注意这里第一个参数不用地址
pthread_join(th2, NULL);
return 0;
}
编译运行：
[yg@ubuntu ~/arnold_test/20211013_pthread_test]5$ gcc pthread_test1.c -lpthread
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$
[yg@ubuntu ~/arnold_test/20211013_pthread_test]6$ ./a.out
运行结果：也可以看到有交替的
· Demo3 随机5000个数字，线程一加前2500个，线程2加后2500个，然后两个结果求和
我的垃圾实现- -
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int arr[5000]; //创建一个包含大小为5000的整型数组
int s1; //前2500个数字和，分配给线程1去加
int s2; //后2500个数字和，分配给线程2去加
void* myfunc(void* args){
int index = *(int*)args;
//printf("%d\n", index);
int count = 0;
int sum = 0;
while(count<2500){
sum+=arr[index];
index++;
count++;
}
if(index-2500==0){
s1+=sum;
}else{
s2+=sum;
}
return NULL;
}
int main(){
//设置随机数种子（否则每次都一样结果）
srand(time(0));
int i;
for(i=0;i<5000;i++){
//给数组赋值
arr[i]=rand()%50; //rand()特别大，所以要取余
//printf("%d\n", arr[i]);
//arr[i]=i+1; //测试1到5000和是12502500，没问题
//printf("%d\n",arr[i]);
}
pthread_t th1;
pthread_t th2;
int a = 0;
int b = 2500;
pthread_create(&th1, NULL, myfunc, &a);
pthread_create(&th2, NULL, myfunc, &b);
pthread_join(th1, NULL);
pthread_join(th2, NULL);
printf("s1 = %d, s2 = %d\n", s1, s2);
return 0;
}
编译运行结果：因为是对50取余，所以每个结果接近（0~49）* 2500 = 24.5 * 2500 = 61250
[yg@ubuntu ~/arnold_test/20211013_pthread_test]127$ gcc pthread_test1.c -lpthread
[yg@ubuntu ~/arnold_test/20211013_pthread_test]128$ ./a.out
s1 = 60163, s2 = 61379
[yg@ubuntu ~/arnold_test/20211013_pthread_test]129$ ./a.out
s1 = 62296, s2 = 60521
[yg@ubuntu ~/arnold_test/20211013_pthread_test]130$ ./a.out
s1 = 61066, s2 = 61084
[yg@ubuntu ~/arnold_test/20211013_pthread_test]131$ ./a.out
s1 = 61051, s2 = 61582
[yg@ubuntu ~/arnold_test/20211013_pthread_test]132$
老师的实现（通过传入结构体指针，获取返回值结果）
1、可以为每个线程单独写一个函数，这样虽然简答，但代码重复多
2、只写一个函数，在外部定义包含需要相加的数字下标起止点的结构体，同时在结构体中定义接收计算结果的变量，将结构体作为线程函数的第四个参数传入
代码实现：
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int arr[5000]; //创建一个包含大小为5000的整型数组
int s1; //前2500个数字和，分配给线程1去加
int s2; //后2500个数字和，分配给线程2去加
//typedef struct _MY_ARGS{
typedef struct{
int start;
int end;
int result;
}MY_ARGS;
void* myfunc(void* args){
MY_ARGS* p = (MY_ARGS*)args;
int sum = 0; //不要忘记赋初始值（！）
int i;
for(i= p->start; i < p->end; i++){
sum+=arr[i];
//printf("%d\n", arr[i]);
}
p->result = sum;
return NULL;
}
int main(){
//设置随机数种子（否则每次都一样结果）
srand(time(0));
int i;
for(i=0;i<5000;i++){
//给数组赋值
arr[i]=rand()%50; //rand()特别大，所以要取余
//printf("%d\n", arr[i]);
//arr[i]=i+1; //测试1到5000和是12502500，没问题
//printf("%d\n",arr[i]);
}
pthread_t th1;
pthread_t th2;
MY_ARGS args1 = {0, 2500, 0};
MY_ARGS args2 = {2500, 5000, 0};
pthread_create(&th1, NULL, myfunc, &args1);
pthread_create(&th2, NULL, myfunc, &args2);
pthread_join(th1, NULL);
pthread_join(th2, NULL);
printf("s1 = %d, s2 = %d\n", args1.result, args2.result);
return 0;
}
编译运行结果：
[yg@ubuntu ~/arnold_test/20211013_pthread_test]182$ gcc pthread_test1.c -lpthread
[yg@ubuntu ~/arnold_test/20211013_pthread_test]183$ ./a.out
s1 = 63337, s2 = 59578
[yg@ubuntu ~/arnold_test/20211013_pthread_test]184$ ./a.out
s1 = 60652, s2 = 60818
[yg@ubuntu ~/arnold_test/20211013_pthread_test]185$ ./a.out
s1 = 61265, s2 = 62009
[yg@ubuntu ~/arnold_test/20211013_pthread_test]186$ ./a.out
s1 = 61265, s2 = 62009
[yg@ubuntu ~/arnold_test/20211013_pthread_test]187$ ./a.out
s1 = 61912, s2 = 62479
[yg@ubuntu ~/arnold_test/20211013_pthread_test]188$
· Demo4 两个线程同时提取数组元素相加（数组元素为1~5000）
不加锁试试
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int arr[5000]; //创建一个包含大小为5000的整型数组
int sum = 0; //两个线程同时对sum进行操作
//typedef struct _MY_ARGS{
typedef struct{
int start;
int end;
}MY_ARGS;
void* myfunc(void* args){
MY_ARGS* p = (MY_ARGS*)args;
int i;
for(i= p->start; i < p->end; i++){
sum+=arr[i];
//printf("%d\n", arr[i]);
}
return NULL;
}
int main(){
//设置随机数种子（否则每次都一样结果）
srand(time(0));
int i;
for(i=0;i<5000;i++){
//给数组赋值
//arr[i]=rand()%50; //rand()特别大，所以要取余
//printf("%d\n", arr[i]);
arr[i]=i+1; //测试1到5000和是12502500，没问题
//printf("%d\n",arr[i]);
}
pthread_t th1;
pthread_t th2;
MY_ARGS args1 = {0, 2500};
MY_ARGS args2 = {2500, 5000};
pthread_create(&th1, NULL, myfunc, &args1);
pthread_create(&th2, NULL, myfunc, &args2);
pthread_join(th1, NULL);
pthread_join(th2, NULL);
printf("sum = %d", sum);
return 0;
}
运行编译结果：
[yg@ubuntu ~/arnold_test/20211013_pthread_test]207$ gcc pthread_test1.c -lpthread
[yg@ubuntu ~/arnold_test/20211013_pthread_test]208$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]209$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]210$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]211$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]212$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]213$ ./a.out
可以看到，结果不全尽然是12502500，说明两个线程其中一个在把元素加到sum变量的同时，另一个线程也在把另一个元素往sum里面加，没排队按顺序加，导致结果不对
加mutex锁🔒（pthread_mutex_t lock;）
pthread_mutex_init() 锁初始化函数
代码示例（加 / 解线程锁：pthread_mutex_lock(&lock); pthread_mutex_unlock(&lock);）
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
int arr[5000]; //创建一个包含大小为5000的整型数组
int sum = 0; //两个线程同时对sum进行操作
pthread_mutex_t lock; //创建mutex锁🔒
//typedef struct _MY_ARGS{
typedef struct{
int start;
int end;
}MY_ARGS;
void* myfunc(void* args){
MY_ARGS* p = (MY_ARGS*)args;
int i;
//int a;
for(i= p->start; i < p->end; i++){
pthread_mutex_lock(&lock); //锁住代码
//a=sum;
sum+=arr[i];
//printf("%d", sum-a-arr[i]); //验证不加锁时会不按顺序乱加sum（如果出现不为0的数表示线程乱窜了）
pthread_mutex_unlock(&lock); //解锁代码
//printf("%d\n", arr[i]);
}
return NULL;
}
int main(){
//设置随机数种子（否则每次都一样结果）
srand(time(0));
int i;
for(i=0;i<5000;i++){
//给数组赋值
//arr[i]=rand()%50; //rand()特别大，所以要取余
//printf("%d\n", arr[i]);
arr[i]=i+1; //测试1到5000和是12502500，没问题
//printf("%d\n",arr[i]);
}
pthread_t th1;
pthread_t th2;
pthread_mutex_init(&lock, NULL); //初始化mutex锁🔒
MY_ARGS args1 = {0, 2500};
MY_ARGS args2 = {2500, 5000};
pthread_create(&th1, NULL, myfunc, &args1);
pthread_create(&th2, NULL, myfunc, &args2);
pthread_join(th1, NULL);
pthread_join(th2, NULL);
printf("sum = %d", sum);
return 0;
}
运行编译结果：
[yg@ubuntu ~/arnold_test/20211013_pthread_test]346$ gcc pthread_tedt1.c -lpthread
[yg@ubuntu ~/arnold_test/20211013_pthread_test]347$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]348$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]349$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]350$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]351$ ./a.out
sum = 12502500[yg@ubuntu ~/arnold_test/20211013_pthread_test]352$
查看运行时间：看不出来，数字太小了（反正作者的意思就是，加锁解锁也需要消费时间！）
[yg@ubuntu ~/arnold_test/20211013_pthread_test]352$ time ./a.out
sum = 12502500
real 0m0.001s
user 0m0.001s
sys 0m0.000s
假共享（伪共享）（False Sharing）：在多核cpu中，因为多线程不同核缓存的不一致，需要同步导致的时间延误？
https://www.bilibili.com/video/BV1kt411z7ND?p=4
这里面作者把两个线程计算结果分别存到同一个整型数组的连续两个位置，长度较短，于是RAM复制到Cache缓存的时候就会整体复制过去，，同步时就会产生较大的时间延误
怎么让它不整体复制呢，方法就是增加字符数组长度（增大到它不想整体复制为止），比如将第一个结果存到第一个int下，将第二个结果存到第101个int下，