[英]Multithreading doesn't work as expected
I am reading an input line by line from stdin. 我正在从stdin逐行读取输入。 I am sending each line to a threaded function.
我将每一行发送到一个线程函数。 But I can see only output of the first input.
但是我只能看到第一个输入的输出。 How can I see output of each input?
如何查看每个输入的输出? Here is the code
这是代码
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <string>
#include <string.h>
#include <iostream>
#include <unistd.h>
using namespace std;
pthread_mutex_t lock;
void *print_message_function( void *ptr );
main()
{
pthread_t mythread[10];
const char *arrays[10];
int irets[10];
string line;
int k = 0;
while(getline(cin,line))
{
if(!line.empty())
{
arrays[k] = line.c_str();
irets[k] = pthread_create( &mythread[k], NULL, print_message_function, (void*) arrays[k]);
usleep(100);
k++;
}
}
for(int i = 0; i < 10; i++)
{
pthread_join( mythread[i], NULL);
}
pthread_mutex_destroy(&lock);
exit(0);
}
void *print_message_function( void *ptr )
{
pthread_mutex_lock(&lock);
char *message;
message = (char *) ptr;
printf("first %s \n", message);
sleep(1);
printf("second %s \n", message);
pthread_mutex_unlock(&lock);
}
Here is the output I get : 这是我得到的输出:
first hello1
second
first
second
first
second
first
second
first
second
first
second
first
second
first
second
first
second
first
second
The input is: 输入为:
hello1
hello2
hello3
hello4
hello5
hello6
hello7
hello8
hello9
hello10
I want to get: 我想得到:
first hello1
second hello1
first hello2
second hello2
first hello3
second hello3
first hello4
second hello4
first hello5
second hello5
first hello6
second hello6
first hello7
second hello7
first hello8
second hello8
first hello9
second hello9
first hello10
second hello10
arrays[k] = line.c_str();
This is not doing what you think it does... and since this is what you give to your print_message
functions... 这并没有按照您认为的那样做...并且由于这是您给
print_message
函数提供的...
Change const char *arrays[10];
更改
const char *arrays[10];
to string arrays[10];
string arrays[10];
and arrays[k] = line.c_str();
和
arrays[k] = line.c_str();
to arrays[k] = line;
到
arrays[k] = line;
and (void*) arrays[k]
to (void*)arrays[k].c_str()
. 和
(void*) arrays[k]
至(void*)arrays[k].c_str()
。
The problem is that once the line
changes to the next value previous arrays[k]
points to a meaningless piece of memory. 问题是,一旦该
line
更改为下一个值,则先前的arrays[k]
指向无意义的内存。 You have to save the value of line
in order to enable the thread to access it. 您必须保存
line
的值才能使线程访问它。
The result of std::string::c_str()
is only guaranteedly available as the std::string
does not change and does not get destructed (when you do a new getline you invalidated the result of the previous c_str()
. If you cant to keep the const char*
for more time than that, you will need to take a copy. Like: std::string::c_str()
的结果仅能保证可用,因为std::string
不会改变且不会被破坏(当您执行新的getline时,会使先前c_str()
的结果无效。无法将const char*
保存更长的时间,您将需要复制一个副本,例如:
arrays[k] = malloc(line.size()+1); //Plus 1 because of the \0 on the end of the string
memcpy(arrays[k],line.c_str(),line.size()+1);
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