如何从 child 进程更新共享数据？
How do I update shared data from a child process?
我正在尝试制作一个分叉 5 children 的程序。 children all access shared memory to read in a number and increase it by 1 until the number reaches 100. children 能够读入共享内存中的值，但它们没有更新它。我做错了什么？
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <string.h>
#include <sched.h>
int main (int argc, char* argv[])
{
//create shared memory with children and copy into it the starting number 0
void* shmem = mmap(NULL, 4, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, -1, 0);
char number[10] = "0";
memcpy(shmem, number, sizeof(number));
int i;
for(i = 0; i < 5; ++i)
{
int childPID = fork();
if(childPID == 0)
{
//read a number in from shared memory
int origNum = atoi(shmem);
if(origNum < 100)
{
int newNum = origNum + 1;
//cast the new number to a string to be sent to shared memory
char newNumString[10];
sprintf(newNumString, "%i", origNum);
//send the new number to shared memory
memcpy(shmem, newNumString, sizeof(newNumString));
printf("I'm child %i. PID: %i. PPID: %i. Read in: %i and printed out %i\n", (i + 1), getpid(), getppid(), origNum, newNum);
printf("Current value in shmem: %s\n", shmem);
//yield process to allow other processes to run
sched_yield();
}
else
{
exit(0);
}
}
else
{
printf("I'm a parent. PID: %i. Num: %s\n", getpid(), shmem);
}
}
}
最明显的问题是：
sprintf(newNumString, "%i", origNum);
从 'old' 值设置 'new' 字符串，因此共享内存中的数据永远不会更新为 'new' 值。
但是，还有其他几个问题涉及 'race' 条件。
这可以通过具有 'share' 属性的互斥锁来纠正。
注意：没有 'shared' 属性，多个进程访问同一个互斥量是未定义的行为。
警告：我没有包括检查调用系统函数的所有返回值，尤其是那些处理互斥锁设置的函数。您应该添加该检查。
警告：我没有包含破坏互斥锁的代码。您应该添加该声明。
警告：我没有包含释放内存映射内存的代码。您应该添加该声明。
存在子进程未全部退出就退出父进程的问题。这导致僵尸进程，而子进程曾经是。
有'problem'使用'magic'号。 'magic' 数字使代码更难理解、调试等。
在下面的代码中，我记录了为什么要包含每个头文件。这使得某些活动更容易完成。
我留下了一些原始代码（注释掉）来说明一些更改。
我在需要时使用了 'cast'，以避免来自编译器的 'implicit conversion' 警告（ gcc on linux ）
以下代码更正了问题，包括问题评论中列出的问题。
#include <stdio.h> // printf(), perror(), sprintf()
#include <stdlib.h> // atoi(), exit(), EXIT_FAILURE
#include <sys/mman.h> // mmap(), MAP_FAILED
#include <string.h> // strcpy(), memset(),
#include <sched.h> // sched_yield()
#include <unistd.h> // fork(), pid_t, getpid(), getppid()
#include <pthread.h> // pthread_mutex_lock(),
// pthread_mutex_unlock(),
// pthread_mutexattr_init(),
// pthread_mutex_init(),
// pthread_mutexattr_setpshared(),
// PTHREAD_PROCESS_SHARED,
// pthread_mutex_t,
// pthread_mutexattr_t
#include <sys/types.h>
#include <sys/wait.h> // waitpid()
#define MAX_CHILDREN 5
#define MAX_NUM_LEN 10
struct shared
{
char number[ MAX_NUM_LEN ];
pthread_mutex_t mutex;
pthread_mutexattr_t attr;
};
struct shared *shmem;
//int main (int argc, char* argv[])
int main( void )
{
//create shared memory with children and copy into it the starting number 0
if( MAP_FAILED == (shmem = (struct shared*)mmap(NULL, sizeof( struct shared), PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, -1, 0) ) )
{
perror( "mmap failed" );
exit( EXIT_FAILURE );
}
// implied else, mmap successful
//char number[10] = "0";
memset( shmem, '[=11=]', sizeof( struct shared ) );
//memcpy(shmem, number, sizeof(number));
// setup the mutex, include the 'shared' attribute
pthread_mutexattr_init(&shmem->attr);
pthread_mutexattr_setpshared( &shmem->attr, PTHREAD_PROCESS_SHARED );
pthread_mutex_init(&shmem->mutex, &shmem->attr );
//int i;
pid_t childPID[ MAX_CHILDREN ] = {0};
for( int i = 0; i < 5; ++i )
{
childPID[i] = fork();
switch( childPID[i] )
{
case -1:
perror( "fork failed" );
break;
case 0:
{ // then child63
while( 1 )
{
pthread_mutex_lock( &shmem->mutex );
//read a number in from shared memory
int origNum = atoi( shmem->number );
if( origNum < 100 )
{
int newNum = origNum + 1;
//cast the new number to a string to be sent to shared memory
char newNumString[ MAX_NUM_LEN ] = {'[=11=]'};
sprintf(newNumString, "%i", newNum);
//send the new number to shared memory
strcpy( &shmem->number[0], newNumString );
printf( "I'm child %i. PID: %i. PPID: %i. Read in: %i and printed out %i\n",
(i + 1),
getpid(),
getppid(),
origNum,
newNum );
printf( "Current value in shmem: %s\n", shmem->number );
// allow other processes to access the number in shared memory
pthread_mutex_unlock( &shmem->mutex );
//yield process to allow other processes to run
sched_yield();
}
else
{
// allow other processes to access the number in shared memory
pthread_mutex_unlock( &shmem->mutex );
exit(0);
}
}
}
break;
default:
// then parent
printf("I'm a parent. PID: %i. Num: %s\n", getpid(), (char*)shmem);
break;
} // end switch
} // end for each child
for( int i=0; i< MAX_CHILDREN; i++ )
{
int status;
if( -1 != childPID[i] )
{ // then child process exists
waitpid( childPID[i], &status, 0 );
}
}
} // end function: main
这里是更正程序输出的摘录：
I'm a parent. PID: 25961. Num:
I'm a parent. PID: 25961. Num: 1
I'm child 1. PID: 25962. PPID: 25961. Read in: 0 and printed out 1
I'm a parent. PID: 25961. Num: 1
Current value in shmem: 1
I'm child 2. PID: 25963. PPID: 25961. Read in: 1 and printed out 2
I'm a parent. PID: 25961. Num: 2
Current value in shmem: 2
I'm child 2. PID: 25963. PPID: 25961. Read in: 2 and printed out 3
Current value in shmem: 3
I'm a parent. PID: 25961. Num: 3
I'm child 4. PID: 25965. PPID: 25961. Read in: 3 and printed out 4
Current value in shmem: 4
...
I'm child 4. PID: 25965. PPID: 25961. Read in: 95 and printed out 96
Current value in shmem: 96
I'm child 3. PID: 25964. PPID: 25961. Read in: 96 and printed out 97
Current value in shmem: 97
I'm child 4. PID: 25965. PPID: 25961. Read in: 97 and printed out 98
Current value in shmem: 98
I'm child 3. PID: 25964. PPID: 25961. Read in: 98 and printed out 99
Current value in shmem: 99
I'm child 4. PID: 25965. PPID: 25961. Read in: 99 and printed out 100
Current value in shmem: 100
我正在尝试制作一个分叉 5 children 的程序。 children all access shared memory to read in a number and increase it by 1 until the number reaches 100. children 能够读入共享内存中的值，但它们没有更新它。我做错了什么？
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <string.h>
#include <sched.h>
int main (int argc, char* argv[])
{
//create shared memory with children and copy into it the starting number 0
void* shmem = mmap(NULL, 4, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, -1, 0);
char number[10] = "0";
memcpy(shmem, number, sizeof(number));
int i;
for(i = 0; i < 5; ++i)
{
int childPID = fork();
if(childPID == 0)
{
//read a number in from shared memory
int origNum = atoi(shmem);
if(origNum < 100)
{
int newNum = origNum + 1;
//cast the new number to a string to be sent to shared memory
char newNumString[10];
sprintf(newNumString, "%i", origNum);
//send the new number to shared memory
memcpy(shmem, newNumString, sizeof(newNumString));
printf("I'm child %i. PID: %i. PPID: %i. Read in: %i and printed out %i\n", (i + 1), getpid(), getppid(), origNum, newNum);
printf("Current value in shmem: %s\n", shmem);
//yield process to allow other processes to run
sched_yield();
}
else
{
exit(0);
}
}
else
{
printf("I'm a parent. PID: %i. Num: %s\n", getpid(), shmem);
}
}
}
最明显的问题是：
sprintf(newNumString, "%i", origNum);
从 'old' 值设置 'new' 字符串，因此共享内存中的数据永远不会更新为 'new' 值。
但是，还有其他几个问题涉及 'race' 条件。 这可以通过具有 'share' 属性的互斥锁来纠正。
注意：没有 'shared' 属性，多个进程访问同一个互斥量是未定义的行为。
警告：我没有包括检查调用系统函数的所有返回值，尤其是那些处理互斥锁设置的函数。您应该添加该检查。
警告：我没有包含破坏互斥锁的代码。您应该添加该声明。
警告：我没有包含释放内存映射内存的代码。您应该添加该声明。
存在子进程未全部退出就退出父进程的问题。这导致僵尸进程，而子进程曾经是。
有'problem'使用'magic'号。 'magic' 数字使代码更难理解、调试等。
在下面的代码中，我记录了为什么要包含每个头文件。这使得某些活动更容易完成。
我留下了一些原始代码（注释掉）来说明一些更改。
我在需要时使用了 'cast'，以避免来自编译器的 'implicit conversion' 警告（ gcc on linux ）
以下代码更正了问题，包括问题评论中列出的问题。
#include <stdio.h> // printf(), perror(), sprintf()
#include <stdlib.h> // atoi(), exit(), EXIT_FAILURE
#include <sys/mman.h> // mmap(), MAP_FAILED
#include <string.h> // strcpy(), memset(),
#include <sched.h> // sched_yield()
#include <unistd.h> // fork(), pid_t, getpid(), getppid()
#include <pthread.h> // pthread_mutex_lock(),
// pthread_mutex_unlock(),
// pthread_mutexattr_init(),
// pthread_mutex_init(),
// pthread_mutexattr_setpshared(),
// PTHREAD_PROCESS_SHARED,
// pthread_mutex_t,
// pthread_mutexattr_t
#include <sys/types.h>
#include <sys/wait.h> // waitpid()
#define MAX_CHILDREN 5
#define MAX_NUM_LEN 10
struct shared
{
char number[ MAX_NUM_LEN ];
pthread_mutex_t mutex;
pthread_mutexattr_t attr;
};
struct shared *shmem;
//int main (int argc, char* argv[])
int main( void )
{
//create shared memory with children and copy into it the starting number 0
if( MAP_FAILED == (shmem = (struct shared*)mmap(NULL, sizeof( struct shared), PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, -1, 0) ) )
{
perror( "mmap failed" );
exit( EXIT_FAILURE );
}
// implied else, mmap successful
//char number[10] = "0";
memset( shmem, '[=11=]', sizeof( struct shared ) );
//memcpy(shmem, number, sizeof(number));
// setup the mutex, include the 'shared' attribute
pthread_mutexattr_init(&shmem->attr);
pthread_mutexattr_setpshared( &shmem->attr, PTHREAD_PROCESS_SHARED );
pthread_mutex_init(&shmem->mutex, &shmem->attr );
//int i;
pid_t childPID[ MAX_CHILDREN ] = {0};
for( int i = 0; i < 5; ++i )
{
childPID[i] = fork();
switch( childPID[i] )
{
case -1:
perror( "fork failed" );
break;
case 0:
{ // then child63
while( 1 )
{
pthread_mutex_lock( &shmem->mutex );
//read a number in from shared memory
int origNum = atoi( shmem->number );
if( origNum < 100 )
{
int newNum = origNum + 1;
//cast the new number to a string to be sent to shared memory
char newNumString[ MAX_NUM_LEN ] = {'[=11=]'};
sprintf(newNumString, "%i", newNum);
//send the new number to shared memory
strcpy( &shmem->number[0], newNumString );
printf( "I'm child %i. PID: %i. PPID: %i. Read in: %i and printed out %i\n",
(i + 1),
getpid(),
getppid(),
origNum,
newNum );
printf( "Current value in shmem: %s\n", shmem->number );
// allow other processes to access the number in shared memory
pthread_mutex_unlock( &shmem->mutex );
//yield process to allow other processes to run
sched_yield();
}
else
{
// allow other processes to access the number in shared memory
pthread_mutex_unlock( &shmem->mutex );
exit(0);
}
}
}
break;
default:
// then parent
printf("I'm a parent. PID: %i. Num: %s\n", getpid(), (char*)shmem);
break;
} // end switch
} // end for each child
for( int i=0; i< MAX_CHILDREN; i++ )
{
int status;
if( -1 != childPID[i] )
{ // then child process exists
waitpid( childPID[i], &status, 0 );
}
}
} // end function: main
这里是更正程序输出的摘录：
I'm a parent. PID: 25961. Num:
I'm a parent. PID: 25961. Num: 1
I'm child 1. PID: 25962. PPID: 25961. Read in: 0 and printed out 1
I'm a parent. PID: 25961. Num: 1
Current value in shmem: 1
I'm child 2. PID: 25963. PPID: 25961. Read in: 1 and printed out 2
I'm a parent. PID: 25961. Num: 2
Current value in shmem: 2
I'm child 2. PID: 25963. PPID: 25961. Read in: 2 and printed out 3
Current value in shmem: 3
I'm a parent. PID: 25961. Num: 3
I'm child 4. PID: 25965. PPID: 25961. Read in: 3 and printed out 4
Current value in shmem: 4
...
I'm child 4. PID: 25965. PPID: 25961. Read in: 95 and printed out 96
Current value in shmem: 96
I'm child 3. PID: 25964. PPID: 25961. Read in: 96 and printed out 97
Current value in shmem: 97
I'm child 4. PID: 25965. PPID: 25961. Read in: 97 and printed out 98
Current value in shmem: 98
I'm child 3. PID: 25964. PPID: 25961. Read in: 98 and printed out 99
Current value in shmem: 99
I'm child 4. PID: 25965. PPID: 25961. Read in: 99 and printed out 100
Current value in shmem: 100