编写一个演示共享内存的非常基本的C程序。程序需要继续计算两个输入的总和，直到用户执行^C（此处未显示）。
到目前为止，我的父母捕获用户输入，然后告诉孩子计算总和，然后父母打印总和（如作业所示）。
附上我的代码。
// Fork the process
while(1) {
// Declare a shared memory integer array
int *shared_ints = mmap(NULL, shared_seg_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
int x;
int y;
shared_ints[3] = 0;
if((pid = fork()) < 0) {
printf("Fork error \n");
exit(-1);
} else if(pid > 0 && shared_ints[3] == 0) { // Parent code
// Ask user for first input
printf("Enter your first number:");
scanf("%d", &x);
printf("\n");
// Ask user for second input
printf("Enter your second number:");
scanf("%d", &y);
printf("\n");
// Assign first value to shared memory
shared_ints[0] = x;
// Assign second value to shared memory
shared_ints[1] = y;
// Tell child that it can compute the sum now
shared_ints[3] = 1;
// Trap parent in a while-loop while the child
// computes the sum
while(shared_ints[3] == 1);
// Child has completed calculating the sum and
// the parent can print the sum
if(shared_ints[3] == 2) {
printf("The sum is: %d", shared_ints[2]);
printf("\n");
}
} else { // Child code
// Wait for parent to accept input values
while(shared_ints[3] == 0);
// Calculate the sum
shared_ints[2] = shared_ints[0] + shared_ints[1];
// Tell parent sum has been calculated
shared_ints[3] = 2;
}
}
总和计算有效，直到我进行总和计算的第四次迭代（这是输出）：
Created shared memory object /my_shared_memory
Shared memory segment allocated correctly (16 bytes).
Enter your first number:1
Enter your second number:2
The sum is: 3
Enter your first number:Enter your first number:3
Enter your second number:4
The sum is: 7
Enter your first number:Enter your first number:5
Enter your second number:6
The sum is: 11
Enter your first number:Enter your first number:7
Enter your second number:8
Enter your second number:9
Enter your second number:
我看到的一个有趣的错误是，在打印总和后，程序会按照此处的建议请求第一次输入两次：Enter your first number:Enter your first number:3，然后在第四次求和迭代中，它会多次请求第二个数字在计算总和之前。
答案 0 :(得分：3)
麻烦的是你在外while循环的每次迭代中都要分叉，所以第一次，你有一个孩子，然后你有一个孩子，认为它已经长大了，是父母（以及知道它的父母是父母）。随着你的继续，它会变得更糟。
另一个问题是您在外部while循环的每次迭代中分配共享内存。我不确定它确实是内存泄漏，但它可能不是一个好主意。
修复：
mmap()和fork()移到while (1)循环之外。main()内，请 - 从用户读取，传输给子，从子进程读取结果，然后继续。这使用了标头"stderr.h"以及我编写的代码中的err_setarg0()和err_syserr()函数，因为错误报告至关重要，这使得它变得简单。你应该可以通过该标题的工作版本和那些函数找到我的SO代码。
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include "stderr.h"
static void be_childish(int *shared_ints);
static void be_parental(int *shared_ints);
int main(int argc, char **argv)
{
const char *file = "fidget";
int shared_seg_size = 4 * sizeof(int);
err_setarg0(argv[0]);
if (argc > 1)
file = argv[1];
int fd = open(file, O_RDWR|O_CREAT, 0600);
if (fd < 0)
err_syserr("Failed to open file %s for read/write\n", file);
/* Assume sizeof(int) == 4 */
if (write(fd, "abcdefghijklmnop", shared_seg_size) != shared_seg_size)
err_syserr("Failed to write %d bytes to %s\n", shared_seg_size, file);
int *shared_ints = mmap(NULL, shared_seg_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (shared_ints == 0)
err_syserr("Failed to mmap file %s\n", file);
shared_ints[3] = 0;
int pid;
if ((pid = fork()) == -1)
err_syserr("Failed to fork\n");
else if (pid == 0)
be_childish(shared_ints);
else
be_parental(shared_ints);
return 0;
}
static void be_childish(int *shared_ints)
{
while (1)
{
// Wait for parent to generate input values
while (shared_ints[3] == 0 || shared_ints[3] == 2)
{
printf("Child: %d\n", shared_ints[3]);
usleep(500000);
}
if (shared_ints[3] != 1)
{
printf("Child: exiting\n");
return;
}
// Calculate the sum
shared_ints[2] = shared_ints[0] + shared_ints[1];
printf("Child: calculated %d + %d = %d\n", shared_ints[0], shared_ints[1], shared_ints[2]);
// Tell parent sum has been calculated
shared_ints[3] = 2;
}
}
static void be_parental(int *shared_ints)
{
while (1)
{
int x;
int y;
// Ask user for first input
printf("Enter your first number:");
if (scanf("%d", &x) != 1)
{
printf("Parent: exiting\n");
shared_ints[3] = -1; /* Tell child to exit */
return;
}
printf("\n");
// Ask user for second input
printf("Enter your second number:");
if (scanf("%d", &y) != 1)
{
printf("Parent: exiting\n");
shared_ints[3] = -1; /* Tell child to exit */
return;
}
printf("\n");
// Assign first value to shared memory
shared_ints[0] = x;
// Assign second value to shared memory
shared_ints[1] = y;
// Tell child that it can compute the sum now
shared_ints[3] = 1;
// Trap parent in a while-loop while the child
// computes the sum
while (shared_ints[3] == 1)
{
printf("Parent: %d\n", shared_ints[3]);
usleep(500000);
}
// Child has completed calculating the sum and
// the parent can print the sum
if (shared_ints[3] == 2)
{
printf("The sum is: %d", shared_ints[2]);
printf("\n");
}
else
{
printf("Parent: unexpected control %d - exiting\n", shared_ints[2]);
shared_ints[3] = -1; /* Tell child to exit */
return;
}
}
}
此代码在busy-wait循环中进行调试，每个读取周期延迟0.5秒。您可以调整一下以适合自己，并在确定它为您工作后输掉输出。请注意，除非您指定要创建/销毁的备用名称，否则代码会销毁文件fidget。它不会在文件退出时删除它;它可能应该。
$ ./dualproc
Enter your first number:Child: 0
1
Enter your second number:Child: 0
2
Parent: 1
Child: calculated 1 + 2 = 3
Child: 2
The sum is: 3
Enter your first number:Child: 2
Child: 2
3
Enter your second number:Child: 2
4
Parent: 1
Child: calculated 3 + 4 = 7
Child: 2
The sum is: 7
Enter your first number:Child: 2
q
Parent: exiting
$ Child: exiting
$
父母在孩子出生前退出;那里没什么大惊喜。如果您愿意，可以升级代码以使用wait()等待孩子退出。
答案 1 :(得分：1)
注意：在父案件退出且孩子仍在等待父母输入值的极端情况下出现问题 - 如果用户在shared_ints [3]！= 0
时中断了程序，则会发生这种情况为了展示@Jonathan Leffler的优秀答案，我的建议是：
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.3.15/angular.min.js"></script>
<div ng-app="myApp">
<div ng-controller='MyController'>
<div ng-repeat="people in peoples">
{{people.name}}
<button ng-click="addOne(people)" class="button">Try it again</button>
{{people.likes}}
</div>
</div>答案 2 :(得分：0)
在big while循环的每次迭代中，父进程和子进程在此行中调用fork()：
if((pid = fork()) < 0) {
每次迭代后，您都会反复生成更多子进程，并且可能会创建意外的交互。