我需要幫助來實現信號量

Question

我有一個想法，但我不知道我該怎么做，從一個帶有 fork () 的父進程我創建了 N 個子進程。 為了讓子進程一次執行某個操作，我想使用一系列信號量，這些信號量設置為 0，並且只有與必須執行該操作的子進程對應的信號量為 1。我該怎么辦？ 我在c89工作。

Answer 1

我為你創建了一個信號量系統。 如果您需要任何澄清，我將很樂意回答。

這是代碼：

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <string.h>

void fatherHandler();             //Used for acquiring the children termination statuses by the father
void resourceRequestedHandler(int sig, siginfo_t *info, void *ucontext);  //Used by the father to tell any children which requested it the status of the resource
void resourceReleasedHandler();   //Used by the father to set the resource status to free when a child has finished its job
void resourceAvailable();         //Used internally by a child which requested the resource status and that found it available
void resourceUnavailable();         //Used internally by a child which requested the resource status and that found it unavailable

int n;                    //Number of children
int status;               //The child view of the semaphore status
int semaphore;            //The resource's semaphore, tells us if we can access the resource or not
int terminatedChildren;   //Used by the father to know how many children has finished their job

int main(int argc, char * argv[]) {
  int i;          //for-loop index
  int pid;        //The pid of the nth child
  struct sigaction sa;

  i = 0;
  n = 3;                 //For example purposes only
  terminatedChildren = 0;

  semaphore = 0;          //The resource is not available before setup is finished

  /* Use the sa_sigaction field because the handles has two additional parameters */
  sa.sa_sigaction = &resourceRequestedHandler;

  /* The SA_SIGINFO flag tells sigaction() to use the sa_sigaction field, not       sa_handler. */
  sa.sa_flags = SA_SIGINFO;

  sigaction(SIGUSR1, &sa, NULL);                         //Used by a child to acquire the resource -> semaphore = 0
  signal(SIGUSR2, resourceReleasedHandler);              //Used by a child to release the resource -> semaphore = 1
  signal(SIGCHLD, fatherHandler);                        //Used to acquire the children's termination state

  /* N children are created */
  while(i < n) {
    pid = fork();
    if (pid == 0)
      i = n;
    else
      i++;
  }

  if(pid > 0) {         //Father's code
    semaphore = 1;      //The resource becomes available
  } else if(pid == 0) { //Child's code
    status = 0;         //The semaphore status -> not available by default
    
    signal(SIGCHLD, SIG_DFL);
    signal(SIGUSR1, resourceAvailable);
    signal(SIGUSR2, resourceUnavailable);

    while(!status) { //While the resource isn't available
      printf("Child %d -> waiting\n", getpid());  //Demo purposes only
      kill(getppid(), SIGUSR1);                   //The child asks the father wheter the resource has become available or not
      sleep(2);                                   //Demo purposes only
    }

  /*
   * Here you should put the code you want every child to execute
   * when they have access to the resource.
   * I just do a printf();
   */
   printf("Child %d did what it had to do !\n", getpid());
   kill(getppid(), SIGUSR2);                //The child tells the father the resource is now available again

   sleep(2);                //Demo purposes only
   exit(1);                 //Child gets terminated
  } else if (pid < 0) {       //Fork failure
    perror("Fork error:");
    exit(-1);
  }

  for(;;);      //The father just waits for its children
  exit(0);
}

void resourceRequestedHandler(int sig, siginfo_t *siginfo, void *context) {
  char outputMessage[200];                //Demo purposes only
  int signalNumber;                    //The number of the signal we'll send to the child: SIGUSR1 if semaphore = 1, SIGUSR2 otherwise
  int childPid = siginfo->si_pid;      //We understand which child requested the semaphore status

  signalNumber = (semaphore == 1) ? SIGUSR1 : SIGUSR2;
  if(semaphore == 1) semaphore = 0;      //If the resource was available and it's been requested it becomes unavailable: semaphore gets set to 0

  sprintf(outputMessage, "Resource requested by %d, responded with resource status %d\n", childPid, signalNumber);   //Demo purposes only
  write(STDOUT_FILENO, outputMessage, strlen(outputMessage));    //Demo purposes only

  kill(childPid, signalNumber);          //We tell the child whether the resource is available or not
}

void resourceReleasedHandler() {
    semaphore = 1;          //If the resource is released the semaphore gets set to 1
}

void fatherHandler() {
  char outputMessage[200];
  int childTerminationStatus;
  int childPid;

  childPid = wait(&childTerminationStatus);                       //The father waits for the childrens' termination states

  sprintf(outputMessage, "Terminated child %d with status %d\n", childPid, childTerminationStatus);
  write(STDOUT_FILENO, outputMessage, strlen(outputMessage));

  terminatedChildren++;
  if(terminatedChildren == n)
   exit(0);                  //All children terminated
}

void resourceAvailable() {
  char outputMessage[200];                                              //Demo purposes only
  sprintf(outputMessage, "Child %d -> resource Available\n", getpid()); //Demo purposes only
  write(STDOUT_FILENO, outputMessage, strlen(outputMessage));           //Demo purposes only

  status = 1;
}

void resourceUnavailable() {
  char outputMessage[200];                                              //Demo purposes only 
  sprintf(outputMessage, "Child %d -> resource Unavailable\n", getpid()); //Demo purposes only
  write(STDOUT_FILENO, outputMessage, strlen(outputMessage));           //Demo purposes only

  status = 0;
}

注意：代碼中的任何sleep()都存在，因為 linux 在短時間內處理多個信號的方式。 如果您不需要打印任何內容，則可以忽略它們。

如果您想快速測試解決方案，這是一個在線 C 編譯器，只需將代碼粘貼到那里並點擊“運行”： https://repl.it/languages/c

這是一個 output 示例：

這可能是一個更優雅的解決方案： https://www.geeksforgeeks.org/use-posix-semaphores-c/