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共有メモリを使用してプロセス間でセマフォを共有する方法

N個のクライアントプロセスを1つのサーバーと同期する必要があります。これらのプロセスは、3つのセマフォを宣言したメイン関数によって分岐されます。 POSIXセマフォを使用することにしましたが、これらのプロセス間でそれらを共有する方法がわかりません。共有メモリは正しく動作するはずだと思っていましたが、いくつか質問があります。

  • セグメントに適切なメモリ領域を割り当てるにはどうすればよいですか？
  • 必要なスペースを正確に割り当てるために、shmgetの_size_t_フィールドでsizeof(sem_t)を使用できますか？
  • この状況に似た例はありますか？
csynchronizationclient-serversemaphoreshared
23
Sicioldr

名前付きPOSIXセマフォを簡単に共有できます

  • セマフォの名前を選択してください

    #define SNAME "/mysem"

  • 使用する sem_open with O_CREATそれらを作成するプロセスで

    sem_t *sem = sem_open(SNAME, O_CREAT, 0644, 3); /* Initial value is 3. */

  • 他のプロセスでセマフォを開く

    sem_t *sem = sem_open(SEM_NAME, 0); /* Open a preexisting semaphore. */

共有メモリの使用を主張する場合、それは確かに可能です。

int fd = shm_open("shmname", O_CREAT, O_RDWR);
ftruncate(fd, sizeof(sem_t));
sem_t *sem = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE,
    MAP_SHARED, fd, 0);

sem_init(sem, 1, 1);

私は上記をテストしていないので、それは完全に奇抜になる可能性があります。

48
cnicutar

親（プロデューサー）がN個の子（消費者）スレッドを生成し、グローバル配列を使用してそれらの間でデータを通信するサンプルアプリケーションを作成しました。グローバル配列は循環メモリであり、データが配列に書き込まれると保護されます。

ヘッダーファイル：

#define TRUE 1
#define FALSE 0

#define SUCCESS 0
#define FAILURE -1

/*
 * Function Declaration.
 */
void Parent( void );
int CalcBitmap(unsigned int Num);
void InitSemaphore( void );
void DeInitSemaphore( void );
int ComputeComplexCalc( int op1, int op2);

/*
 * Thread functions.
 */
void *Child( void *arg );
void *Report( void *arg1 );

/*
 * Macro Definition.
 */
#define INPUT_FILE  "./input.txt"
#define NUM_OF_CHILDREN 10
#define SIZE_CIRCULAR_BUF 256
#define BUF_SIZE 128
//Set the bits corresponding to all threads.
#define SET_ALL_BITMAP( a ) a = uiBitmap;     
//Clears the bit corresponding to a thread after
//every read. 
#define CLEAR_BIT( a, b ) a &= ~( 1 << b );   

/*
 * Have a dedicated semaphore for each buffer
 * to synchronize the acess to the shared memory
 * between the producer (parent) and the consumers. 
 */
sem_t ReadCntrMutex;
sem_t semEmptyNode[SIZE_CIRCULAR_BUF];

/*
 * Global Variables.
 */
unsigned int uiBitmap = 0;
//Counter to track the number of processed buffers.
unsigned int Stats;                           
unsigned int uiParentCnt = 0;
char inputfile[BUF_SIZE];
int EndOfFile = FALSE;

/*
 * Data Structure Definition. ( Circular Buffer )
 */
struct Message
{
    int id;
    unsigned int BufNo1;
    unsigned int BufNo2;
    /* Counter to check if all threads read the buffer. */
    unsigned int uiBufReadCntr;
};
struct Message ShdBuf[SIZE_CIRCULAR_BUF];

ソースコード：

    /*
# 
# ipc_communicator is a IPC binary where a parent
# create ten child threads, read the input parameters
# from a file, sends that parameters to all the children.
# Each child computes the operations on those input parameters
# and writes them on to their own file.
#
#  Author: Ashok Vairavan              Date: 8/8/2014
*/

/*
 * Generic Header Files.
 */
#include "stdio.h"
#include "unistd.h"
#include "string.h"
#include "pthread.h"
#include "errno.h"
#include "malloc.h"
#include "fcntl.h"
#include "getopt.h"
#include "stdlib.h"
#include "math.h"
#include "semaphore.h"

/*
 * Private Header Files.
 */
#include "ipc*.h"

/*
 * Function to calculate the bitmap based on the 
 * number of threads. This bitmap is used to 
 * track which thread read the buffer and the
 * pending read threads.
 */
int CalcBitmap(unsigned int Num)
{
   unsigned int uiIndex;

   for( uiIndex = 0; uiIndex < Num; uiIndex++ )
   {
      uiBitmap |= 1 << uiIndex;
   }
   return uiBitmap;
}
/*
 * Function that performs complex computation on
 * numbers.
 */
int ComputeComplexCalc( int op1, int op2)
{
    return sqrt(op1) + log(op2);
}

/*
 * Function to initialise the semaphores. 
 * semEmptyNode indicates if the buffer is available
 * for write. semFilledNode indicates that the buffer
 * is available for read.
 */
void InitSemaphore( void )
{
   unsigned int uiIndex=0;

   CalcBitmap(NUM_OF_CHILDREN);

   sem_init( &ReadCntrMutex, 0, 1);
   while( uiIndex<SIZE_CIRCULAR_BUF )
   {
    sem_init( &semEmptyNode[uiIndex], 0, 1 );
        uiIndex++;
   }
   return;
}

/* 
 * Function to de-initilaise semaphore.
 */
void DeInitSemaphore( void )
{
   unsigned int uiIndex=0;

   sem_destroy( &ReadCntrMutex);
   while( uiIndex<SIZE_CIRCULAR_BUF )
   {
    sem_destroy( &semEmptyNode[uiIndex] );
        uiIndex++;
   }
   return;
}

/* Returns TRUE if the parent had reached end of file */
int isEOF( void )
{
   return __sync_fetch_and_add(&EndOfFile, 0);
}

/*
 * Thread functions that prints the number of buffers
 * processed per second.
 */ 
void *Report( void *arg1 )
{
   int CumStats = 0;

   while( TRUE )
   {
      sleep( 1 );
      CumStats += __sync_fetch_and_sub(&Stats, 0);
      printf("Processed Per Second: %d Cumulative Stats: %d\n", 
               __sync_fetch_and_sub(&Stats, Stats), CumStats);
   }
   return NULL;
}

/*
 * Function that reads the data from the input file
 * fills the shared buffer and signals the children to
 * read the data.
 */
void Parent( void )
{
   unsigned int op1, op2;
   unsigned int uiCnt = 0;

   /* Read the input parameters and process it in
    * while loop till the end of file.
    */

   FILE *fp = fopen( inputfile, "r" );
   while( fscanf( fp, "%d %d", &op1, &op2 ) == 2 )
   {
     /* Wait for the buffer to become empty */
     sem_wait(&semEmptyNode[uiCnt]);

     /* Access the shared buffer */
     ShdBuf[uiCnt].BufNo1 = op1;
     ShdBuf[uiCnt].BufNo2 = op2;

     /* Bitmap to track which thread read the buffer */
     sem_wait( &ReadCntrMutex );
     SET_ALL_BITMAP( ShdBuf[uiCnt].uiBufReadCntr );
     sem_post( &ReadCntrMutex );

     __sync_fetch_and_add( &uiParentCnt, 1);
     uiCnt = uiParentCnt % SIZE_CIRCULAR_BUF;
   }

   /* If it is end of file, indicate it to other
    * children using global variable.
    */ 
   if( feof( fp ) )
   {
      __sync_add_and_fetch(&EndOfFile, TRUE);
      fclose( fp ); 
      return;
   }

   return;
}

/* 
 * Child thread function which takes threadID as an
 * argument, creates a file using threadID, fetches the
 * parameters from the shared memory, computes the calculation,
 * writes the output to their own file and will release the 
 * semaphore when all the threads read the buffer.
 */
void *Child( void *ThreadPtr )
{
   unsigned int uiCnt = 0, uiTotalCnt = 0;
   unsigned int op1, op2;
   char filename[BUF_SIZE];
   int ThreadID;

   ThreadID = *((int *) ThreadPtr);
   free( ThreadPtr );

   snprintf( filename, BUF_SIZE, "Child%d.txt", ThreadID );

   FILE *fp = fopen( filename, "w" );
   if( fp == NULL )
   {
      perror( "fopen" );
      return NULL;
   }

   while (TRUE)
   {
      /* Wait till the parent fills the buffer */
      if( __sync_fetch_and_add( &uiParentCnt, 0 ) > uiTotalCnt )
      {
         /* Access the shared memory */
         op1 = ShdBuf[uiCnt].BufNo1;
         op2 = ShdBuf[uiCnt].BufNo2;

         fprintf(fp, "%d %d = %d\n", op1, op2, 
                               ComputeComplexCalc( op1, op2) ); 

         sem_wait( &ReadCntrMutex );
         ShdBuf[uiCnt].uiBufReadCntr &= ~( 1 << ThreadID );
         if( ShdBuf[uiCnt].uiBufReadCntr == 0 )
         {
            __sync_add_and_fetch(&Stats, 1);

            /* Release the semaphore lock if 
               all readers read the data */
            sem_post(&semEmptyNode[uiCnt]);
         }
         sem_post( &ReadCntrMutex );

         uiTotalCnt++;
         uiCnt = uiTotalCnt % SIZE_CIRCULAR_BUF;

         /* If the parent reaches the end of file and 
            if the child thread read all the data then break out */
         if( isEOF() && ( uiTotalCnt ==  uiParentCnt ) )
         {
            //printf("Thid %d p %d c %d\n", ThreadID, uiParentCnt, uiCnt );
            break;
         }
      }
      else
      {
         /* Sleep for ten micro seconds before checking 
            the shared memory */
         usleep(10);
      }
   }
   fclose( fp );
   return NULL;
}

void usage( void )
{
    printf(" Usage:\n");
    printf("         -f - the absolute path of the input file where the input parameters are read from.\n\t\t Default input file: ./input.txt");
    printf("         -?  - Help Menu. \n" );
    exit(1);
}

int main( int argc, char *argv[])
{
   pthread_attr_t ThrAttr;
   pthread_t ThrChild[NUM_OF_CHILDREN], ThrReport;
   unsigned int uiThdIndex = 0;
   unsigned int *pThreadID;
   int c;

   strncpy( inputfile, INPUT_FILE, BUF_SIZE );

   while( ( c = getopt( argc, argv, "f:" )) != -1 )
   {
       switch( c )
       {
           case 'f':
                strncpy( inputfile, optarg, BUF_SIZE );
                break;

           default:
                usage();
        }
   }

   if( access( inputfile, F_OK ) == -1 )
   {
      perror( "access" );
      return FAILURE;
   } 

   InitSemaphore();

   pthread_attr_init( &ThrAttr );
   while( uiThdIndex< NUM_OF_CHILDREN )
   {
      /* Allocate the memory from the heap and pass it as an argument
       * to each thread to avoid race condition and invalid reference
       * issues with the local variables.
       */
      pThreadID = (unsigned int *) malloc( sizeof( unsigned int ) );
      if( pThreadID == NULL )
      {
     perror( "malloc" );
     /* Cancel pthread operation */
     return FAILURE;
      }
      *pThreadID = uiThdIndex;
      if( pthread_create( 
             &ThrChild[uiThdIndex], NULL, &Child, pThreadID) != 0 )
      {
    printf( "pthread %d creation failed. Error: %d\n", uiThdIndex, errno);
    perror( "pthread_create" );
    return FAILURE;
      }
      uiThdIndex++;
   }

   /* Report thread reports the statistics of IPC communication
    * between parent and childs threads.
    */
   if( pthread_create( &ThrReport, NULL, &Report, NULL) != 0 )
   {
    perror( "pthread_create" );
    return FAILURE;
   }

   Parent();

   uiThdIndex = 0;
   while( uiThdIndex < NUM_OF_CHILDREN )
   {
      pthread_join( ThrChild[uiThdIndex], NULL );
      uiThdIndex++;
   }

   /*
    * Cancel the report thread function when all the 
    * children exits.
    */
   pthread_cancel( ThrReport );

   DeInitSemaphore();

   return SUCCESS;
}
4
Ashok Vairavan

共有メモリのアプローチがうまくいくかどうかは本当に疑わしい。

知る限り、セマフォは実際には単なるハンドルであり、開いているプロセスに対して有効です。実際のセマフォ情報はカーネルメモリにあります。

そのため、他のプロセスで同じハンドル値を使用しても機能しません。

1
Ketan

はい、2つのプロセス間で名前付きセマフォを使用できます。ファイルを開くことができます。

snprintf(sem_open_file, 128, "/sem_16");
ret_value = sem_open((const char *)sem_open_file, 0, 0, 0);

if (ret_value == SEM_FAILED) {
    /*
     * No such file or directory
     */
    if (errno == ENOENT) {
        ret_value = sem_open((const char *)sem_open_file,
                           O_CREAT | O_EXCL, 0777, 1);
     }
}

そして、他のプロセスがこれを使用できます。

1
rahul