Como usar memória compartilhada com Linux em C

Eu tenho um pequeno problema com um dos meus projetos.

Tenho tentado encontrar um exemplo bem documentado de uso de memória compartilhada, fork()mas sem sucesso.

Basicamente, o cenário é que quando o usuário inicia o programa, eu preciso armazenar dois valores na memória compartilhada: current_path que é um char * e um file_name que também é char * .

Dependendo dos argumentos do comando, um novo processo é iniciado fork()e esse processo precisa ler e modificar a variável current_path armazenada na memória compartilhada enquanto a variável file_name é somente leitura.

Existe um bom tutorial sobre memória compartilhada com código de exemplo (se possível) que você pode me indicar?

Existem duas abordagens: shmgete mmap. Vou falar sobre mmap, já que é mais moderno e flexível, mas você pode dar uma olhada man shmget( ou neste tutorial ) se preferir usar as ferramentas de estilo antigo.

A mmap()função pode ser usada para alocar buffers de memória com parâmetros altamente personalizáveis ​​para controlar o acesso e as permissões, e para apoiá-los com o armazenamento do sistema de arquivos, se necessário.

A função a seguir cria um buffer na memória que um processo pode compartilhar com seus filhos:

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

void* create_shared_memory(size_t size) {
  // Our memory buffer will be readable and writable:
  int protection = PROT_READ | PROT_WRITE;

  // The buffer will be shared (meaning other processes can access it), but
  // anonymous (meaning third-party processes cannot obtain an address for it),
  // so only this process and its children will be able to use it:
  int visibility = MAP_SHARED | MAP_ANONYMOUS;

  // The remaining parameters to `mmap()` are not important for this use case,
  // but the manpage for `mmap` explains their purpose.
  return mmap(NULL, size, protection, visibility, -1, 0);
}

A seguir está um programa de exemplo que usa a função definida acima para alocar um buffer. O processo pai escreverá uma mensagem, bifurcará e então aguardará que seu filho modifique o buffer. Ambos os processos podem ler e gravar na memória compartilhada.

#include <string.h>
#include <unistd.h>

int main() {
  char parent_message[] = "hello";  // parent process will write this message
  char child_message[] = "goodbye"; // child process will then write this one

  void* shmem = create_shared_memory(128);

  memcpy(shmem, parent_message, sizeof(parent_message));

  int pid = fork();

  if (pid == 0) {
    printf("Child read: %s\n", shmem);
    memcpy(shmem, child_message, sizeof(child_message));
    printf("Child wrote: %s\n", shmem);

  } else {
    printf("Parent read: %s\n", shmem);
    sleep(1);
    printf("After 1s, parent read: %s\n", shmem);
  }
}

Aqui está um exemplo de memória compartilhada:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

#define SHM_SIZE 1024  /* make it a 1K shared memory segment */

int main(int argc, char *argv[])
{
    key_t key;
    int shmid;
    char *data;
    int mode;

    if (argc > 2) {
        fprintf(stderr, "usage: shmdemo [data_to_write]\n");
        exit(1);
    }

    /* make the key: */
    if ((key = ftok("hello.txt", 'R')) == -1) /*Here the file must exist */ 
{
        perror("ftok");
        exit(1);
    }

    /*  create the segment: */
    if ((shmid = shmget(key, SHM_SIZE, 0644 | IPC_CREAT)) == -1) {
        perror("shmget");
        exit(1);
    }

    /* attach to the segment to get a pointer to it: */
    data = shmat(shmid, NULL, 0);
    if (data == (char *)(-1)) {
        perror("shmat");
        exit(1);
    }

    /* read or modify the segment, based on the command line: */
    if (argc == 2) {
        printf("writing to segment: \"%s\"\n", argv[1]);
        strncpy(data, argv[1], SHM_SIZE);
    } else
        printf("segment contains: \"%s\"\n", data);

    /* detach from the segment: */
    if (shmdt(data) == -1) {
        perror("shmdt");
        exit(1);
    }

    return 0;
}

Passos :

1. Use ftok para converter um nome de caminho e um identificador de projeto em uma chave IPC do System V

2. Use shmget que aloca um segmento de memória compartilhada

3. Use shmat para anexar o segmento de memória compartilhada identificado por shmid ao espaço de endereço do processo de chamada

4. Faça as operações na área de memória

5. Desanexar usando shmdt

Isso inclui o uso de memória compartilhada

#include<sys/ipc.h>
#include<sys/shm.h>

int shmid;
int shmkey = 12222;//u can choose it as your choice

int main()
{
  //now your main starting
  shmid = shmget(shmkey,1024,IPC_CREAT);
  // 1024 = your preferred size for share memory
  // IPC_CREAT  its a flag to create shared memory

  //now attach a memory to this share memory
  char *shmpointer = shmat(shmid,NULL);

  //do your work with the shared memory 
  //read -write will be done with the *shmppointer
  //after your work is done deattach the pointer

  shmdt(&shmpointer, NULL);

experimente este exemplo de código, eu testei, fonte: http://www.makelinux.net/alp/035

#include <stdio.h> 
#include <sys/shm.h> 
#include <sys/stat.h> 

int main () 
{
  int segment_id; 
  char* shared_memory; 
  struct shmid_ds shmbuffer; 
  int segment_size; 
  const int shared_segment_size = 0x6400; 

  /* Allocate a shared memory segment.  */ 
  segment_id = shmget (IPC_PRIVATE, shared_segment_size, 
                 IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR); 
  /* Attach the shared memory segment.  */ 
  shared_memory = (char*) shmat (segment_id, 0, 0); 
  printf ("shared memory attached at address %p\n", shared_memory); 
  /* Determine the segment's size. */ 
  shmctl (segment_id, IPC_STAT, &shmbuffer); 
  segment_size  =               shmbuffer.shm_segsz; 
  printf ("segment size: %d\n", segment_size); 
  /* Write a string to the shared memory segment.  */ 
  sprintf (shared_memory, "Hello, world."); 
  /* Detach the shared memory segment.  */ 
  shmdt (shared_memory); 

  /* Reattach the shared memory segment, at a different address.  */ 
  shared_memory = (char*) shmat (segment_id, (void*) 0x5000000, 0); 
  printf ("shared memory reattached at address %p\n", shared_memory); 
  /* Print out the string from shared memory.  */ 
  printf ("%s\n", shared_memory); 
  /* Detach the shared memory segment.  */ 
  shmdt (shared_memory); 

  /* Deallocate the shared memory segment.  */ 
  shmctl (segment_id, IPC_RMID, 0); 

  return 0; 
} 

Aqui está um exemplo de mmap:

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

/*
 * pvtmMmapAlloc - creates a memory mapped file area.  
 * The return value is a page-aligned memory value, or NULL if there is a failure.
 * Here's the list of arguments:
 * @mmapFileName - the name of the memory mapped file
 * @size - the size of the memory mapped file (should be a multiple of the system page for best performance)
 * @create - determines whether or not the area should be created.
 */
void* pvtmMmapAlloc (char * mmapFileName, size_t size, char create)  
{      
  void * retv = NULL;                                                                                              
  if (create)                                                                                         
  {                                                                                                   
    mode_t origMask = umask(0);                                                                       
    int mmapFd = open(mmapFileName, O_CREAT|O_RDWR, 00666);                                           
    umask(origMask);                                                                                  
    if (mmapFd < 0)                                                                                   
    {                                                                                                 
      perror("open mmapFd failed");                                                                   
      return NULL;                                                                                    
    }                                                                                                 
    if ((ftruncate(mmapFd, size) == 0))               
    {                                                                                                 
      int result = lseek(mmapFd, size - 1, SEEK_SET);               
      if (result == -1)                                                                               
      {                                                                                               
        perror("lseek mmapFd failed");                                                                
        close(mmapFd);                                                                                
        return NULL;                                                                                  
      }                                                                                               

      /* Something needs to be written at the end of the file to                                      
       * have the file actually have the new size.                                                    
       * Just writing an empty string at the current file position will do.                           
       * Note:                                                                                        
       *  - The current position in the file is at the end of the stretched                           
       *    file due to the call to lseek().  
              *  - The current position in the file is at the end of the stretched                    
       *    file due to the call to lseek().                                                          
       *  - An empty string is actually a single '\0' character, so a zero-byte                       
       *    will be written at the last byte of the file.                                             
       */                                                                                             
      result = write(mmapFd, "", 1);                                                                  
      if (result != 1)                                                                                
      {                                                                                               
        perror("write mmapFd failed");                                                                
        close(mmapFd);                                                                                
        return NULL;                                                                                  
      }                                                                                               
      retv  =  mmap(NULL, size,   
                  PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0);                                     

      if (retv == MAP_FAILED || retv == NULL)                                                         
      {                                                                                               
        perror("mmap");                                                                               
        close(mmapFd);                                                                                
        return NULL;                                                                                  
      }                                                                                               
    }                                                                                                 
  }                                                                                                   
  else                                                                                                
  {                                                                                                   
    int mmapFd = open(mmapFileName, O_RDWR, 00666);                                                   
    if (mmapFd < 0)                                                                                   
    {                                                                                                 
      return NULL;                                                                                    
    }                                                                                                 
    int result = lseek(mmapFd, 0, SEEK_END);                                                          
    if (result == -1)                                                                                 
    {                                                                                                 
      perror("lseek mmapFd failed");                  
      close(mmapFd);                                                                                  
      return NULL;                                                                                    
    }                                                                                                 
    if (result == 0)                                                                                  
    {                                                                                                 
      perror("The file has 0 bytes");                           
      close(mmapFd);                                                                                  
      return NULL;                                                                                    
    }                                                                                              
    retv  =  mmap(NULL, size,     
                PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0);                                       

    if (retv == MAP_FAILED || retv == NULL)                                                           
    {                                                                                                 
      perror("mmap");                                                                                 
      close(mmapFd);                                                                                  
      return NULL;                                                                                    
    }                                                                                                 

    close(mmapFd);                                                                                    

  }                                                                                                   
  return retv;                                                                                        
}