c - 在c中制作共享数据结构

Question

我在我的 C 程序中创建了一个数据结构，如下所示，

typedef struct {
  int *array;
  size_t used;
  size_t size;
} Array;

void initArray(Array *a, size_t initialSize) {
  a->array = (int *)malloc(initialSize * sizeof(int));
  a->used = 0;
  a->size = initialSize;
}

void insertArray(Array *a, int element) {
  if (a->used == a->size) {
    a->size *= 2;
    a->array = (int *)realloc(a->array, a->size * sizeof(int));
  }
  a->array[a->used++] = element;
}

void freeArray(Array *a) {
  free(a->array);
  a->array = NULL;
  a->used = a->size = 0;
}

然后我使用以下方法从外部文本文件向该数据结构添加一些数据，

Array read_ints (const char* file_name)
{
  Array numbers;
  initArray(&numbers,5);

  FILE* file = fopen (file_name, "r");
  int i = 0;
  int count = 0;

  fscanf (file, "%d,", &i);  
  insertArray(&numbers,i);  
  while (!feof (file))
    {  
      //printf ("%d ", i);
      fscanf (file, "%d,", &i);
      insertArray(&numbers,i);      
    }
  fclose (file);
  return numbers;        
}

现在我需要做的是，我需要使“Array”数据结构成为共享内存部分，以便我程序中的子进程和父进程都可以访问该数据结构。我不知道如何使它成为共享内存。我知道 shmget() 系统调用可用于在 UNIX 环境中获取共享内存。但我看不到如何在这种情况下使用该系统调用。请帮助我。

Answer 1

主要代码——shm-master.c

#include "posixver.h"
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <unistd.h>
#include "so-stderr.h"

enum { DEFAULT_SHM_SIZE = 65536 };
enum { DEFAULT_FTOK_ID = 0 };
static const char default_filename[] = "/etc/passwd";

static const char usestr[] = "[-adx][-f file][-s size][-i id]";
static const char optstr[] = "adf:s:x";

int main(int argc, char **argv)
{
    int aflag = 0;
    int xflag = 0;
    int dflag = 0;
    int id = DEFAULT_FTOK_ID;
    size_t size = DEFAULT_SHM_SIZE;
    const char *file = default_filename;
    int opt;

    err_setarg0(argv[0]);
    while ((opt = getopt(argc, argv, optstr)) != -1)
    {
        switch (opt)
        {
        case 'a':
            aflag = 1;
            break;
        case 'd':
            dflag = 1;
            break;
        case 'f':
            file = optarg;
            break;
        case 'i':
            id = atoi(optarg);
            break;
        case 's':
            size = strtoull(optarg, 0, 0);
            if (size == 0)
                err_error("Invalid size (%s) evaluates to zero\n", optarg);
            break;
        case 'x':
            xflag = 1;
            break;
        default:
            err_usage(usestr);
        }
    }

    if (aflag + dflag + xflag > 1)
        err_error("%d of 3 mutually exclusive options -a, -d and -x specified\n", aflag + dflag + xflag);

    printf("ID: %d, File: %s\n", id, file);
    key_t key = ftok(file, id);
    printf("Key: 0x%.8" PRIX64 "\n", (uint64_t)key);
    int shmflg = S_IRUSR | S_IWUSR;
    if (!aflag && !dflag)
        shmflg |= IPC_CREAT;
    if (xflag)
        shmflg |= IPC_EXCL;

    int shmid = shmget(key, size, shmflg);
    if (shmid < 0)
        err_syserr("Failed to get shared memory ID: ");
    printf("ShmID: %d\n", shmid);

    if (dflag)
    {
        struct shmid_ds buf;
        int rc = shmctl(shmid, IPC_RMID, &buf);
        if (rc < 0)
            err_syserr("Failed to delete shared memory: ");
        printf("Shared memory removed\n");
    }
    else
    {
        void *space = shmat(shmid, 0, 0);
        if (space == (void *)-1)
            err_syserr("Failed to attach to shared memory: ");
        printf("Shared memory allocated at 0x%" PRIXPTR "\n", (uintptr_t)space);
        memset(space, '\0', size);
        int rc = shmdt(space);
        if (rc != 0)
            err_syserr("Failed to detach from shared memory: ");
        printf("Detached from shared memory\n");
    }

    return 0;
}

库代码——so-stderr.h

#ifndef SO_STDERR_H_INCLUDED
#define SO_STDERR_H_INCLUDED

extern void err_setarg0(const char *arg0);
extern void err_error(const char *fmt, ...);
extern void err_syserr(const char *fmt, ...);

#endif /* SO_STDERR_H_INCLUDED */

库代码——so-stderr.c

#include "so-stderr.h"
#include <errno.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static const char *argv0 = "**undefined**";

void err_setarg0(const char *arg0)
{
  argv0 = arg0;
}

void err_error(const char *fmt, ...)
{
  fprintf(stderr, "%s: ", argv0);
  va_list args;
  va_start(args, fmt);
  vfprintf(stderr, fmt, args);
  va_end(args);
  exit(EXIT_FAILURE);
}

void err_syserr(const char *fmt, ...)
{
  int errnum = errno;
  fprintf(stderr, "%s: ", argv0);
  va_list args;
  va_start(args, fmt);
  vfprintf(stderr, fmt, args);
  va_end(args);
  if (errnum != 0)
    fprintf(stderr, "(%d: %s)", errnum, strerror(errnum));
  putc('\n', stderr);
  exit(EXIT_FAILURE);
}

配置头文件——posixver.h

您可以在许多系统上将更高的版本号调整为 700(对于 POSIX 2008/2013)，但在 Mac OS X 上这可能不是一个好主意，即使是 10.10.3 Yosemite。

#ifndef JLSS_ID_POSIXVER_H
#define JLSS_ID_POSIXVER_H

#if !defined(_XOPEN_SOURCE) && !defined(_POSIX_C_SOURCE)
#if __STDC_VERSION__ >= 199901L
#define _XOPEN_SOURCE 600
#else
#define _XOPEN_SOURCE 500
#endif

#endif

在所有情况下，“真实”代码都包含(一些)注释和对正在发生的事情的其他解释。生产 stderr.h 和 stderr.c 比显示的最小版本更复杂，但对于许多用途，显示的内容等同于生产版本。

示例运行

$ ./shm-master -H
./shm-master: invalid option -- 'H'
Usage: ./shm-master [-adx][-f file][-s size][-i id]
$ ./shm-master -ax
./shm-master: 2 of 3 mutually exclusive options -a, -d and -x specified
$ ./shm-master -dx
./shm-master: 2 of 3 mutually exclusive options -a, -d and -x specified
$ ./shm-master -da
./shm-master: 2 of 3 mutually exclusive options -a, -d and -x specified
$ ./shm-master -dax
./shm-master: 3 of 3 mutually exclusive options -a, -d and -x specified
$ ipcs -m | grep -v '^0x00000000 '

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x620010f7 0          root       660        557920     4
0x63002725 32769      root       666        82164      3

$ ./shm-master -x
ID: 0, File: /etc/passwd
Key: 0x0000009F
ShmID: 44793901
Shared memory allocated at 0x7F29AC43A000
Detached from shared memory
$ ipcs -m | grep -v '^0x00000000 '

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x620010f7 0          root       660        557920     4
0x63002725 32769      root       666        82164      3
0x0000009f 44793901   jleffler   600        65536      0

$ ./shm-master -d
ID: 0, File: /etc/passwd
Key: 0x0000009F
ShmID: 44793901
Shared memory removed
$ ipcs -m
$ grep -v '^0x00000000 '

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x620010f7 0          root       660        557920     4
0x63002725 32769      root       666        82164      3

$ ./shm-master -f /home/jleffler/soq/shm-master -a
./shm-master: Failed to get shared memory ID: (2: No such file or directory)
ID: 0, File: /home/jleffler/soq/shm-master
Key: 0x00010FB9
$ ./shm-master -f /home/jleffler/soq/shm-master -d
./shm-master: Failed to get shared memory ID: (2: No such file or directory)
ID: 0, File: /home/jleffler/soq/shm-master
Key: 0x00010FB9
$ ./shm-master -f /home/jleffler/soq/shm-master -x
ID: 0, File: /home/jleffler/soq/shm-master
Key: 0x00010FB9
ShmID: 44826669
Shared memory allocated at 0x7FA1488CA000
Detached from shared memory
$ ./shm-master -f /home/jleffler/soq/shm-master -d
ID: 0, File: /home/jleffler/soq/shm-master
Key: 0x00010FB9
ShmID: 44826669
Shared memory removed
$ ./shm-master -f /home/jleffler/soq/shm-master -x
ID: 0, File: /home/jleffler/soq/shm-master
Key: 0x00010FB9
ShmID: 44859437
Shared memory allocated at 0x7F93005EC000
Detached from shared memory
$ shmid=$(./shm-master -f /home/jleffler/soq/shm-master -a sed -n '/ShmID: /s///p')
$ ipcs -m -i $shmid

Shared memory Segment shmid=44859437
uid=199484      gid=5000        cuid=199484     cgid=5000
mode=0600       access_perms=0600
bytes=65536     lpid=31202      cpid=31200      nattch=0
att_time=Fri Apr 17 11:37:06 2015
det_time=Fri Apr 17 11:37:06 2015
change_time=Fri Apr 17 11:37:06 2015

$ ./shm-master -f /home/jleffler/soq/shm-master -d
ID: 0, File: /home/jleffler/soq/shm-master
Key: 0x00010FB9
ShmID: 44859437
Shared memory removed
$

顺便说一句，ipcs 选项-i id 是对 ipcs 的 POSIX 规范的 Linux 扩展。 ，并且该选项在 Mac OS X (BSD) 等平台上不可用。最接近的等效项是 ipcs -m -a | grep "$shmid"，这并不完美。 grep -v '^0x00000000 ' 操作消除了私有(private)共享内存段(在我进行测试的机器上使用了很多共享内存段)。