引用'void *'指针

Question

我正在尝试创建一个C程序，其中使用POSIX信号量函数通过在父级和子级之间交替来增加计数器的数量。到目前为止，考虑到这是我第一次使用信号量函数，因此我在使用它时遇到了麻烦，因此，如果我的程序有任何问题，一些指针也将非常有用。

到目前为止，我当前的问题是使用解引用void *指针，由于Linux程序员的手册，mmap是空指针，所以我不确定如何继续。

gcc -ansi -I -Wl -Wall -DLINUX -D_GNU_SOURCE    devzeroMyEdit.c  -L /lib -pthread -lrt -o devzeroMyEdit
devzeroMyEdit.c: In function ‘main’:
devzeroMyEdit.c:37:16: warning: dereferencing ‘void *’ pointer
  sem_init(&shmp->parent, 1, 0);
                ^~
devzeroMyEdit.c:37:16: error: request for member ‘parent’ in something not a structure or union
devzeroMyEdit.c:38:16: warning: dereferencing ‘void *’ pointer
  sem_init(&shmp->child, 1, 1);        /* Child first */
                ^~
devzeroMyEdit.c:38:16: error: request for member ‘child’ in something not a structure or union
devzeroMyEdit.c:40:13: warning: implicit declaration of function ‘fork’ [-Wimplicit-function-declaration]
  if ((pid = fork()) < 0) {
             ^~~~
devzeroMyEdit.c:45:18: warning: dereferencing ‘void *’ pointer
    sem_wait(&shmp->child);
                  ^~
devzeroMyEdit.c:45:18: error: request for member ‘child’ in something not a structure or union
devzeroMyEdit.c:50:18: warning: dereferencing ‘void *’ pointer
    sem_post(&shmp->parent);
                  ^~
devzeroMyEdit.c:50:18: error: request for member ‘parent’ in something not a structure or union
devzeroMyEdit.c:56:18: warning: dereferencing ‘void *’ pointer
    sem_wait(&shmp->parent);
                  ^~
devzeroMyEdit.c:56:18: error: request for member ‘parent’ in something not a structure or union
devzeroMyEdit.c:61:18: warning: dereferencing ‘void *’ pointer
    sem_post(&shmp->child);
                  ^~
devzeroMyEdit.c:61:18: error: request for member ‘child’ in something not a structure or union
<builtin>: recipe for target 'devzeroMyEdit' failed
make: *** [devzeroMyEdit] Error 1

我的程序：

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <semaphore.h>

#define NLOOPS  1000
#define SIZE    sizeof(long)    /* size of shared memory area */

struct shmbuf{
    sem_t child;
    sem_t parent;
};

static int update(long *ptr)
{
    return((*ptr)++);       /* return value before increment */
}

int main(void)
{
    int     fd, i, counter;
    pid_t   pid;
    void    *shmp;

    if ((fd = shm_open("/dev/zero", O_RDWR, ALLPERMS)) < 0)
        perror("open error");

    if ((shmp = mmap(0, sizeof(struct shmbuf), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED)
        perror("mmap error");

    sem_init(&shmp->parent, 1, 0);
    sem_init(&shmp->child, 1, 1);        /* Child first */

    if ((pid = fork()) < 0) {
        perror("fork error");
    } else if (pid > 0) {                /* Parent */
        for (i = 0; i < NLOOPS; i += 2) {

            sem_wait(&shmp->child);

            if ((counter = update((long *)shmp)) != i)
                printf("parent: expected %d, got %d", i, counter);

            sem_post(&shmp->parent);
        }
    } else {                             /* Child */
        for (i = 1; i < NLOOPS + 1; i += 2) {

            sem_wait(&shmp->parent);

            if ((counter = update((long *)shmp)) != i)
                printf("child: expected %d, got %d", i, counter);

            sem_post(&shmp->child);
        }
    }

    return(0);
}

Answer 1

shmp的类型显然应该是struct shmbuf*，而不是void*。取消引用空指针没有任何意义。

[shmp = mmap(0, ...)是合法的，即使shmp的类型不是void*，因为将空指针安全地提升为任何其他指针类型。

所以只需将void *shmp更改为struct shmbuf *shmp。

奖励：您也忘记了#include <unistd.h>。