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。如果我的程序有任何问题,那么某些指针将非常有用。
到目前为止,孩子似乎根本没有增加,而且我不确定为什么会这样。该程序在刚开始时也表现得很奇怪,因为它留在孩子中,然后再交替给父母。
到目前为止的程序结果:
GOT EM - parent: expected 0, got 0
GOT EM - child: expected 1, got 1
child: expected 3, got 1
child: expected 5, got 1
child: expected 7, got 1
child: expected 9, got 1
child: expected 11, got 1
child: expected 13, got 1
child: expected 15, got 1
child: expected 17, got 1
child: expected 19, got 1
child: expected 21, got 1
child: expected 23, got 1
child: expected 25, got 1
child: expected 27, got 2
child: expected 29, got 2
child: expected 31, got 2
child: expected 33, got 2
child: expected 35, got 2
child: expected 37, got 2
GOT EM - parent: expected 2, got 2
child: expected 39, got 2
child: expected 41, got 3
GOT EM - parent: expected 4, got 4
child: expected 43, got 4
GOT EM - parent: expected 6, got 6
child: expected 45, got 6
GOT EM - parent: expected 8, got 8
child: expected 47, got 8
GOT EM - parent: expected 10, got 10
child: expected 49, got 10
我的程序:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.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;
struct shmbuf *shmp;
if ((fd = open("/dev/zero", O_RDWR)) < 0)
perror("open error");
ftruncate(fd, sizeof(struct shmbuf));
if ((shmp = mmap(0, sizeof(struct shmbuf), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED)
perror("mmap error");
close(fd);
sem_init(&shmp->child, 1, 0);
sem_init(&shmp->parent, 1, 1); /* Parent first */
if ((pid = fork()) < 0) {
perror("fork error");
} else if (pid > 0) { /* parent */
for (i = 0; i < NLOOPS; i += 2) {
sem_wait(&shmp->parent);
if ((counter = update((long *)shmp)) != i)
printf(" parent: expected %d, got %d\n", i, counter);
else
printf("GOT EM - parent: expected %d, got %d\n", i, counter);
sem_post(&shmp->child);
}
} else { /* child */
for (i = 1; i < NLOOPS + 1; i += 2) {
sem_wait(&shmp->child);
if ((counter = update((long *)shmp)) != i)
printf(" child: expected %d, got %d\n", i, counter);
else
printf("GOT EM - child: expected %d, got %d\n", i, counter);
sem_post(&shmp->parent);
}
}
exit(0);
}
update((long *)shmp)
该强制转换是错误的,毫无意义,是造成问题的根源。您不能仅将信号量之一的内存用作自己的计数器。您需要将自己的计数器字段添加到struct
中,然后将指针传递给该字段。
以下建议的代码:
ftruncate()
返回的值的警告>>对以下问题进行更正:
关于:
static int update(long *ptr) { return((*ptr)++); /* return value before increment */ }
ptr被声明为指向long,但是返回的类型为int。
关于:
if ((pid = fork()) < 0) perror("fork error");
这是一个'show stopper',因此下一条语句应为:
exit( EXIT_FAILURE );
不要像执行
fork()
一样成功地继续执行程序。
关于:
if ((shmp = mmap(0, sizeof(struct shmbuf), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED) perror("mmap error");
此错误是'show stopper',所以下一条语句应该是:
exit( EXIT_FAILURE );
没有像执行mmap()一样成功地继续执行程序。
关于:
if ((fd = open("/dev/zero", O_RDWR)) < 0) perror("open error");
此错误是'显示停止符',因此,当发生此错误时,下一条语句应为:
exit( EXIT_FAILURE );
不要像执行
open()
一样成功地继续执行程序
现在,建议的代码:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.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( int *ptr)
{
return((*ptr)++); /* return value before increment */
}
int main(void)
{
int fd;
int i;
int counter = 0;
pid_t pid;
struct shmbuf *shmp;
if ((fd = open("/dev/zero", O_RDWR)) < 0)
{
perror("open error");
exit( EXIT_FAILURE );
}
ftruncate(fd, sizeof(struct shmbuf));
if ((shmp = mmap(0, sizeof(struct shmbuf), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED)
{
perror("mmap error");
close(fd);
exit( EXIT_FAILURE );
}
close(fd);
sem_init(&shmp->child, 1, 0);
sem_init(&shmp->parent, 1, 1); /* Parent first */
if ((pid = fork()) < 0)
{
perror("fork error");
munmap( shmp, sizeof( struct shmbuf ) );
exit( EXIT_FAILURE );
}
else if (pid > 0)
{ /* parent */
for (i = 0; i < NLOOPS; i += 2)
{
sem_wait(&shmp->parent);
if ((counter = update( &counter )) != i)
printf(" parent: expected %d, got %d\n", i, counter);
else
printf("GOT EM - parent: expected %d, got %d\n", i, counter);
sem_post(&shmp->child);
}
}
else
{ /* child */
for (i = 1; i < NLOOPS + 1; i += 2)
{
sem_wait(&shmp->child);
if ((counter = update( &counter ) != i))
printf(" child: expected %d, got %d\n", i, counter);
else
printf("GOT EM - child: expected %d, got %d\n", i, counter);
sem_post(&shmp->parent);
}
}
exit(0);
}