Postgresql源码（91）POSIX匿名信号量初始化与使用流程总结

struct PGPROC
{
	...
	PGSemaphore sem;
	...         |     -------- 是否共享，如果是需要在共享内存中获取newsem
};              |    |
                v    v  v----------- 初始值：1
if (sem_init(newsem, 1, 1) < 0)
...

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <semaphore.h>

#define PG_CACHE_LINE_SIZE 128
#define PG_SEM_REF(x) (&(x)->sem_padded.pgsem)

typedef union SemTPadded
{
	sem_t pgsem;
	char pad[PG_CACHE_LINE_SIZE];
} SemTPadded;

typedef struct PGSemaphoreData
{
	SemTPadded sem_padded;
} PGSemaphoreData;

typedef struct PGSemaphoreData *PGSemaphore;
static PGSemaphore sharedSemas;

int main(int argc, char *argv[])
{
	sem_t *newsem;
	int errStatus;

	sharedSemas = mmap(NULL, sizeof(PGSemaphoreData), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
	if (sharedSemas == MAP_FAILED)
	{
		fprintf(stderr, "mmap() failed\n");
		exit(EXIT_FAILURE);
	}

	newsem = PG_SEM_REF(sharedSemas);
	if (sem_init(newsem, 1, 1) < 0)
	{
		fprintf(stderr, "sem_init() failed\n");
		exit(EXIT_FAILURE);
	}
	sem_wait(newsem);

	/*Parent and child share mapping*/
	switch (fork())
	{
	case -1:
		fprintf(stderr, "fork() failed\n");
		exit(EXIT_FAILURE);

	case 0:
		printf("[child]mmap address %p\n", sharedSemas);
		do
		{
			printf("[child]sem_post...\n");
			errStatus = sem_post(newsem);
		} while (errStatus < 0 && errno == EINTR);
		printf("[child]exit\n");
		exit(EXIT_SUCCESS);

	default:
		printf("[parent]mmap address %p\n", sharedSemas);
		do
		{
			printf("[parent]start semi_wait...\n");
			errStatus = sem_wait(newsem);
		} while (errStatus < 0 && errno == EINTR);
		printf("[parent]stop semi_wait...\n");
		if (wait(NULL) == -1)
		{
			fprintf(stderr, "wait() failed\n");
			exit(EXIT_FAILURE);
		}
		printf("[parent]stop waiting child exit\n");

		if (munmap(sharedSemas, sizeof(int)) == -1)
		{
			fprintf(stderr, "munmap() failed\n");
			exit(EXIT_FAILURE);
		}
		printf("[parent]exit\n");
		exit(EXIT_SUCCESS);
	}
}

// gcc -o main1 -Wall -g -ggdb -O0 -g3 -gdwarf-2 main1.c -lpthread

[parent]mmap address 0x7f69797d4000
[parent]start semi_wait...
[child]mmap address 0x7f69797d4000
[child]sem_post...
[child]exit
[parent]stop semi_wait...
[parent]stop waiting child exit
[parent]exit