undefided reference to 'sem_init' 等信号量问题

解决办法：在编译时加上 -pthread,

eg: gcc -pthread sem.c -o sem

#include<stdio.h>

#include<errno.h>

#include<semaphore.h>

static int shared = 0;

static sem_t sharedsem;

int initshared(int val)

{

if(sem_init(&sharedsem,0,1) == -1)

return -1;

shared = val;

return 0;

}

int getshared(int *sval)

{

while(sem_wait(&sharedsem) == -1)

if(errno != EINTR)

return -1;

*sval = shared;

return sem_post(&sharedsem);

}

int incshared()

{

while(sem_wait(&sharedsem) == -1)

if(errno != EINTR)

return -1;

shared++;

return sem_post(&sharedsem);

}

int main()

{

int sval;

initshared(1);

getshared(&sval);

printf("sval is %d\n",sval);

incshared();

printf("shared is %d\n",shared);

return 0;

}