Question

I am getting a segmentation fault while running this code. I can’t work out why this is happening – can anyone see a possible reason? (I have already got and initialized the semaphore’s shared memory.)

My code:

   #include<stdlib.h>    #include<sys/types.h>    #include<sys/shm.h>    #include<sys/ipc.h>    #include<stdio.h>    #include<sys/sem.h>     union semun     {    int val;    struct semid_ds *buf;    unsigned short *array;    } arg;      int main()   {   key_t semkey;   int shmid,semid,nsem,sops;   struct sembuf buf[1];    char *ptrr,*shm,c,*s;    semkey=ftok("/home/mawia/abc.c",'a');    printf("entered into main of producer\n");   if(semkey<0)   {    perror("ftok");    exit(1);   }    shmid=shmget(semkey,30,0777);    if(shmid<0)   {    printf("error");    perror("shmget");    exit(1);   }    shm=shmat(shmid,0,0);   if(shm==(char *) -1)   {   perror("shm");   exit(1);   }     s=shm;   semid=semget(semkey,1,0777);  if(semid<0)  {   printf("error");   perror("semget");   exit(0);  }   ptrr=shm+1;  *s='w';  printf("going to check the value 0th semaphores\n");  buf[0].sem_num=0;  buf[0].sem_op=0;   buf[0].sem_flg=0;  buf[1].sem_num=0;  buf[1].sem_op=1;  buf[1].sem_flg=0;   printf("entered the critical region\n");  //printf("waiting to enter the buffer zone...");  semop(semid,buf,2);   printf("entered the critical region\v");  if(*s!='r')  {   printf("\nPRODUCER IS PRODUCING\n\n\n");    printf("ENTER DATA\n");    while((c=getchar())!='\n')   {     *ptrr++=c;   }   *ptrr='\0';   *s='r';  }     else    printf("RESOURCE IS FULL:CAN'T PRODUCE");   //printf("produced enough for  the consumer \nexiting from the buffer area now...");  buf[0].sem_num=0;  buf[0].sem_op=-1;  buf[0].sem_flg=0;   semop(semid,buf,1);   ptrr=shm+1;    if(!strcmp(ptrr,"exit"))   {   printf("exiting...");   exit(0);   }   sleep(1);   return 0;   } 

Answer 1

After a quick glance (very quick), i would say that it MAY be caused by

struct sembuf buf[1];  /*some other code*/  buf[1].sem_num=0; buf[1].sem_op=1; buf[1].sem_flg=0; 

You are accessing memory outside of the buffer. buf[1] reserves memory in the stack for only one struct sembuf, you are trying to use 2. In that case, you should use

 struct sembuf buf[2]