CODEWITHSUNDEEP
cserversegmentation-faultclient
Dawid Janas
Dawid Janas

Reputation: 1

Segmention fault

I am trying to run program with shared memory. My code as below.

Producer:

int main() {
    const int SIZE = 4096;

    const char *name="OS";
    const char *message_0="Hello";
    const char *message_1="World";

    int fd;

    char *wsk;

    fd=shm_open(name,O_CREAT | O_RDWR, 0666);

    ftruncate(fd,SIZE);

    wsk=(char *)
            mmap(0,SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd , 0);

sprintf(wsk,"%s",message_0);
wsk += strlen(message_0);
sprintf(wsk,"%s",message_1);
wsk += strlen(message_1);
return 0; }

Consumer:

int main(){
const int SIZE=4096;

const char *name= "OS";

int fd;

char *wsk;

fd=shm_open(name,O_RDONLY,0666);

wsk=(char *)
        mmap(0,SIZE,PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

printf("%s", (char *)wsk);

shm_unlink(name);

return 0; }

But I am getting segmention fault. I think it can be problem with pointers but I can't find resolve.I am using macOs.Thanks in advance.

Upvotes: 0

Views: 113

Answers (1)

xbjfk
xbjfk

Reputation: 11

The reason for the consumer exiting with a segmentation fault is due to permissions. fd is opened with O_RDONLY (read only), whereas mmap is called on fd with PROT_READ | PROT_WRITE. This causes mmap to error with EACCESS (permission denied). When a function like mmap errors, it returns a negative number. This means when you try to print wsk, you are trying to print a string at memory address -1 which obviously is not possible.

Upvotes: 1

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