fix computer errors

I am doing C program to create shared memory object. While compiling, I have got the above error. Please some one help me in solving this.
Here is my program:
#include
#include
#include
#include
#define SHMSZ 27
main()
{
char c;
int shmid;
key_t key;
char *shm, *s;
/*
* We’ll name our shared memory segment
* “5678″.
*/
key = 5678;
/*
* Create the segment.
*/
if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {
perror("shmget");
exit(1);
}
/*
* Now we attach the segment to our data space.
*/
if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
perror("shmat");
exit(1);
}
/*
* Now put some things into the memory for the
* other process to read.
*/
s = shm;
for (c = 'a'; c <= 'z'; c )
*s = c;
*s = NULL;
/*
* Finally, we wait until the other process
* changes the first character of our memory
* to '*', indicating that it has read what
* we put there.
*/
while (*shm != '*')
sleep(1);
exit(0);
}