semaphore 를 이용해서 프로그램 짜봤는데 이해가 안되고 오류가 많이뜨네요

프로그램 이해가 잘안됨니다 주석좀 부탁드림니다...
오류가 많이 나요 뭐 빠진게 있습니까?
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서버부분

#include
#include
#include
#include

#include "shm.h"

int shmid, clisem, servsem;
mesg *mesgptr;

main()
{
if ( (shmid = shmget(SHMKEY, sizeof(Mesg), PERMS | IPC_CREAT)) < 0)
err_sys("server: can't get shared memory");
if ( (mesgptr = (Mesg *) shmat(shmid, (char *) 0, 0)) == (Mesg *) -1)
err_sys("server: can't attach shared memory");

if( (clisem = sem_create(SEMKEY1, 1)) <0)
err_sys("server: can't create client samaphore");
if( (servsem = sem_create(SEMKEY2, 0)) < 0)
err_sys("server: can't create server semaphore");

server();

if(shmdt(mesgptr) < 0)
err_sys("server: can't detach shared memory");

sem_close(clisem);
sem_close(servsem);

exit(0);
}

server()
{
int m, filefd;
char errmesg[256], *sys_err_str();

sem_wait(servsem);

mesgptr->mesg_data[mesgptr->mesg_len] = '\0';

if( (filefd = open(mesgptr->mesg_data, 0)) < 0) {
sprintf(errmesg, ": can't open, %s\n", sys_err_str());
strcat(mesgptr->mesg_data, errmesg);
mesgptr->mesg_len = strlen(mesgptr->mesg_data);
sem_signal(clisem);
sem_wait(servsem);
} else {
while ( (n = read(filefd, mesgptr->mesg_data, MAXMESGDATA-1))

mesgptr->mesg_len = n;
sem_signal(clisem);
sem_wait(servsem);
}
close(filefd);
if (n < 0)
err_sys("server: read error");
}

mesgptr->mesg_len = 0;
sem_signal(clisem);
}

=====================================================
클라이언트 부분

#include
#include
#include
#include

#include "shm.h"

int shmid, clisem, servsem;
mesg *mesgptr;

main()
{
if ( (shmid = shmget(SHMKEY, sizeof(Mesg), 0)) < 0)
err_sys("client: can't get shared memory segment");
if ( (mesgptr = (Mesg *) shmat(shmid, (char *) 0, 0)) == (Mesg *) -1)
err_sys("client: can't attach shared memory segment");

if ( (clisem = sem_open(SEMKEY1)) < 0)
err_sys("client: can't open client semaphore");
if ( (servsem = sem_open(SEMKEY2)) < 0)
err_sys("client: can't open server semaphore");

client();

if (shmot(mesgptr) < 0)
err_sys("client: can't detach shared memory");
if (shmotl(shmid, IPC_RMID, (struct shmid_ds *) 0) < 0)
err_sys("client: can't remove shared memory");

sem_close(clisem);
sem_close(servsem);

exit(0);
}

client()
{
int n;

sem_wait(clisem);
if(fgets(mesgptr->mesg_data, MAXMESGDATA, stdin) == NULL)
err_sys("filename read error");

n = strlen(mesgptr->mesg_data);
if (mesgptr->mesg_data[n-1] == '\n')
n--;

mesgptr->mesg_len = n;
sem_signal(servsem);

sem_wait(clisem);
while( (n = mesgptr->mesg_len) > 0) {
if (write(1, mesgptr->mesg_data, n) != n)
err_sys("data write error");
sem_signal(servsem);
sem_wait(clisem);
}

if (n < 0)
err_sys("data read error");
}