C Program to lock file using semaphores.

Write a C Program to lock file using semaphores.
Using semaphores, We can control access to files, shared memory and other things. The basic functionality of a semaphore is that you can either set it, check it, or wait until it clears then set it (“test-n-set”). In C semaphores functions defined in the sys/sem library. Read more about C Programming Language . and read the C Programming Language (2nd Edition) by K and R.

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#include <stdio.h>
#include <sys/file.h>
#include <error.h>
#include <sys/sem.h>
#define MAXBUF 100
#define KEY 1216
#define SEQFILE "seq_file"
int semid,fd;
void my_lock(int);
void my_unlock(int);
union semnum
{
int val;
struct semid_ds *buf;
short *array;
}arg;
int main()
{
int child, i,n, pid, seqno;
char buff[MAXBUF+1];
pid=getpid();
if((semid=semget(KEY, 1, IPC_CREAT | 0666))= = -1)
{
perror("semget");
exit(1);
}
arg.val=1;
if(semctl(semid,0,SETVAL,arg)<0)
perror("semctl");
if((fd=open(SEQFILE,2))<0)
{
perror("open");
exit(1);
}
pid=getpid();
for(i=0;i<2;i++)
{
my_lock(fd);
lseek(fd,01,0);
if((n=read(fd,buff,MAXBUF))<0)
{
perror("read");
exit(1);
}
printf("pid:%d, Seq no:%dn", pid, seqno);
seqno++;
sprintf(buff,"%dn", seqno);
n=strlen(buff);
lseek(fd,01,0);
if(write(fd,buff,n)!=n)
{
perror("write");
exit(1);
}
sleep(1);
my_unlock(fd);
}
}
void my_lock(int fd)
{
struct sembuff sbuf=(0, -1, 0);
if(semop(semid, &sbuf, 1)= =0)
printf("Locking: Resource…n");
else
printf("Error in Lockn");
}
void my_unlock(int fd)
{
struct sembuff sbuf=(0, 1, 0);
if(semop(semid, &sbuf, 1)= =0)
printf("UnLocking: Resource…n");
else
printf("Error in Unlockn");
}
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