Lock and IPC (Inter-Process Communication) (Chap 12, 14 in the book “Advanced Programming in the UNIX Environment”) Acknowledgement : Prof. Y. Moon at Kangwon Nat’l Univ.

Lock a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution Advisory lock Other processes can still access (read or write) to a locked resource There is a way to check whether a resource is locked Mandatory lock Prevent access to a locked resource

Advisory Locking #include <fcntl.h> int flock(int fd, int operation); Returns: 0 if OK, -1 otherwise Applies or removes an advisory lock on the file associated with the file descriptor fd Operation can be LOCK_SH LOCK_EX LOCK_NB : non-blocking mode. Use together with other lock modes by OR ‘|’ LOCK_UN Locks entire file

Advisory Record Locking #include <fcntl.h> int fcntl(int fd, int cmd, struct flock* lock); Returns: depends on cmd if OK, -1 on error struct flock { short l_type; /* F_RDLCK, F_WRLCK, or F_UNLCK */ off_t l_start; /* offset in bytes from l_whence */ short l_whence; /* SEEK_SET, SEEK_CUR, SEEK_END */ off_t l_len; /* length, in bytes; 0 means “lock to EOF */ pid_t l_pid; /* returned by F_GETLK */ } Lock types are : F_RDLCK : non-exclusive (read) lock; fails if write lock exists F_WRLCK : exclusive (write) lock; fails if any lock exists F_UNLCK : releases our lock on specified range

Setting and Clearing a lock struct flock fl; int fd; fl.l_type = F_WRLCK; /* F_RDLCK, F_WRLCK, F_UNLCK */ fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */ fl.l_start = 0; /* Offset from l_whence */ fl.l_len = 0; /* length, 0 = to EOF */ fl.l_pid = getpid(); /* our PID */ fd = open("filename", O_WRONLY); fcntl(fd, F_SETLKW, &fl); /* F_GETLK, F_SETLK, F_SETLKW */ fl.l_type = F_UNLCK; /* tell it to unlock the region */ fcntl(fd, F_SETLK, &fl); /* set the region to unlocked */ cmd F_SETLKW: obtain lock F_SETLK: obtain lock but no wait if it cannot obtain the lock F_GETLK: only check to see if there is a lock

Non-Blocking(Asynchronous) I/O A form of I/O that permits subsequent processing to continue before current processing has finished. Non Blocking I/O function returns immediately Two ways for specifying non-blocking I/O When calling “open” and specify O_NONBLOCK flag For a file descriptor that is already open, call fcntl to turn on O_NONBLOCK file status flag.

IPC (Inter-Process Communication) Pipes FIFOs Message Queue Shared Memory Semaphores

Contents Pipes FIFOs System V IPC APUE (Interprocess Communication Message Queues Shared Memory Semaphores

IPC using Pipes IPC using regular files IPC using pipes unrelated processes can share fixed size lack of synchronization IPC using pipes for transmitting data between related processes can transmit an unlimited amount of data automatic synchronization on open()

Pipes in a UNIX Shell In a UNIX shell, the pipe symbol is: | (the vertical bar) In a shell, UNIX pipes look like: $ ls -al | more where the standard output of the program at the left (i.e., the producer) becomes the standard input of the program at the right (i.e., the consumer). We can have longer pipes: $ ls –l | grep *.c | wc > output.file

Example (1/2) $ who | sort

Example (2/2)

Data transmitting Types of pipes IPC using Pipes APUE (Interprocess Communication Data transmitting data is written into pipes using the write() system call data is read from a pipe using the read() system call automatic blocking when full or empty Types of pipes (unnamed) pipes named pipes (FIFOs)

In UNIX, pipes are the oldest form of IPC. Limitations of Pipes: Half duplex (data flows in one direction) Can only be used between processes that have a common ancestor (Usually used between the parent and child processes) Processes cannot pass pipes and must inherit them from their parent If a process creates a pipe, all its children will inherit it

Pipes (2/4) Two file descriptors are returned through the fd argument APUE (Interprocess Communication #include <unistd.h> int pipe(int fd[2]) Returns: 0 if OK, -1 on error Two file descriptors are returned through the fd argument fd[0]: can be used to read from the pipe, and fd[1]: can be used to write to the pipe Anything that is written on fd[1] may be read by fd[0]. This is of no use in a single process. However, between processes, it gives a method of communication The pipe() system call gives parent-child processes a way to communicate with each other.

Pipes (3/4) parent child parent child pipe kernel pipe kernel fd[0] parent closes fd[0] child closes fd[1] parent  child: parent closes fd[1] child closes fd[0] parent fd[0] child parent fd[1] child fd[1] fd[0] pipe kernel pipe kernel

Read from a pipe with write end closed: (fd[1]이 close된 경우) Pipes (4/4) Read from a pipe with write end closed: (fd[1]이 close된 경우) returns 0 to indicate EOF Write to a pipe with read end closed: (fd[0]가 close된 경우) SIGPIPE generated, write() returns error (errno == EPIPE)

example: pipe.c (1/2) #include <stdio.h> // pipe.c #define READ 0 #define WRITE 1 char* phrase = "Stuff this in your pipe and smoke it"; main( ) { int fd[2], bytesRead; char message[100]; pipe(fd); if (fork() == 0) { // child close(fd[READ]); write(fd[WRITE], phrase, strlen(phrase)+1); fprintf(stdout, "[%d, child] write completed.\n", getpid()); close(fd[WRITE]); } else { // parent bytesRead = read(fd[READ], message, 100); fprintf(stdout, "[%d, parent] read completed.\n", getpid()); printf("Read %d bytes: %s\n", bytesRead,message);

example: pipe.c (2/2) 실행 결과

Contents Pipes FIFOs System V IPC Message Queues Shared Memory Semaphores

FIFOs are "named pipes" that can be used between unrelated processes. APUE (Interprocess Communication Pipes can be used only between related processes. (e.g., parent and child processes) FIFOs are "named pipes" that can be used between unrelated processes. A type of file stat.st_mode == FIFO Test with S_ISFIFO() macro

Creating FIFOs is similar to creating a file. APUE (Interprocess Communication #include <sys/types.h> #include <sys/stat.h> int mkfifo(const char *pathname, mode_t mode); Returns: 0 if OK, -1 on error Creating FIFOs is similar to creating a file. pathname: filename mode: permissons, same as for open() function Using a FIFO is similar to using a file. we can open, close, read, write, unlink, etc., to the FIFO.

if FIFO opened without O_NONBLOCK flag FIFOs (3/3) if FIFO opened without O_NONBLOCK flag an open for read-only blocks until some other process opens the FIFO for writing an open for write-only blocks until some other process opens the FIFO for reading if O_NONBLOCK is specified (nonblocking) an open for read-only returns immediately if no process has the FIFO open for writing an open for write-only returns an error (errno=ENXIO) if no process has the FIFO open for reading Like a pipe, if we write to a FIFO that no process has open for reading, the signal SIGPIPE is generated. When the last writer for a FIFO closes the FIFO, an end of file (EOF) is generated for the reader of the FIFO.

Uses of FIFOs Used by shell commands to pass data from one shell pipeline to another, without creating intermediate files. Used in client-server application to pass data between clients and server.

Using FIFOs to Duplicate Output Streams tee(1) copies its standard input to both its standard output and to the file named on its command line. $ mkfifo fifo1 $ prog3 < fifo1 & $ prog1 < infile | tee fifo1 | prog2 fifo1 prog3 infile prog1 tee prog2

An Example using a FIFO APUE (Interprocess Communication

Client-Server Communication Using a FIFO Server creates a “well-known” FIFO to communicate with clients. client well-known FIFO read request server write request . Problem: Server can't reply clients using a single “well-known” FIFO

Contents Pipes FIFOs System V IPC Message Queues Shared Memory Semaphores (간단히)

System V IPC Message Queues Shared Memory Semaphores Send and receive amount of data called “messages”. The sender classifies each message with a type. Shared Memory Shared memory allows two or more processes to share a given region of memory. Readers and writers may use semaphore for synchronization. Semaphores Process synchronization and resource management For example, a semaphore might be used to control access to a device like printer.

Identifiers & Keys Identifier: each IPC structure has a nonnegative integer Key: when creating an IPC structure, a key must be specified (key_t) id = xxxget(key, …) How to access the same IPC?  key in a common header Define a key in a common header Client and server agree to use that key Server creates a new IPC structure using that key Problem when the key is already in use (msgget, semget, shmget returns error) Solution: delete existing key, create a new one again!

IPC System Calls msg/sem/shm get msg/sem/shm ctl msg/sem/shm op APUE (Interprocess Communication msg/sem/shm get Create new or open existing IPC structure. Returns an IPC identifier msg/sem/shm ctl Determine status, set options and/or permissions Remove an IPC identifier msg/sem/shm op Operate on an IPC identifier For example(Message queue) add new msg to a queue (msgsnd) receive msg from a queue (msgrcv)

Permission Structure ipc_perm is associated with each IPC structure. Defines the permissions and owner. struct ipc_perm { uid_t uid; /* owner's effective user id */ gid_t gid; /* owner's effective group id */ uid_t cuid; /* creator's effective user id */ gid_t cgid; /* creator’s effective group id */ mode_t mode; /* access modes */ ulong seq; /* slot usage sequence number */ key_t key; /* key */ };

Linked list of messages Message Queues (1/2) Linked list of messages Stored in kernel Identified by message queue identifier (in kernel) msgget Create a new queue or open existing queue. msgsnd Add a new message to a queue msgrcv Receive a message from a queue Fetching order: based on type

Message Queues (2/2) Each queue has a structure struct msqid_ds { struct ipc_perm msg_perm; struct msg *msg_first; /* ptr to first msg on queue */ struct msg *msg_last; /* ptr to last msg on queue */ ulong msg_cbytes; /* current # bytes on queue */ ulong msg_qnum; /* # msgs on queue */ ulong msg_qbytes; /* max # bytes on queue */ pid_t msg_lspid; /* pid of last msgsnd() */ pid_t msg_lrpid; /* pid of last msgrcv() */ time_t msg_stime; /* last-msgsnd() time */ time_t msg_rtime; /* last-msgrcv() time */ time_t msg_ctime; /* last-change time */ }; We can get the structure using msgctl() function. Actually, however, we don’t need to know the structure in detail.

msgget() Create new or open existing queue flag : ipc_perm.mode #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgget(key_t key, int flag); Returns: msg queue ID if OK, -1 on error Create new or open existing queue flag : ipc_perm.mode Example msg_qid = msgget(DEFINED_KEY, IPC_CREAT | 0666);

msgctl() Performs various operations on a queue #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgctl(int msqid, int cmd, struct msqid_ds *buf); Returns: 0 if OK, -1 on error Performs various operations on a queue cmd = IPC_STAT: fetch the msqid_ds structure for this queue, storing it in buf cmd = IPC_SET: set the following four fields from buf: msg_perm.uid, msg_perm.gid, msg_perm.mode, and msg_qbytes cmd = IPC_RMID: remove the message queue.

msgsnd() msgsnd() places a message at the end of the queue. #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgsnd(int msqid, const void *ptr, size_t nbytes, int flag); Returns: 0 if OK, -1 on error msgsnd() places a message at the end of the queue. ptr: pointer that points to a message nbytes: length of message data if flag = IPC_NOWAIT: IPC_NOWAIT is similar to the nonblocking I/O flag for file I/O. Structure of messages struct mymesg { long mtype; /* positive message type */ char mtext[512]; /* message data, of length nbytes */ };

msgrcv() msgrcv() retrieves a message from a queue. #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgrcv(int msqid, void *ptr, size_t nbytes, long type, int flag); Returns: data size in message if OK, -1 on error msgrcv() retrieves a message from a queue. type == 0: the first message on the queue is returned type > 0: the first message on the queue whose message type equals type is returned type < 0: the first message on the queue whose message type is the lowest value less than or equal to the absolute value of type is returned flag may be given by IPC_NOWAIT

example: sender.c receiver.c (1/4) #include <stdio.h> // sender.c #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> #define DEFINED_KEY 0x10101010 main(int argc, char **argv) { int msg_qid; struct { long mtype; char content[256]; } msg; fprintf(stdout, "=========SENDER==========\n"); if((msg_qid = msgget(DEFINED_KEY, IPC_CREAT | 0666)) < 0) { perror("msgget: "); exit(-1); } msg.mtype = 1; while(1) { memset(msg.content, 0x0, 256); gets(msg.content); if(msgsnd(msg_qid, &msg, sizeof(msg.content), 0) < 0) { perror("msgsnd: "); exit(-1);

example: sender.c receiver.c (2/4) #include <stdio.h> // receiver.c #include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> #define DEFINED_KEY 0x10101010 main(int argc, char **argv) { int msg_qid; struct { long mtype; char content[256]; } msg; fprintf(stdout, "=========RECEIVER==========\n"); if((msg_qid = msgget(DEFINED_KEY, IPC_CREAT | 0666)) < 0) { perror("msgget: "); exit(-1); } while(1) { memset(msg.content, 0x0, 256); if(msgrcv(msg_qid, &msg, 256, 0, 0) < 0) { perror("msgrcv: "); exit(-1); puts(msg.content);

example: sender.c receiver.c (3/4) APUE (Interprocess Communication

example: sender.c receiver.c (4/4)

Allows multiple processes to share a region of memory Shared Memory Allows multiple processes to share a region of memory Fastest form of IPC: no need of data copying between client & server If a shared memory segment is attached It become a part of a process data space, and shared among multiple processes Readers and writers may use semaphore to synchronize access to a shared memory segment

Shared Memory Segment Structure APUE (Interprocess Communication Each shared memory has a structure struct shmid_ds { struct ipc_perm shm_perm; struct anon_map *shm_amp; /* pointer in kernel */ int shm_segsz; /* size of segment in bytes */ ushort shm_lkcnt; /* # of times segment is being locked */ pid_t shm_lpid; /* pid of last shmop() */ pid_t shm_cpid; /* pid of creator */ ulong shm_nattch; /* # of current attaches */ ulong shm_cnattch; /* used only for shminfo() */ time_t shm_atime; /* last-attach time */ time_t shm_dtime; /* last-detach time */ time_t shm_ctime; /* last-change time */ }; We can get the structure using shmctl() function. Actually, however, we don’t need to know the structure in detail.

shmget() Obtain a shared memory identifier #include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> int shmget(key_t key, int size, int flag); Returns: shared memory ID if OK, -1 on error Obtain a shared memory identifier size: is the size of the shared memory segment flag: ipc_perm.mode Example shmId = shmget(key, size, PERM|IPC_CREAT|IPC_EXCL|0666);

shmctl() Performs various shared memory operations #include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> int shmctl(int shmid, int cmd, struct shmid_ds *buf); Returns: 0 if OK, -1 on error Performs various shared memory operations cmd = IPC_STAT: fetch the shmid_ds structure into buf cmd = IPC_SET: set the following three fields from buf: shm_perm.uid, shm_perm.gid, and shm_perm.mode cmd = IPC_RMID: remove the shared memory segment set from the system

shmat() Attached a shared memory to an address #include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> void *shmat (int shmid, void *addr, int flag); Returns: pointer to shared memory segment if OK, -1 on error Attached a shared memory to an address flag = SHM_RDONLY: the segment is read-only addr==0: at the first address selected by the kernel (recommended!) addr!=0: at the address given by addr

Memory Layout high address command-line arguments and environment variables stack heap 0xf7fffb2c 0xf77e86a0 0xf77d0000 shared memory shared memory of 100,000 bytes 0x0003d2c8 0x00024c28 malloc of 100,000 bytes uninitialized data (bss) 0x0003d2c8 0x00024c28 array[] of 40,000 bytes initialized data text low address

shmdt() Detach a shared memory segment #include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> void shmdt (void *addr); Returns: 0 if OK, -1 on error Detach a shared memory segment

example: tshm.c (1/2) #include <sys/types.h> // tshm.c #include <sys/ipc.h> #include <sys/shm.h> #define ARRAY_SIZE 100000 #define MALLOC_SIZE 100000 #define SHM_SIZE 100000 err_sys(char *p) { perror(p); exit(-1); } char array[ARRAY_SIZE]; /* uninitialized data = bss */ int main(void) { int shmid; char *ptr, *shmptr; printf("array[] from %x to %x\n", &array[0], &array[ARRAY_SIZE]); printf("stack around %x\n", &shmid); if ((ptr = malloc(MALLOC_SIZE)) == NULL) err_sys("malloc error"); printf("malloced from %x to %x\n", ptr, ptr+MALLOC_SIZE); if ((shmid = shmget(0x01010101, SHM_SIZE, IPC_CREAT | 0666)) < 0) err_sys("shmget error"); if ((shmptr = shmat(shmid, 0, 0)) == (void *) -1) err_sys("shmat error"); printf("shared memory attached from %x to %x\n", shmptr, shmptr+SHM_SIZE); // if (shmctl(shmid, IPC_RMID, 0) < 0) err_sys("shmctl error"); exit(0); }

example: tshm.c (2/2)

Semaphore a special variable (sv) upon which only two operations are allowed P(sv) : wait if sv is greater than zero, decrement sv. If sv is zero, suspend execution of the process V(sv) : signal If some other process has been suspended for waiting sv, make it resume execution If no process is suspended waiting for sv, increment sv. They are used to ensure that a single executing process has exclusive access to a resource. Critical Section Binary Semaphore

Semaphore semaphore sv = 1; loop forever { P(sv); critical section code; V(sv); non-critical code section }

To obtain a shared resource: Semaphores A counter to provide access to shared data object for multiple processes To obtain a shared resource: 1. Test semaphore that controls the resource 2. If value > 0, value--, grant use 3. If value == 0, sleep until value > 0 4. Release resource, value ++ Step 1, 2 must be an atomic operation

Semaphore Structure Each semaphore has a structure struct semid_ds { struct ipc_perm sem_perm; struct sem *sem_base; /*ptr to first semaphore in set */ ushort sem_nsems; /* # of semaphors in set */ time_t sem_otime; /* last-semop() time */ time_t sem_ctime; /* last-change time */ }; struct sem { ushort semval; /* semaphore value, always >= 0 */ pid_t sempid; /* pid for last operation */ ushort semncnt; /* # processes awaiting semval > currval */ ushort semzcnt; /* # processes awaiting semval = 0 */ We can get the structure using semctl() function. Actually, however, we don’t need to know the structure in detail.

semget() Obtain a semaphore ID #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semget(key_t key, int nsems, int flag); Returns: semaphore ID if OK, -1 on error Obtain a semaphore ID nsems: sem_nsens (# of semaphores in set) flag: ipc_perm.mode

semctl() APUE (Interprocess Communication #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semctl(int semid, int semnum, int cmd, union semun arg); union semun { int val; /* for SETVAL */ struct semid_ds *buf; /* for IPC_START and IPC_SET */ ushort *array; /* for GETALL and SETALL */ }; To use semaphore, please refer to the textbook and manuals related semaphore.

ipcs, ipcrm ipcs: checking status of System V IPC APUE (Interprocess Communication ipcs: checking status of System V IPC $ ipcs // check information of IPC (q, m, s) $ ipcs –q ($ ipcs –qa) // check information of Message Queue $ ipcs –m ($ ipcs –ma) // check information of Shared Memory $ ipcs –s ($ ipcs –sa) // check information of Semaphore ipcrm: delete a defined IPC $ ipcrm –q id // Delete Message Queue $ ipcrm –m id // Delete Shared Memory $ ipcrm –s id // Delete Semaphore

Memory mapped file I/O APUE (Interprocess Communication #include <sys/types.h> #include <sys/mman.h> caddr_t mmap (caddr_t addr, size_t len, int prot, int flag, int filedes, off_t off); Returns: starting address of mapped region if OK, -1 on error int munmap(caddr_t addr, size_t len); Returns : 0 if OK, -1 on error Memory mapped I/O lets us map a file on disk into a buffer in memory so that when we fetch bytes from the buffer, the corresponding bytes of the file are read. Similarly, when we store data in the buffer, the corresponding bytes are automatically written to the file. This lets us perform I/O without using read or write. To use this feature we have to tell the kernel to map a given file to a region in memory.

Memory mapped file I/O flag argument MAP_FIXED : the return value must equal addr MAP_SHARED : store operation modify the mapped file. Thus shared by others. MAP_PRIVATE: make a copy of original file and modify the copied file. therefore, it does not affect others. high address command-line arguments and environment variables stack heap length Memory-mapped portion of file Starting address Memory-mapped portion of file file : uninitialized data (bss) offset initialized data text low address