// semaphore.h // Copyright 2016 Robin.Rowe@CinePaint.org // License open source MIT #ifndef semaphore_h #define semaphore_h #ifndef __cplusplus #error C++11 required #endif #include "unistd.h" #include "../portable/stub.h" #include #include #include #include #include #include // #define TRACE_SEM_T class sem_t { unsigned flagOverwrite; std::mutex sem_mutex; std::condition_variable sem_cv; std::atomic posts; std::string name; unsigned semNumber; std::chrono::milliseconds GetDelay(const struct timespec* deadline) { struct timespec now; if (!deadline || clock_gettime(CLOCK_REALTIME, &now) == -1) { return std::chrono::milliseconds(0); } time_t delay = 1000 * (deadline->tv_sec - now.tv_sec); delay += (deadline->tv_nsec - now.tv_nsec) / (1000*1000); if(delay<=0) { return std::chrono::milliseconds(0); } return std::chrono::milliseconds(delay); } bool IsPosted() { const int i = posts.fetch_sub(1) - 1; if(i>=0) { #ifdef TRACE_SEM_T printf("sem_t #%u consumed:%i\n",semNumber,int(posts)); #endif return true; } posts.fetch_add(1); return false; } public: ~sem_t() { flagOverwrite=0; #ifdef TRACE_SEM_T static unsigned i; semNumber = ++i; printf("sem_t #%u destroyed\n",semNumber); #endif } sem_t() : posts(0) , flagOverwrite(6009) { #ifdef TRACE_SEM_T static unsigned i; semNumber = ++i; printf("sem_t #%u created\n",semNumber); #endif } static sem_t* sem_open(const char *name, int oflag) { sem_t* st = new sem_t; st->name = name; return st; } static sem_t* sem_open(const char *name, int oflag,mode_t mode, unsigned int value) { sem_t* st = new sem_t; st->name = name; st->posts.exchange(value); return st; } int sem_init(int pshared, unsigned int value) { if(6009 !=flagOverwrite) { puts("ERROR: Windows sem_t memory overwrite"); return -1; } posts.exchange(value); return 0; } static int sem_close(sem_t *st) { delete st; return 0; } int sem_destroy() { return 0; } int sem_getvalue(sem_t *restrict, int *restrict2) { *restrict2=0; return posts; } int sem_trywait() { #ifdef TRACE_SEM_T printf("sem_t #%u trywait:%i\n",semNumber,int(posts)); #endif return posts > 0 ? 0:-1; } int sem_wait() { if(IsPosted()) { return 0; } std::unique_lock lk(sem_mutex); while(!IsPosted()) { #ifdef TRACE_SEM_T printf("sem_t #%u wait:%i\n",semNumber,int(posts)); #endif sem_cv.wait(lk); // Restart if interrupted } return 0; } int sem_timedwait(const struct timespec* ts) { if(IsPosted()) { return 0; } std::unique_lock lk(sem_mutex); std::chrono::milliseconds delay(GetDelay(ts)); while(delay.count() && !IsPosted()) { #ifdef TRACE_SEM_T printf("sem_t #%u timedwait(%lli):%i\n",semNumber,delay.count(),int(posts)); #endif if(std::cv_status::timeout==sem_cv.wait_for(lk,delay)) { return 0; } delay = GetDelay(ts); // errno = EINTR; // return -1; } return 0; } int sem_post() { posts.fetch_add(1); #ifdef TRACE_SEM_T printf("sem_t #%u post:%i\n",semNumber,int(posts)); #endif sem_cv.notify_one(); return 0; } void sem_name_win32(const char* name) { this->name = name; } static int sem_unlink(const char *) { STUB_NEG(sem_unlink); } }; inline int sem_close(sem_t *st) { return sem_t::sem_close(st); } inline int sem_destroy(sem_t *st) { if(!st) { return -1; } return st->sem_destroy(); } inline int sem_getvalue(sem_t *st, int *val) { if(!st) { return -1; } return st->sem_getvalue(st,val); } inline int sem_init(sem_t *st, int pshared, unsigned int value) { if(!st) { return -1; } return st->sem_init(pshared,value); } inline void sem_name_win32(sem_t *st, const char* name) { if(!st) { return; } return st->sem_name_win32(name); } inline sem_t* sem_open(const char *name, int oflag) { return sem_t::sem_open(name,oflag); } inline sem_t* sem_open(const char *name, int oflag,mode_t mode, unsigned int value) { return sem_t::sem_open(name,oflag,mode,value); } inline int sem_post(sem_t* st) { if(!st) { return -1; } return st->sem_post(); } inline int sem_timedwait(sem_t* st, const struct timespec* ts) { if(!st) { return -1; } return st->sem_timedwait(ts); } inline int sem_trywait(sem_t* st) { if(!st) { return -1; } return st->sem_trywait(); } inline int sem_unlink(const char* name) { return sem_t::sem_unlink(name); } inline int sem_wait(sem_t* st) { if(!st) { return -1; } return st->sem_wait(); } /* Linux man: SEM_OVERVIEW(7) Linux Programmer's Manual SEM_OVERVIEW(7) NAME top sem_overview - overview of POSIX semaphores DESCRIPTION top POSIX semaphores allow processes and threads to synchronize their actions. A semaphore is an integer whose value is never allowed to fall below zero. Two operations can be performed on semaphores: increment the semaphore value by one (sem_post(3)); and decrement the semaphore value by one (sem_wait(3)). If the value of a semaphore is currently zero, then a sem_wait(3) operation will block until the value becomes greater than zero. POSIX semaphores come in two forms: named semaphores and unnamed semaphores. Named semaphores A named semaphore is identified by a name of the form /somename; that is, a null-terminated string of up to NAME_MAX-4 (i.e., 251) characters consisting of an initial slash, followed by one or more characters, none of which are slashes. Two processes can operate on the same named semaphore by passing the same name to sem_open(3). The sem_open(3) function creates a new named semaphore or opens an existing named semaphore. After the semaphore has been opened, it can be operated on using sem_post(3) and sem_wait(3). When a process has finished using the semaphore, it can use sem_close(3) to close the semaphore. When all processes have finished using the semaphore, it can be removed from the system using sem_unlink(3). Unnamed semaphores (memory-based semaphores) An unnamed semaphore does not have a name. Instead the semaphore is placed in a region of memory that is shared between multiple threads (a thread-shared semaphore) or processes (a process-shared semaphore). A thread-shared semaphore is placed in an area of memory shared between the threads of a process, for example, a global variable. A process-shared semaphore must be placed in a shared memory region (e.g., a System V shared memory segment created using shmget(2), or a POSIX shared memory object built created using shm_open(3)). Before being used, an unnamed semaphore must be initialized using sem_init(3). It can then be operated on using sem_post(3) and sem_wait(3). When the semaphore is no longer required, and before the memory in which it is located is deallocated, the semaphore should be destroyed using sem_destroy(3). The remainder of this section describes some specific details of the Linux implementation of POSIX semaphores. Versions Prior to kernel 2.6, Linux supported only unnamed, thread-shared semaphores. On a system with Linux 2.6 and a glibc that provides the NPTL threading implementation, a complete implementation of POSIX semaphores is provided. Persistence POSIX named semaphores have kernel persistence: if not removed by sem_unlink(3), a semaphore will exist until the system is shut down. Linking Programs using the POSIX semaphores API must be compiled with cc -pthread to link against the real-time library, librt. Accessing named semaphores via the filesystem On Linux, named semaphores are created in a virtual filesystem, normally mounted under /dev/shm, with names of the form sem.somename. (This is the reason that semaphore names are limited to NAME_MAX-4 rather than NAME_MAX characters.) Since Linux 2.6.19, ACLs can be placed on files under this directory, to control object permissions on a per-user and per-group basis. NOTES top System V semaphores (semget(2), semop(2), etc.) are an older semaphore API. POSIX semaphores provide a simpler, and better designed interface than System V semaphores; on the other hand POSIX semaphores are less widely available (especially on older systems) than System V semaphores. EXAMPLE top An example of the use of various POSIX semaphore functions is shown in sem_wait(3). SEE ALSO top sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7) COLOPHON top This page is part of release 4.06 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2015-08-08 SEM_OVERVIEW(7) ----- sem_wait() decrements (locks) the semaphore pointed to by sem. If the semaphore's value is greater than zero, then the decrement proceeds, and the function returns, immediately. If the semaphore currently has the value zero, then the call blocks until either it becomes possible to perform the decrement (i.e., the semaphore value rises above zero), or a signal handler interrupts the call. sem_trywait() is the same as sem_wait(), except that if the decrement cannot be immediately performed, then call returns an error (errno set to EAGAIN) instead of blocking. sem_timedwait() is the same as sem_wait(), except that abs_timeout specifies a limit on the amount of time that the call should block if the decrement cannot be immediately performed. The abs_timeout argument points to a structure that specifies an absolute timeout in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). This structure is defined as follows: struct timespec { time_t tv_sec; // Seconds long tv_nsec; // Nanoseconds [0 .. 999999999] }; All of these functions return 0 on success. Error is -1. */ #endif