\nHeader Files Needed
\n<\/h5>\n\r\n\r\n#include <sys\/sem.h>\t\t\/\/Used for semaphores\r\n<\/code><\/pre>\n\n \n<\/p>\n
\nDefining The Semaphore Your Header File
\n<\/h5>\n\r\n\r\n\/\/----- SEMAPHORE -----\r\n\/\/On linux systems this union is probably already defined in the included sys\/sem.h, but if not use this default basic definition:\r\nunion semun {\r\n\tint val;\r\n\tstruct semid_ds *buf;\r\n\tunsigned short *array;\r\n};\r\n\r\n\/\/FUNCTIONS:\r\nstatic int semaphore1_get_access(void);\r\nstatic int semaphore1_release_access(void);\r\n\/\/VARIABLES:\r\nstatic int semaphore1_id;\r\n<\/code><\/pre>\n\nThe Functions
\n<\/h5>\n\r\n\r\n\r\n\/\/***********************************************************\r\n\/\/***********************************************************\r\n\/\/********** WAIT IF NECESSARY THEN LOCK SEMAPHORE **********\r\n\/\/***********************************************************\r\n\/\/***********************************************************\r\n\/\/Wait if necessary and then change the semaphore by –1. This is the "wait" operation\r\nstatic int semaphore1_get_access(void)\r\n{\r\n\tstruct sembuf sem_b;\r\n\tsem_b.sem_num = 0;\r\n\tsem_b.sem_op = -1; \/* P() *\/\r\n\tsem_b.sem_flg = SEM_UNDO;\r\n\tif (semop(semaphore1_id, &sem_b, 1) == -1)\t\t\/\/Wait until free\r\n\t{\r\n\t\tfprintf(stderr, "semaphore1_get_access failed\\n");\r\n\t\treturn(0);\r\n\t}\r\n\treturn(1);\r\n}\r\n\r\n\/\/***************************************\r\n\/\/***************************************\r\n\/\/********** RELEASE SEMAPHORE **********\r\n\/\/***************************************\r\n\/\/***************************************\r\n\/\/Setting the semaphore back to available. This is the "release" operation.\r\n\r\nstatic int semaphore1_release_access(void)\r\n{\r\n\tstruct sembuf sem_b;\r\n\tsem_b.sem_num = 0;\r\n\tsem_b.sem_op = 1; \/* V() *\/\r\n\tsem_b.sem_flg = SEM_UNDO;\r\n\tif (semop(semaphore1_id, &sem_b, 1) == -1)\r\n\t{\r\n\t\tfprintf(stderr, "semaphore1_release_access failed\\n");\r\n\t\treturn(0);\r\n\t}\r\n\treturn(1);\r\n}\r\n<\/code><\/pre>\n\nCreate The Semaphore
\n<\/h5>\n\r\n\r\n\t\/\/----------------------------\r\n\t\/\/----- CREATE SEMAPHORE -----\r\n\t\/\/----------------------------\r\n\tprintf("Creating semaphore...\\n");\r\n\tsemaphore1_id = semget((key_t)12345, 1, 0666 | IPC_CREAT);\t\t\/\/<<<<< SET THE SEMPAHORE KEY (Semaphore key, number of semaphores required, flags)\r\n\t\/\/\tSemaphore key\r\n\t\/\/\t\tUnique non zero integer (usually 32 bit). Needs to avoid clashing with another other processes semaphores (you just have to pick a random value and hope - ftok() can help with this but it still doesn't guarantee to avoid colision)\r\n\r\n\t\/\/Initialize the semaphore using the SETVAL command in a semctl call (required before it can be used)\r\n\tunion semun sem_union_init;\r\n\tsem_union_init.val = 1;\r\n\tif (semctl(semaphore1_id, 0, SETVAL, sem_union_init) == -1)\r\n\t{\r\n\t\tfprintf(stderr, "Creating semaphore failed to initialize\\n");\r\n\t\texit(EXIT_FAILURE);\r\n\t}\r\n<\/code><\/pre>\n\nUsing The Semaphore
\n<\/h5>\n\r\n\r\n\t\/\/----- SEMAPHORE GET ACCESS -----\r\n\tif (!semaphore1_get_access())\r\n\t\texit(EXIT_FAILURE);\r\n\r\n\t\/\/... do the interlocked action ...\r\n\r\n\t\/\/----- SEMAPHORE RELEASE ACCESS -----\r\n\tif (!semaphore1_release_access())\r\n\t\texit(EXIT_FAILURE);\r\n<\/code><\/pre>\n\nDeleting The Semaphore
\n<\/h5>\n\r\n\r\n\t\/\/----------------------------\r\n\t\/\/----- DELETE SEMAPHORE -----\r\n\t\/\/----------------------------\r\n\t\/\/It's important not to unintentionally leave semaphores existing after program execution. It also may cause problems next time you run the program.\r\n\tunion semun sem_union_delete;\r\n\tif (semctl(semaphore1_id, 0, IPC_RMID, sem_union_delete) == -1)\r\n\t\tfprintf(stderr, "Failed to delete semaphore\\n");\r\n<\/code><\/pre>\n\n \n<\/p>\n
\n <\/p>\n","protected":false},"excerpt":{"rendered":"
Semaphores allow processes to avoid clashes when accessing shared resources, for instance shared memory. Generating A Random Key Humans aren't very good at picking random values so use a site like this one: www.random.org\/integers\/ It won't do a 32bit 1 to 2147483647, but it will do 1 to 1000000000 Simple Example Of Creating & Using […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[45],"tags":[],"class_list":["post-756","post","type-post","status-publish","format-standard","hentry","category-semaphores"],"_links":{"self":[{"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/posts\/756","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/comments?post=756"}],"version-history":[{"count":5,"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/posts\/756\/revisions"}],"predecessor-version":[{"id":1764,"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/posts\/756\/revisions\/1764"}],"wp:attachment":[{"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/media?parent=756"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/categories?post=756"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/raspberry-projects.com\/pi\/wp-json\/wp\/v2\/tags?post=756"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}