\nIf you need unrelated processes to be able to exchange data you can do this using FIFOs, often referred to as named pipes. A named pipe is a special type of file (everything is a file in linux!) that exists as a name in the file system but behaves like unnamed pipes. Once created data can be passed through it, i.e. to pass data to your application or read data from it.\n<\/p>\n
\r\n\r\n#include <unistd.h>\r\n#include <stdlib.h>\r\n#include <stdio.h>\r\n#include <fcntl.h>\r\n#include <sys\/types.h>\r\n#include <sys\/stat.h>\r\n\r\n#define OUR_INPUT_FIFO_NAME "\/tmp\/my_fifo"\r\n\r\n\r\n\tint our_input_fifo_filestream = -1;\r\n\r\n\r\n\t\/\/--------------------------------------\r\n\t\/\/----- CREATE A FIFO \/ NAMED PIPE -----\r\n\t\/\/--------------------------------------\r\n\tint result;\r\n\r\n\tprintf("Making FIFO...\\n");\r\n\tresult = mkfifo(OUR_INPUT_FIFO_NAME, 0777);\t\t\/\/(This will fail if the fifo already exists in the system from the app previously running, this is fine)\r\n\tif (result == 0)\r\n\t{\r\n\t\t\/\/FIFO CREATED\r\n\t\tprintf("New FIFO created: %s\\n", OUR_INPUT_FIFO_NAME);\r\n\t}\r\n\r\n\tprintf("Process %d opening FIFO %s\\n", getpid(), OUR_INPUT_FIFO_NAME);\r\n\tour_input_fifo_filestream = open(OUR_INPUT_FIFO_NAME, (O_RDONLY | O_NONBLOCK));\r\n\t\t\t\t\t\/\/Possible flags:\r\n\t\t\t\t\t\/\/\tO_RDONLY - Open for reading only.\r\n\t\t\t\t\t\/\/\tO_WRONLY - Open for writing only.\r\n\t\t\t\t\t\/\/\tO_NDELAY \/ O_NONBLOCK (same function) - Enables nonblocking mode. When set read requests on the file can return immediately with a failure status\r\n\t\t\t\t\t\/\/\t\t\t\t\t\t\t\t\t\t\tif there is no input immediately available (instead of blocking). Likewise, write requests can also return\r\n\t\t\t\t\t\/\/\t\t\t\t\t\t\t\t\t\t\timmediately with a failure status if the output can't be written immediately.\r\n\tif (our_input_fifo_filestream != -1)\r\n\t\tprintf("Opened FIFO: %i\\n", our_input_fifo_filestream);\r\n\r\n<\/code><\/pre>\n\nNote that although this code asks for a mode of 0777, it will be altered by the user mask (umask) setting just as in normal file creation.\n<\/p>\n
\nA program may not open a FIFO for reading and writing with the mode O_RDWR. Use seperate pipes if you want to read and write.\n<\/p>\n
\nChecking For Received Bytes
\n<\/h4>\n\r\n\r\n\t\t\/\/---------------------------------------------\r\n\t\t\/\/----- CHECK FIFO FOR ANY RECEIVED BYTES -----\r\n\t\t\/\/---------------------------------------------\r\n\t\t\/\/ Read up to 255 characters from the port if they are there\r\n\t\tif (our_input_fifo_filestream != -1)\r\n\t\t{\r\n\t\t\tunsigned char rx_buffer[256];\r\n\t\t\tint rx_length = read(our_input_fifo_filestream, (void*)rx_buffer, 255);\t\t\/\/Filestream, buffer to store in, number of bytes to read (max)\r\n\t\t\tif (rx_length < 0)\r\n\t\t\t{\r\n\t\t\t\t\/\/An error occured (this can happen)\r\n\t\t\t\t\/\/printf("FIFO Read error\\n");\r\n\t\t\t}\r\n\t\t\telse if (rx_length == 0)\r\n\t\t\t{\r\n\t\t\t\t\/\/No data waiting\r\n\t\t\t}\r\n\t\t\telse\r\n\t\t\t{\r\n\t\t\t\t\/\/Bytes received\r\n\t\t\t\trx_buffer[rx_length] = '\\0';\r\n\t\t\t\tprintf("FIFO %i bytes read : %s\\n", rx_length, rx_buffer);\r\n\t\t\t}\r\n\t\t}\r\n<\/code><\/pre>\n\nClosing The FIFO
\n<\/h4>\n\r\n\r\n\t\/\/----- CLOSE THE FIFO -----\r\n\t(void)close(our_input_fifo_filestream);\r\n<\/code><\/pre>\n\nClose FIFO From Command Line
\n<\/h4>\n
\nThis can be needed when debugging. Normally you don't have to close a fifo to be able to use it again but it is possible to sometimes cause a fifo to not work properly when used again ( maybe by not having emptied it before your application closes?), but using this will kill it completely ready for the next time your app is run to re-create it.\n<\/p>\n
\r\n\r\nsudo rm \/tmp\/my_fifo\r\n<\/code><\/pre>\n\nSending data to your applications FIFO
\n<\/h4>\n\nSending from a command prompt
\n<\/h5>\n\r\n\r\necho "ABCD" > \/tmp\/my_fifo\r\n<\/code><\/pre>\n\nSending from C++
\n<\/h5>\n\r\n\r\n\tchar buffer[] = "ABCD";\r\n\twrite(our_input_fifo_filestream, (void*)buffer, strlen(buffer));\r\n<\/code><\/pre>\n\nSending bytes from a PHP web page
\n<\/h5>\n\r\n\r\n\t<?php\r\n\tsystem("sudo sh -c 'echo \\"ABCD\\" > tmp\/my_fifo'");\r\n\t?>\r\n<\/code><\/pre>\n\nPHP Web page Access
\n<\/h4>\n
\nSee our PHP web page example here<\/a>\n<\/p>\n\nUsing pipes between main application and child threads
\n<\/h4>\n
\nSee this example here<\/a>.\n<\/p>\n\nOther Methods Of Passing Data Between Processes
\n<\/h4>\n