Inter-process communication using pipes in C

• Within the operating system kernel, a pipe is a vector of memory, typically 4096 bytes long

• From within a C program, a pipe is represented as an array of two integer file-descriptors. Writing data to array[0] adds the data to the pipe, and reading from array[1] removes that data.

  #include <unistd.h> int thepipe[2]; char data[1024]; int datasize, nbytes; if(pipe(thepipe) != 0) { perror("cannot create pipe"); exit(EXIT_FAILURE); } datasize = ... nbytes = write( thepipe[0], data, datasize); // write to the pipe nbytes = read( thepipe[1], data, sizeof(data)); // read from the pipe

Pipes have a finite size, typically 4096 bytes, and their typical use affects the scheduling of processes connected by the same pipe:

• A newly created pipe is empty.
• Data written to the writing end is added to the pipe.
• If a process tries to write more data than will fit in the pipe, that writing process is blocked until space becomes available.
• Data read from the reading end is removed from the pipe.
• If a process tries to read more data than is held in the pipe (including if the pipe is empty) then that reading process is blocked until data becomes available.
• A pair of processes writing and reading using the same pipe, 'cause' those processes to (roughly) alternate their execution.