In one ear, out the other…wait.

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Pipes

Pipes allow forked processes to communicate with each other, and is often used to redirect the output of a process into the input of another process.

A common usage pattern for pipes is:

• Create a pipe
• Fork the process (both processes contain the pipe file descriptors)
• Write into the pipe as the parent/child
• Read from the pipe as the child/parent

unistd.h provides STDIN_FILENO and STDOUT_FILENO (and STDERR_FILENO)

int dup2(int oldfd, int newfd)
Close newfd if necessary, and copy oldfd into newfd.

Effectively, the dup2 function sets newfd to oldfd.
For example dup2(fd, STDOUT_FILENO) will set the output to go into fd instead of through the standard output

int pipe(int fd[2])
ret: 0
err: -1

The pipe function allocates a pair of file descriptors in which the first is read from, and the second is written into.
This allows a child process to communicate with the parent process.

FILE *popen(char *Cmd, char *Mode)
ret: FILE*
err: NULL

Message Queues

Message Queues allow for unrelated processes (ie a process that was not forked) to communicate with other processes. As opposed to the buffer stream that pipes provide, message queues separate each message from another.

Note: Message queue names start with a forward slash /

mqueue.h provides the functionality for message queues.

mqd_t mq_open(char *Name, int Flags) - Create/open a message queue
int mq_close(mqd_t *MQ) - Close access to a message queue
int mq_send(mqd_t MQ, char *Msg, int Size, uint Prio) - Block and wait until MQ has space
int mq_receive(mqd_t MQ, char *Msg, int Size, uint *Prio) - Receive highest priority message in MQ

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