Chapter 18 some additional notes

1
Chapter 18 some additional notes

• Interprocess Communication (IPC) in C
• We can create processes using fork()
• Need to exchange data and control information
• Many options are available to use
• Pipes
• Signals
• Message Queues
• Semaphores
• Shared Memory
• Sockets
• Well look at two of them
• pipes and sockets

2
Chapter 18 some additional notes

• Pipe
• Pipe can be used to transfer data
• think of the C pipe like a conduit for data
• Parent and child can both read/write from/to the
  pipe
• Programmer decides
• Parent writes, child reads
• Child writes, parent reads
• Parent and child both read and write
• this is the default
• Pipe is available in the stdio.h
• Syntax is straightforward, for example
• int fd2
• pipe(fd) // pass integer array of size
  2 - for file descriptors

3
Chapter 18 some additional notes

• Pipe and Fork
• Create the pipe first, then fork the child
  process

Parent
Child
Pipe
Kernal
4
Chapter 18 some additional notes

• Pipe
• Read from the pipe?
• Can I read the data in the pipe more than once?
• No
• See pipe_read_test.c example
• Write to the pipe?
• Can I overwrite data that hasnt been read yet?
• No
• See pipe_write_test.c example

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Chapter 18 some additional notes

• Overview of example code
• kcooper/public_html/teaching/5375/IPC_pipe/pipe.c
  
• Here, parent writes to pipe, child reads from
  pipe
• Create, initialize variables
• Create the pipe
• Create the child process with fork
• If child
• close write fd1
• read from pipe
• Else parent
• close read fd0
• write to pipe

Reminder. Parent and child are two separate
processes that can be executed by the CPU
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Chapter 18 some additional notes

• Pipe call
• pipe(fd), where fd is an integer array of size 2
• fd0 read or write?
• fd1 read or write?
• Read call
• read(fd0,buf,100)
• Write call
• write(fd1, something to write , 19) or
• write(fd1, my_char_array, strlen(my_char_array)
  )

fd0 which file descriptor to use buf where to
store the read values 100 how many characters
to read
fd1 which file descriptor to use something to
write what to write 19 how many characters to
write
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Chapter 18 some additional notes

• Synchronizing the reads and writes?
• A simple way is to use sleep call
• simple
• not very precise
• If child writes and parent reads, then could use
  wait call in parent
• simple
• doesnt work the other way around (parent writes,
  child reads)
• More sophisticated solutions are possible
• E.g., use signals
• more advanced
• Note. well stay with the first two options