Network Programming 2014-10-09

2014/10/09 Network Programming Class Note

• Return value of read() equals 0 means that it is the end (blocking mode)
  • pipe: the write end has been closed
  • file: end of file
• create pipe 時 read 的 FD 先於 write 的 FD, e.g. read = 3 and write = 4
• cat f1 | sort | lpr
  • csh
    • csh fork 3 children
    • csh create 2 pipes
    • close unused resources, i.e. 4-6
  • child1 (cat): 不修改到原來程式的前提下竄改 FD table 中的 stdout
    • close 1
    • dup 4: to replace stdout (FD table round-robin)
    • close 3-6: avoid side effect (referencing)
    • exec cat
  • child2 (sort): 不修改到原來程式的前提下竄改 FD table 中的 stdin & stdout
    • close 1-2
    • dup 3: to replace stdin
    • dup 6: to replace stdout
    • close 3-6
    • exec sort
  • child3 (lpr): 不修改到原來程式的前提下竄改 FD table 中的 stdin
    • close 2
    • dup 5
    • close 3-6
    • exec lpr

FD table in above example

+---+---------------+
| 0 | stdin         |
+---+---------------+
| 1 | stdout        |
+---+---------------+
| 2 | stderr        |
+---+---------------+
| 3 | pipe 1 read   |
+---+---------------+
| 4 | pipe 1 write  |
+---+---------------+
| 5 | pipe 2 read   |
+---+---------------+
| 6 | pipe 2 write  |
+---+---------------+

• If you do not close unused read end or write end of a pipe, you may encounter some throny problems.