1 / 23

Stream Socket Programming

Stream Socket Programming. Idioms and pitfalls. Stream Socket Characteristics. Transmissions across a stream socket are considered to be a continuous stream of bytes. Any other structure must be created by the applications participating in the communication.

janice
Download Presentation

Stream Socket Programming

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Stream Socket Programming Idioms and pitfalls

  2. Stream Socket Characteristics • Transmissions across a stream socket are considered to be a continuous stream of bytes. • Any other structure must be created by the applications participating in the communication. • "Message" boundaries are not guaranteed to be preserved. • Most applications DO want to communicate in terms of a series of separate messages.

  3. Application Protocols • Rules the processes involved in your application use to communicate. • Includes: • Allowed message types (formats) • Rules about when each message type can be sent. • Programming language structures don't always map exactly onto your message formats

  4. Banking Example • Report total deposit amount (in pennies), total withdrawal amount (in pennies), number of deposits, number of withdrawals. • Design decision #1: encoding scheme • Character strings of digits • Binary integer values

  5. Character Encoding • Advantages • No limit on size of values that can be encoded • No byte ordering issues • Disadvantages • Inefficient • Easy to get buffer size wrong or waste space • Must be careful about delimiters

  6. Binary numeric encoding • Advantages • Uses fewer bits to represent a given value • Fields in message are a fixed number of bytes • Disadvantages • Byte ordering is significant – must use hton/ntoh • Building structs to represent messages isn't always straightforward (alignment issues)

  7. Alignment rules • Compilers lay out structs to maximize alignment. • Fields are allocated in the order they are declared • A data value is aligned if its address is a multiple of its size (e.g. 32 bit ints – 4 bytes – at addresses divisible by 4) • A struct is aligned if its address is a multiple of the size of the largest data type it contains. • Unnamed padding bytes are added to keep struct members aligned.

  8. Example struct mixedData { char Data1; short Data2; int Data3; char Data4; }; Total data bytes = 1 + 2 + 4 + 1 = 8

  9. Example struct MixedData /* after compilation */ { char Data1; char Padding0[1]; /* So following 'short' is on a 2 byte boundary */ short Data2; int Data3; char Data4; char Padding1[3]; }; Total bytes = 1 + 1 + 2 + 4 + 1 + 3 = 12 Size is a multiple of sizeof(int)

  10. To avoid padding: struct mixedData2 { int Data3; short Data2; char Data1; char Data4; }; Data items declared in order of decreasing size. Assuming actual space needed is a multiple of size of largest data item, no padding needed. sizeof(mixedData2) = 8

  11. Bank Example struct bankMsg { int depositAmt; short depositCnt; int withdrawAmt; short withdrawCnt; }; • Total data size is 4 + 2 + 4 + 2 bytes = 12 bytes • On RHEL 5, using gcc, sizeof(bankMsg) = 16. Why?

  12. No padding needed: struct bankMsg { int depositAmt; int withdrawAmt; short depositCnt; short withdrawCnt; }; You can also add the padding to your definition so it is explicit. (Recall sockaddr_in)

  13. Parsing received messages • If the fields are fixed size, we can just send and receive the associated struct: struct bankMsg msg; void *buffer = (void *) &msg; msg.depositAmt = 2324234; msg.withdrawAmt = 2232344; msg.depositCnt = 50; msg.withdrawCnt = 42; send(s, &msg, sizeof(bankMsg), 0);

  14. Parsing received messages • In the receiving process: struct bankMsg msg; void *buffer = (void *) &msg; int rbytes, rv; ... for (rbytes = 0; rbytes < sizeof(msg); rbytes += rv){ if ((rv = recv(s, buffer + rbytes, sizeof(msg) – rbytes, 0)) < 0) /* handle error */ } /* Fix byte order! */ msg.depositAmt = ntohl(msg.depositAmt); ...

  15. Portability/Compatibility • The C/C++ standards don't specify alignment rules – left up to compiler implementations • Say you don't pay attention to padding when you define message format structs in your code. What can go wrong?

  16. Delimited char strings • Sending multiple messages consisting of variable-length delimited character strings is tricky. • Doing a receive may give you bytes from more than one message. • Depending on how you have structured your parsing routines, it may be complicated to track which bytes of a receive buffer you have parsed.

  17. Delimited char strings • One solution: for each delimited string you expect, receive one byte at a time until you receive the delimiter. • Leaves subsequent strings waiting in the received stream for subsequent calls to recv(). • Downside: slower than multi-byte receives, but if you don't know how many characters to expect, you're kind of stuck.

  18. Working with core dumps • When a program crashes, it can create a file containing the image of the address space of the process at the time of the crash – a "core dump" • Debuggers can examine these files and show you precisely where the error occurred. Good for tracking down segmentation faults.

  19. Requirements • You must compile your program with –g to preserve symbol table information for the debugger • You need to be allowed to create core files in your account. • Use the ulimit –c command to check. If return is 0, no core files will be created. • Change with "ulimit –c unlimited" • Caution: you can only do this once per login session, you can't switch back and forth.

  20. Using core dumps • If a core dump is created, you will see "(core dumped)" in the error message • Linux creates a file called core.n, where n is a unique number • To examine the core file, use gdb (ddd also works): gdb executable-namecorefile-name

  21. What you see • gdb reports where the error occurred, e.g. #0 0x08048370 in a (p=0x0) at test_core.c:11 • int y = *p; • a is the method name • p is the variable that caused the problem • test_core is the name of the executable, 11 is the line number • you can use gdb to examine variable values, etc.

  22. Caveats • Core files are big, and because of the Linux naming convention, you will create a separate one every time a program crashes. • Pay attention to creation dates, and make sure you're examining the latest core dump. • Periodically delete core files. Make core.* one of the things the "clean" target in your makefile cleans up • Don't set ulimit to "unlimited" unless you need to examine a core dump.

  23. A reasonable tutorial • http://www.network-theory.co.uk/articles/gccdebug.html

More Related