Chapter 6. Coding Suggestions

Steve French

Simo Sorce

Andrew Bartlett

Tim Potter

Martin Pool

So you want to add code to Samba ...

One of the daunting tasks facing a programmer attempting to write code for Samba is understanding the various coding conventions used by those most active in the project. These conventions were mostly unwritten and helped improve either the portability, stability or consistency of the code. This document will attempt to document a few of the more important coding practices used at this time on the Samba project. The coding practices are expected to change slightly over time, and even to grow as more is learned about obscure portability considerations. Two existing documents samba/source/internals.doc and samba/source/architecture.doc provide additional information.

The loosely related question of coding style is very personal and this document does not attempt to address that subject, except to say that I have observed that eight character tabs seem to be preferred in Samba source. If you are interested in the topic of coding style, two oft-quoted documents are:

http://lxr.linux.no/source/Documentation/CodingStyle

http://www.fsf.org/prep/standards_toc.html

But note that coding style in Samba varies due to the many different programmers who have contributed.

Following are some considerations you should use when adding new code to Samba. First and foremost remember that:

Portability is a primary consideration in adding function, as is network compatibility with de facto, existing, real world CIFS/SMB implementations. There are lots of platforms that Samba builds on so use caution when adding a call to a library function that is not invoked in existing Samba code. Also note that there are many quite different SMB/CIFS clients that Samba tries to support, not all of which follow the SNIA CIFS Technical Reference (or the earlier Microsoft reference documents or the X/Open book on the SMB Standard) perfectly.

Here are some other suggestions:

  1. use d_printf instead of printf for display text reason: enable auto-substitution of translated language text

  2. use SAFE_FREE instead of free reason: reduce traps due to null pointers

  3. don't use bzero use memset, or ZERO_STRUCT and ZERO_STRUCTP macros reason: not POSIX

  4. don't use strcpy and strlen (use safe_* equivalents) reason: to avoid traps due to buffer overruns

  5. don't use getopt_long, use popt functions instead reason: portability

  6. explicitly add const qualifiers on parm passing in functions where parm is input only (somewhat controversial but const can be #defined away)

  7. when passing a va_list as an arg, or assigning one to another please use the VA_COPY() macro reason: on some platforms, va_list is a struct that must be initialized in each function...can SEGV if you don't.

  8. discourage use of threads reason: portability (also see architecture.doc)

  9. don't explicitly include new header files in C files - new h files should be included by adding them once to includes.h reason: consistency

  10. don't explicitly extern functions (they are autogenerated by "make proto" into proto.h) reason: consistency

  11. use endian safe macros when unpacking SMBs (see byteorder.h and internals.doc) reason: not everyone uses Intel

  12. Note Unicode implications of charset handling (see internals.doc). See pull_* and push_* and convert_string functions. reason: Internationalization

  13. Don't assume English only reason: See above

  14. Try to avoid using in/out parameters (functions that return data which overwrites input parameters) reason: Can cause stability problems

  15. Ensure copyright notices are correct, don't append Tridge's name to code that he didn't write. If you did not write the code, make sure that it can coexist with the rest of the Samba GPLed code.

  16. Consider usage of DATA_BLOBs for length specified byte-data. reason: stability

  17. Take advantage of tdbs for database like function reason: consistency

  18. Don't access the SAM_ACCOUNT structure directly, they should be accessed via pdb_get...() and pdb_set...() functions. reason: stability, consistency

  19. Don't check a password directly against the passdb, always use the check_password() interface. reason: long term pluggability

  20. Try to use asprintf rather than pstrings and fstrings where possible

  21. Use normal C comments / * instead of C++ comments // like this. Although the C++ comment format is part of the C99 standard, some older vendor C compilers do not accept it.

  22. Try to write documentation for API functions and structures explaining the point of the code, the way it should be used, and any special conditions or results. Mark these with a double-star comment start / ** so that they can be picked up by Doxygen, as in this file.

  23. Keep the scope narrow. This means making functions/variables static whenever possible. We don't want our namespace polluted. Each module should have a minimal number of externally visible functions or variables.

  24. Use function pointers to keep knowledge about particular pieces of code isolated in one place. We don't want a particular piece of functionality to be spread out across lots of places - that makes for fragile, hand to maintain code. Instead, design an interface and use tables containing function pointers to implement specific functionality. This is particularly important for command interpreters.

  25. Think carefully about what it will be like for someone else to add to and maintain your code. If it would be hard for someone else to maintain then do it another way.

The suggestions above are simply that, suggestions, but the information may help in reducing the routine rework done on new code. The preceding list is expected to change routinely as new support routines and macros are added.