Wrapping C Constants

When SWIG encounters C preprocessor macros and C declarations that look like constants, it creates Tcl variables with an identical value. Click here to see a SWIG interface with some constant declarations in it.

Accessing Constants from Tcl

Key points

• The values of preprocessor macros are converted into Tcl read-only variables.
• Types are inferred by syntax (e.g., "3" is an integer and "3.5" is a float).
• Character constants such as 'x' are converted into strings.
• C string literals such as "Hello World" are converted into strings.
• Macros that are not fully defined are simply ignored. For example:

  #define EXTERN extern
  

  is ignored because SWIG has no idea what type of variable this would be.

• Expressions are allowed provided that all of their components are defined. Otherwise, the constant is ignored.
• Certain C declarations involving 'const' are also turned into Tcl constants.
• The constants that appear in a SWIG interface file do not have to appear in any sort of matching C source file since the creation of a constant does not require linkage to a stored value (i.e., a value held in a C global variable or memory location).
• Since constants are turned into Tcl variables, you have to use the global statement when accessing from a procedure. For example:

  proc foo {} {
      global ICONST          # Some C constant
      puts $ICONST
  }