B. VB. B. VB. - 1 year ago 64
C Question

Anonymous functions using GCC statement expressions

This question isn't terribly specific; it's really for my own C enrichment and I hope others can find it useful as well.

Disclaimer: I know many will have the impulse to respond with "if you're trying to do FP then just use a functional language". I work in an embedded environment that needs to link to many other C libraries, and doesn't have much space for many more large shared libs and does not support many language runtimes. Moreover, dynamic memory allocation is out of the question. I'm also just really curious.

Many of us have seen this nifty C macro for lambda expressions:

#define lambda(return_type, function_body) \
({ \
return_type __fn__ function_body \
__fn__; \

And an example usage is:

int (*max)(int, int) = lambda (int, (int x, int y) { return x > y ? x : y; });
max(4, 5); // Example

gcc -std=c89 -E test.c
, the lambda expands to:

int (*max)(int, int) = ({ int __fn__ (int x, int y) { return x > y ? x : y; } __fn__; });

So, these are my questions:

  1. What precisely does the line int (*X); declare? Of course, int * X; is a pointer to an integer, but how do these two differ?

  2. Taking a look at the exapnded macro, what on earth does the final
    do? If I write a test function
    void test() { printf("hello"); } test;
    - that immediately throws an error. I do not understand that syntax.

  3. What does this mean for debugging? (I'm planning to experiment myself with this and gdb, but others' experiences or opinions would be great). Would this screw up static analyzers?

Answer Source

This declaration (at block scope):

int (*max)(int, int) =
    int __fn__ (int x, int y) { return x > y ? x : y; }

is not C but is valid GNU C.

It makes use of two gcc extensions:

  1. nested functions
  2. statement expressions

Both nested functions (defining a function inside a compound statement) and statement expressions (({}), basically a block that yields a value) are not permitted in C and come from GNU C.

In a statement expression, the last expression statement is the value of the construct. This is why the nested function __fn__ appears as an expression statement at the end of the statement expression. A function designator (__fn__ in the last expression statement) in a expression is converted to a pointer to a function by the usual conversions. This is the value used to initialize the function pointer max.