Statement in C++ macro

Question

Reading chromium code, found helpful macro for handling EINTR errno of system calls on POSIX compliant systems. Here are the code(base/posix/eintr_wrapper.h):

#define HANDLE_EINTR(x) ({ \ decltype(x) eintr_wrapper_result; \ do { \ eintr_wrapper_result = (x); \ } while (eintr_wrapper_result == -1 && errno == EINTR); \ eintr_wrapper_result; \ })

The question is what is the role of last statement in macro eintr_wrapper_result; ? If we use commas instead of semicolons - it will be clear - to return the result of last operation(comma operator). But what is purpose in this case?

See possible duplicate: Are compund statements (blocks) surrounded by parens expressions in ANSI C? ... as my answer to the that question says: last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. — Shafik Yaghmour
– Shafik Yaghmour, Commented Jun 1, 2015 at 14:55

Quentin · Accepted Answer · 2015-06-01 14:55:19Z

9

This macro uses the Statement-Expressions GCC extension. The last expression in the inner block serves as the value for the whole, once it has been executed, much like the comma operator.

answered Jun 1, 2015 at 14:55

Quentin

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2 Comments

simonwo Over a year ago

For interest, why not just use the comma operator?

Quentin Over a year ago

@simonwo Because do{}while() is not an expression, and thus can't be an operand of the comma operator.

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