选题

How are C data types “supported directly by most computers”?

sources/talk/20150122 How are C data types “supported directly by most computers”.md

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+How are C data types “supported directly by most computers”?
+---------
+
+#Q:
+
+I am reading K&R's *The C Programming Language*” and came across this statement [Introduction, p. 3]:
+
+>Because the data types and control structures provided by C are supported directly by most computers, the run-time library required to implement self-contained programs is tiny.
+
+What does the bolded statement mean? Is there an example of a data type or a control structure that isn't supported directly by a computer?
+
+#A:
+
+Yes, there are data types not directly supported.
+
+On many embedded systems, there is no hardware floating point unit. So, when you write code like this:
+
+```C
+float x = 1.0f, y = 2.0f;
+return x + y;
+```
+
+It gets translated into something like this:
+
+```C
+unsigned x = 0x3f800000, y = 0x40000000;
+return _float_add(x, y);
+```
+
+Then the compiler or standard library has to supply an implementation of `_float_add()`, which takes up memory on your embedded system. If you're counting bytes on a really tiny system, this can add up.
+
+Another common example is 64-bit integers (`long long` in the C standard since 1999), which are not directly supported by 32-bit systems. Old SPARC systems didn't support integer multiplication, so multiplication had to be supplied by the runtime. There are other examples.
+
+##Other languages
+
+By comparison, other languages have more complicated primitives.
+
+For example, a Lisp symbol requires a lot of runtime support, just like tables in Lua, strings in Python, arrays in Fortran, et cetera. The equivalent types in C are usually either not part of the standard library at all (no standard symbols or tables) or they are much simpler and don't require much runtime support (arrays in C are basically just pointers, nul-terminated strings are almost as simple).
+
+##Control structures
+
+A notable control structure missing from C is exception handling. Nonlocal exit is limited to `setjmp()` and `longjmp()`, which just save and restore certain parts of processor state. By comparison, the C++ runtime has to walk the stack and call destructors and exception handlers.
+
+----
+via:[stackoverflow](http://stackoverflow.com/questions/27977522/how-are-c-data-types-supported-directly-by-most-computers/27977605#27977605)
+
+作者：[Dietrich Epp][a]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
+
+[a]:http://stackoverflow.com/users/82294/dietrich-epp