_Complex long int [英] _Complex long int

问题描述

tl;博士;

问_Complex long int中的int是什么意思?为什么它合法?

Q what does int mean in _Complex long int? Why is it legal?

我正在移植一些 32 位代码以保证 64 位安全.在一个地方，我注意到它使用:

I was porting some 32-bit code to be 64-bit safe. In one place I noted it uses:

static __complex__ long int i32X[256];

一个愚蠢的搜索和替换为 int32_t 更改long int"给了我一个编译错误.__complex__ 是一种较旧的 GNUism，已被标准 _Complex 取代.这是重现问题的简短代码片段:

A dumb search and replace changing "long int" for int32_t gave me a compilation error. __complex__ is an older GNUism which has been replaced by the standard _Complex. Here is a short code snippet to reproduce the issue:

#include <complex.h>
#include <inttypes.h>

typedef _Complex long int WhyAmILegal;

typedef _Complex int32_t Snafu;

typedef int32_t _Complex Snafu2;

在gcc下编译给出:

complextest.c:6:26: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’     before ‘Snafu’
typedef _Complex int32_t Snafu;
                      ^~~~~
complextest.c:8:17: error: two or more data types in declaration specifiers
typedef int32_t _Complex Snafu2;

(草案)C11 标准 说:

"共有三种复杂类型，指定为float _Complex、double_Complex, and long double _Complex.43) (复杂类型是有条件的实现不需要支持的功能；见 6.10.8.3.)真正的浮动类型和复杂类型统称为浮动类型."

"There are three complex types, designated as float _Complex, double _Complex, and long double _Complex.43) (Complex types are a conditional feature that implementations need not support; see 6.10.8.3.) The real floating and complex types are collectively called the floating types."

所以这里的错误似乎是当只有浮点复杂类型是合法的时，我试图请求一个整数复杂类型._Complex long的意思其实更像是_Complex long double.我认为这可能是 gcc 解析器中的一个错误，但是

So the bug here would seem to be that I am trying to request an integral complex type when only floating point complex types are legal. The meaning of _Complex long is actually more like _Complex long double. I thought it might be a bug in gcc's parser, however

_Complex long int

在 clang online 下也能很好地编译.

compiles just fine under clang online as well.

那么为什么 int 在这里是合法的".

So why is int 'legal' here.

感谢@Keith-Thompson 指出整数复数是一种 GNU 扩展并且受 clang 支持.

Thanks to @Keith-Thompson's for pointing out that integral complex numbers are a GNU extension and one that is supported by clang.

另一个数据点.以下程序:

Another data point. The following program:

#include <complex.h>
#include <inttypes.h>
#include <stdio.h>

typedef _Complex long int ComplexLong;
typedef _Complex short int ComplexShort;
typedef _Complex int ComplexInt;

int main(void)
{
  fprintf(stderr,"sizeof(_Complex long int)=%d
",sizeof(ComplexLong));
  fprintf(stderr,"sizeof(_Complex int)=%d
",sizeof(ComplexInt));
  fprintf(stderr,"sizeof(_Complex short int)=%d
",sizeof(ComplexShort));
  fprintf(stderr,"sizeof(short int)=%d
",sizeof(short int));
  fprintf(stderr,"sizeof(int)=%d
",sizeof(int));
  fprintf(stderr,"sizeof(long int)=%d
",sizeof(long int));
  return 0;
}

编译使用:

all: complextest complextest32

complextest: complextest.c
    $(CC) -o$@ $<

complextest32: complextest.c
    $(CC) -m32 -o$@ $<

when(使用gcc编译)并运行给出:

when (compiled using gcc) and run gives:

>./complextest
sizeof(_Complex long int)=16
sizeof(_Complex int)=8
sizeof(_Complex short int)=4
sizeof(short int)=2
sizeof(int)=4
sizeof(long int)=8
>./complextest32
sizeof(_Complex long int)=8
sizeof(_Complex int)=8
sizeof(_Complex short int)=4
sizeof(short int)=2
sizeof(int)=4
sizeof(long int)=4

所以 _Complex long int 像 long 不能以架构不变的方式指定.您应该使用 _Complex int 或 _Complex short 以稍微提高可移植性.

So _Complex long int like long cannot be specified in an architecture invariant manner. You should use _Complex int or _Complex short to be slightly more portable.

推荐答案

这是一个 gcc 扩展，你可以通过在(或多或少)符合 C 模式下编译看到:

It's a gcc extension, as you can see by compiling in (more or less) conforming C mode:

$ cat c.c
_Complex double cd;
_Complex long int cli;
$ gcc -c c.c
$ gcc -c -std=c11 -pedantic c.c
c.c:2:1: warning: ISO C does not support complex integer types [-Wpedantic]
 _Complex long int cli;
 ^~~~~~~~
$

这是记录在 gcc 手册中:

ISO C99 支持复杂的浮点数据类型，并作为 GCC 的扩展在 C90 模式和 C++ 中支持它们.GCC 还支持复杂的不属于 ISO C99 的整数数据类型.

ISO C99 supports complex floating data types, and as an extension GCC supports them in C90 mode and in C++. GCC also supports complex integer data types which are not part of ISO C99.

毫无疑问，clang 支持相同的扩展.

clang, not surprisingly, supports the same extension.

要回答添加的问题，_Complex 是一个类型说明符._Complex int32_t 无效，原因与 unsigned int32_t 无效.类型说明符不能应用于 typedef.

To answer the added question, _Complex is a type specifier. _Complex int32_t is invalid for the same reason unsigned int32_t is invalid. Type specifiers can't be applied to typedefs.

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