如何设置2个字节的wchar_t输出? [英] How to set 2 byte wchar_t output?

问题描述

默认情况下，GCC使用4字节的wchar_t.我可以将选项-fshort-wchar设置为在L"string constants"中每个wchar_t获得2个字节.但是，当我将编译器选项设置为源文件时，会收到著名的警告消息

The GCC uses a 4-byte wchar_t by default. I can set the option -fshort-wchar to get 2 bytes per wchar_t in the L"string constants". But when I set the compiler option to my source file I get the famous warning message

foo.o使用2字节wchar_t，但输出将使用4字节wchar_t.在对象之间使用wchar_t值可能会失败

foo.o uses 2-byte wchar_t yet the output is to use 4-byte wchar_t; use of wchar_t values across objects may fail

因为我真的想要2字节wchar_t，所以我也希望输出使用此变体.有链接器选项可以告诉我我想要什么吗?

Since I really want 2-byte wchar_t I also want the output to use this variant. Is there any linker option to tell it what I want?

修改

此警告不会阻止链接器产生有效的输出.但是数十种虚假警告涵盖了其他消息.

This warning doesn't inhibit the linker to produce a valid output. But the dozens of false warnings cover other messages.

推荐答案

在binutils中，您可以在bfd/elf32-arm.c中以以下方式找到此错误消息:

In binutils you can find this error message in bfd/elf32-arm.c as:

警告:％B使用％u字节wchar_t，但输出将使用％u字节 wchar_t;在对象之间使用wchar_t值可能会失败"

"warning: %B uses %u-byte wchar_t yet the output is to use %u-byte wchar_t; use of wchar_t values across objects may fail"

但是，如果您进一步看一下binutils，您会发现输出的wchar_t大小不会在任何地方初始化为4.那么，什么决定了输出wchar_t大小"呢?实际上，赋予ld的第一个对象初始化输出属性.接下来的对象将其属性合并到其中.如果与gcc/g ++链接，它将在内部执行ld，因此请尝试gcc -v并查看如何执行ld.这将使您深入了解它隐式链接到可执行文件中的内部对象文件(除了您自己的文件之外).

However, if you look further at binutils, you'd realize that the output's wchar_t size is not initialized to 4 anywhere. So what determines the "output wchar_t size"? Actually, the first object given to ld initializes the output attributes. The next objects merge their attributes into it. If you link with gcc/g++, it executes ld internally, so try gcc -v and see how ld is executed. This will give you insight into what internal object files (in addition to your own) it implicitly links into your executable.

例如，与gcc链接(例如gcc -v -shared -o libfoobar.so foo.o bar.o)会导致调用:

For example, linking with gcc (e.g. gcc -v -shared -o libfoobar.so foo.o bar.o) results in invocation of:

ld ... crtbegin_so.o foo.o bar.o crtend_so.o ...

即实际链接了以下对象(按顺序):

i.e. the following objects are actually linked (in order):

• crtbegin_so.o(暗含)
• foo.o
• bar.o
• crtend_so.o(暗含)

这是ld的作用:

1. 输出属性集开始为空.
2. 合并 crtbegin_so.o 属性.现在输出属性包含out_attr[Tag_ABI_PCS_wchar_t] == 4
3. 合并 foo.o 属性.如果foo.o是用-fshort-wchar构建的，则in_attr[Tag_ABI_PCS_wchar_t] == 2会导致冲突和您看到的警告.

1. The output attributes set starts off empty.
2. Merging crtbegin_so.o attributes. Now output attributes contain out_attr[Tag_ABI_PCS_wchar_t] == 4
3. Merging foo.o attributes. If foo.o was built with -fshort-wchar, then in_attr[Tag_ABI_PCS_wchar_t] == 2 and this will result in a conflict and the warning you're seeing.

如果要在ld命令行上交换crtbegin_so.o和foo.o，则会收到以下警告:

If you were to swap crtbegin_so.o and foo.o on the ld command line, you'd get the following warning instead:

ld:警告: android-ndk-r9d/platforms/android-16/arch-arm/usr/lib/crtbegin_so.o 使用4字节wchar_t，但输出将使用2字节wchar_t；用于 跨对象的wchar_t值可能会失败

ld: warning: android-ndk-r9d/platforms/android-16/arch-arm/usr/lib/crtbegin_so.o uses 4-byte wchar_t yet the output is to use 2-byte wchar_t; use of wchar_t values across objects may fail

如您所见，这不是输入与输出不兼容的问题，而是链接在一起的两个目标文件之间(可感知的)不兼容的问题.

As you can see, it's not a matter of incompatibility of the input with the output, but rather (perceived) incompatibility between two object files linked together.

我们该怎么办?

• 从2008年起，ld支持--no-wchar-size-warning标志可禁止显示此警告.但是正如您所说，不加选择地禁止警告有其弊端.

• As of 2008, ld supports the --no-wchar-size-warning flag to suppress this warning. But as you said, indiscriminately suppressing warnings has its drawbacks.

您可以使用-fshort-wchar重建工具链.

You can rebuild your toolchain with -fshort-wchar.

如果您确实相信Tag_ABI_PCS_wchar_t不可知，则可以从内部gcc对象二进制文件中剥离Tag_ABI_PCS_wchar_t标记.这可能比重建工具链更容易.为此，您可以使用我曾经写过的此实用程序. (您可能需要解包libgcc.a，更改其目标文件并重新打包.)

You can strip the Tag_ABI_PCS_wchar_t tags from your internal gcc object binaries if you truly believe they're sizeof(wchar_t)-agnostic. This might be easier than rebuilding your toolchain. For that, you can use this utility I once wrote. (You might need to unpack libgcc.a, change its object files and repack it.)

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