为什么我们不能使用预处理器来创建自定义分隔的字符串呢？

Question

#define STR_START "
#define STR_END "

int puts(const char *);

int main() {
    puts(STR_START hello world STR_END);
}

当我用gcc编译它时（注意：与clang类似的错误），它失败了，出现了以下错误：

$ gcc test.c
test.c:1:19: warning: missing terminating " character
test.c:2:17: warning: missing terminating " character
test.c: In function ‘main’:
test.c:7: error: missing terminating " character
test.c:7: error: ‘hello’ undeclared (first use in this function)
test.c:7: error: (Each undeclared identifier is reported only once
test.c:7: error: for each function it appears in.)
test.c:7: error: expected ‘)’ before ‘world’
test.c:7: error: missing terminating " character

这有点让我困惑，所以我在预处理器中运行它：

$ gcc -E test.c
# 1 "test.c"
# 1 ""
# 1 ""
# 1 "test.c"
test.c:1:19: warning: missing terminating " character
test.c:2:17: warning: missing terminating " character

int puts(const char *);

int main() {
    puts(" hello world ");
}

尽管有警告，它仍生成完全有效的代码（粗体文本）！

Answer 1

问题是,即使代码扩展为“hello,world”,预处理器仍不能将其识别为单个字符串文字标记；相反，它被识别为令牌“,hello,,,世界,”的（无效）序列。

N1570:

请注意，在此定义下，'和“都不是标点符号。