为什么在大括号括C $ C $的C块？ [英] Why enclose blocks of C code in curly braces?

问题描述

我在看一些C code，并已注意到它充满了周围的code块没有任何类型的控制结构的这些花括号。看看看的：

I am looking at some C code, and have noticed it is full of these curly braces surrounding blocks of code without any sort of control structure. Take a look-see:

//do some stuff . . .
fprintf(stderr, "%.2f sec\n", (float)(clock() - t) / CLOCKS_PER_SEC);
{
    //a block! why not?
    char *tmp_argv[3];
    tmp_argv[0] = argv[0]; tmp_argv[1] = str; tmp_argv[2] = prefix;
    t = clock();
    fprintf(stderr, "[bwa_index] Convert nucleotide PAC to color PAC... ");
    bwa_pac2cspac(3, tmp_argv);
    fprintf(stderr, "%.2f sec\n", (float)(clock() - t) / CLOCKS_PER_SEC);
}

为什么你会插入这样的块中的code？这是座充满了他们。是否有某种性能优势？有些神秘的东西Ç？为什么???

Why would you insert blocks like this in the code? It is chock full of 'em. Is there some kind of performance benefit? Some mystical C thing? Why???

编辑：：此code。如果从 BWA ，即对齐小序列的生物信息学计划使用 Burrows-Wheeler变换，在任何情况下，你想知道。此$ C $℃实施例不是本应用程序的功能特别相关。

edit: This code if from BWA, a bioinformatics program that aligns small sequences to large reference ones using the Burrows-Wheeler transform, in case any of you were wondering. This code example isn't particularly relevant to the functionality of the application.

推荐答案

在C89，你不能只是做 INT I; 的任何地方;声明是只在代码块的开始有效。

Legacy code needed { } in order to do declarations at all

In C89, you couldn't just do int i; anywhere; declarations were only valid at the beginning of blocks.

所以：

a = 1;
int i; /* error */
i = 2;

...不是有效的，但

...wasn't valid, but

a = 1
if (e) {
  int i;

......很好，因为是一个简单的块。

...was fine, as was a plain block.

由此产生的风格继续即使后声明成为有效的（C99）的块项目的（S），部分原因是惯性，部分是为了向后便携性，还因为它是有道理的，建立新的作用域声明。

The resulting style continued even after declarations became valid (C99) block-item(s), partly by inertia, partly for backwards portability, and also because it makes sense to establish a scope for new declarations.

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