c - 写 3 条指令是否用逗号分隔 `,` 未定义行为？

Question

我想我在某处看到了用逗号分隔的多于 1 条指令 ,是未定义的行为。

那么下面的代码会产生未定义的行为吗？

for (i=0, j=3, k=1; i<3 && j<9 && k<5; i++, j++, k++) {
    printf("%d %d %d\n", i, j, k);
}

因为有 3 条指令用逗号分隔 , :

i++, j++, k++

Answer 1

writing more than 1 instruction separated by comma , is undefined behaviour.

不，这不是一般情况。

在您的情况下，i++, j++, k++是完全有效的。

FWIW，根据 C11 ，第 6.5.17 章，Comma operator (强调我的)

The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; [...]

[注意]:您可能会因为看到类似的东西而感到困惑

  printf("%d %d %d", i++, ++i, i);

一种声明，但请注意，,完全不是逗号运算符(而是提供参数的分隔符)并且不会发生排序。所以，这类陈述是UB。

同样，引用标准，同一章节的脚注 3

^{As indicated by the syntax, the comma operator (as described in this subclause) cannot appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists of initializers).}