C语言 c语言中if(!variable_name)是什么意思
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What does if (! variable_name) mean in c language
提问by Ask_it
Here is the problem
这是问题所在
int main() {
int pid = fork();
if (!pid) {
// condition 1
} else {
// condition 2
}
return 0;
}
What does (!pid)do?
有什么作用(!pid)？
回答by md5
It is equivalent to:
它相当于：
if (!pid != 0) /* ... */
And then:
进而：
if (pid == 0) /* ... */
C11 (n1570), § 6.5.3.3 Unary arithmetic operators
The result of the logical negation operator
!is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has typeint. The expression!Eis equivalent to(0==E).
C11 (n1570), § 6.5.3.3 一元算术运算符
!如果其操作数的值比较不等于 0，则逻辑否定运算符的结果为 0，如果其操作数的值比较等于 0，则结果为 1。结果的类型为int。表达式!E等价于(0==E)。
回答by Roman Bodnarchuk
It means negation. In your case condition 1will be executed in a parent process, condition 2in a child process.
意思是否定。在您的情况下，condition 1将在父进程中执行，condition 2在子进程中。
回答by Luca Davanzo
if(!pid)
Is as you wrote:
就像你写的：
if(pid == 0) {
/* do something */
}
And then:
进而：
if(pid)
is
是
if(pid != 0)
回答by Mihai Stancu
In plain old C there is not boolean data type but there is boolean logic.
在普通的旧 C 中，没有布尔数据类型，但有布尔逻辑。
Numeric values all evaluate to trueexcept for 0 which evaluates to false.
true除了 0 的计算结果为之外，所有数值都计算为false。
The consequence of this is the fact that if you want to test if a condition is true you are in fact comparing it to 0.
这样做的结果是，如果你想测试一个条件是否为真，你实际上是在将它与 0 进行比较。
Comparison operators in C yield a trueor falseresult meaning they return a numeric 1 or 0.
C 中的比较运算符产生 atrue或false结果，这意味着它们返回数字 1 或 0。
The negation operator inverts trueinto false.
否定运算符反转true为false。
回答by Andrejs Cainikovs
!pidexpression will be true if pidequals to zero.
!pid如果pid等于零，则表达式为真。
Basically, it's the same as:
基本上，它与以下内容相同：
if (pid == 0) {
}
回答by Philipp
The !-operator negates a logical condition.
!-运算符否定逻辑条件。
In C, a numerical value of 0 is considered a logical false, any other numerical value a logical true. The !-operator negates a logical condition, so when pid is 0 it's true and when pid is not 0, it's false.
在 C 中，数值 0 被认为是逻辑假，任何其他数值都被认为是逻辑真。!-运算符否定逻辑条件，因此当 pid 为 0 时为真，当 pid 不为 0 时为假。
You could read it as "when there is no pid".
您可以将其读作“当没有 pid 时”。
回答by Boundless
Things that are false: false, 0, null Things that are true: everything else.
错误的事情：false, 0, null 正确的事情：其他一切。
For if !pid to be true, pid would need to be 0, false, or null
如果 !pid 为真，pid 需要为 0、false 或 null
回答by Klas Lindb?ck
(!pid)is equivalent to (pid == 0)
(!pid)相当于 (pid == 0)
Actually, the code in the question is bugged. fork()can return one of three things:
实际上，问题中的代码被窃听了。fork()可以返回以下三件事之一：
>0The process id of the child process. This is returned to the parent.
>0子进程的进程ID。这将返回给父级。
0This is returned to the child.
0这是返回给孩子的。
<0Failure. This is returned to the parent.
<0失败。这将返回给父级。
Check this tutorialfor the proper use of the return value from fork.
检查本教程以正确使用fork.
回答by anishsane
I guess all the answers posted are correct. In one line, (!pid) would be equivalent to (pid==0).
I however take the opportunity to explain how/why it is used here.
我想所有发布的答案都是正确的。在一行中，(!pid) 等价于 (pid==0)。
然而，我借此机会解释如何/为什么在这里使用它。
fork()function will create a subprocess A.K.A. child process. So when the function returns, at that instance there will be 2 processes, which are executing at the same location code-wise. Thus it will return in two copies of the process - we call them as parent & child.
fork()函数将创建一个子进程又名子进程。因此，当函数返回时，在该实例中将有 2 个进程，它们以代码方式在同一位置执行。因此，它将在进程的两个副本中返回——我们称它们为父和子。
The return value of fork()is the PIDof the child, when it is returning in the parent process. & it is = 0 in child process. In your code, pid = fork();will capture this return value.
的返回值fork()是子进程的PID，当它在父进程中返回时。& 在子进程中为 = 0。在您的代码中，pid = fork();将捕获此返回值。
If (pid==0) then you are in child process. If (pid != 0) you are in parent process. Based on whether you are in parent or child, you can change the behavior of your code. (e.g. you may call exec in child & wait in parent.)
如果 (pid==0) 则您处于子进程中。如果 (pid != 0) 你在父进程中。根据您是父级还是子级，您可以更改代码的行为。（例如，您可以在 child 中调用 exec 并在 parent 中等待。）
For more details about fork function, google for fork().
有关 fork 功能的更多详细信息，请 google for fork().
