如何检查scanf返回的字符串是否为null [英] How to check if a string returned by scanf is null

问题描述

我正在使用 scanf()从用户读取输入字符串.

我要检查此字符串是否为NULL(\ 0).

I want to check if this string is NULL (\0) or not.

这是我的代码:

#include<stdio.h>

char *argument; // argument for mycat
scanf("%s", &argument);

if(fork()==0)       // at child
{
    printf("Child process: about to execute \"mycat %s\"\n", &argument);
    fflush(stdout);
    if(strcmp(argument, "") == 0) // <-- Here is the problem
    {

        execlp("mycat", "mycat", &argument, NULL);      // execute child process
    }
    execlp("mycat","mycat", NULL);
}

我正在Red Hat 6.1上使用g ++编译器进行编译

I'm compiling with g++ compiler on Red Hat 6.1

问题是我无法为 if 语句甚至不能与一起使用 argument 取消引用.> strlen().

The problem is that I'm not able to dereference argument either for the if statement or even for use with strlen().

推荐答案

NULL 和 \ 0 并不是同一个东西，尽管它们的求和结果均为零. NULL 是零指针，即我们用于null指针的指针. \ 0 是ASCII码为零的字符，否则称为 NUL (一个'L')，即是一个值为0的 char .

NULL and \0 are not the same thing, though they both evaluate to zero. NULL is a pointer zero, i.e. it's what we use for a null pointer. \0 is the character with ASCII number zero, otherwise known as NUL (one 'L'), i.e. is a char with value 0.

char * 为 NULL (即内存中根本没有字符串)或字符串为空(即仅包含一个 char ，即 \ 0 ，否则称为 NUL ).

There is an important difference between a char * being NULL (i.e. there not being a string in memory at all), or the string being empty (i.e. only containing one char which is \0 otherwise known as NUL).

要测试第一个:

if (!string)

或者如果您想更详细些:

or if you like to be more verbose:

if (string == NULL)

要测试第二个:

if (!string[0])

或者如果您想更详细些:

or if you like to be more verbose:

if (string[0] == 0)

很明显，如果您需要同时测试两者，请先测试第一个，然后再测试第二个，因为如果 string 为 NULL ，则第二个将取消引用空指针.

Obviously if you need to test for both, test for the first then the second, as the second would dereference a null pointer if string was NULL.

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