问题描述
我不清楚字符指针及其工作原理.
I am not so clear on character pointer and how they work.
该程序已构建，但在运行时崩溃.
The program builds, but crashes when I run it.
char *ab = NULL;
//ab = "abc123"; // works fine
sprintf(ab, "abc%d", 123); // this line seems to crash the program
当sprintf将(char * str)作为第一个参数时，我不知道这怎么错.
I don't get how this can be wrong, when sprintf takes in a (char * str) as a first argument.
任何人都可以向我解释一下吗?
Can anyone please explain this to me?
推荐答案
您尚未分配要与ab一起使用的内存.
You have allocated no memory to use with ab.
第一个分配起作用是因为您要向ab分配一个字符串常量:"abc123".常量字符串的内存由编译器代表您提供:您不需要分配此内存.
The first assignment works because you are assigning to ab a string constant: "abc123". Memory for constant strings are provided by the compiler on your behalf: you don't need to allocate this memory.
例如，在将ab与sprintf，您需要使用malloc分配一些内存，并将该空间分配给ab:
Before you can use ab with e.g. sprintf, you'll need to allocate some memory using malloc, and assign that space to ab:
ab = malloc(sizeof(char) * (NUM_CHARS + 1));
然后，只要您使用malloc留出了足够的空间，您的sprintf就可以工作.注意:+ 1用于为空终止符.
Then your sprintf will work so long as you've made enough space using malloc. Note: the + 1 is for the null terminator.
或者，您可以通过将其声明为数组来为ab腾出一些内存:
Alternately you can make some memory for ab by declaring it as an array:
char ab[NUM_CHARS + 1];
在不为ab分配内存的情况下，sprintf调用将尝试写入NULL，这是未定义的行为.这是导致崩溃的原因.
Without allocating memory somehow for ab, the sprintf call will try to write to NULL, which is undefined behavior; this is the cause of your crash.
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