下面的代码在第2行接收到seg错误:

char *str = "string";
str[0] = 'z';  // could be also written as *str = 'z'
printf("%s\n", str);

虽然这个方法非常有效:

char str[] = "string";
str[0] = 'z';
printf("%s\n", str);

用MSVC和GCC测试。


当前回答

因为在第一个例子的上下文中,“whatever”的类型是const char*(即使你将它赋值给一个非const char*),这意味着你不应该尝试写它。

编译器通过将字符串放在内存的只读部分来强制执行这一点,因此写入它会产生段错误。

其他回答

The C FAQ that @matli linked to mentions it, but no one else here has yet, so for clarification: if a string literal (double-quoted string in your source) is used anywhere other than to initialize a character array (ie: @Mark's second example, which works correctly), that string is stored by the compiler in a special static string table, which is akin to creating a global static variable (read-only, of course) that is essentially anonymous (has no variable "name"). The read-only part is the important part, and is why the @Mark's first code example segfaults.

当您试图访问不可访问的内存时,会导致分割错误。

Char *str是一个指向不可修改的字符串的指针(这是导致segfault的原因)。

而char str[]是一个数组,可以修改。

像“String”这样的字符串文字可能在可执行文件的地址空间中作为只读数据分配(通过编译器)。当你去触摸它时,它会害怕你在它的泳衣区,并让你知道一个隔离错误。

在第一个例子中,你得到一个指向const数据的指针。在第二个示例中,使用const数据的副本初始化一个7个字符的数组。

// create a string constant like this - will be read only
char *str_p;
str_p = "String constant";

// create an array of characters like this 
char *arr_p;
char arr[] = "String in an array";
arr_p = &arr[0];

// now we try to change a character in the array first, this will work
*arr_p = 'E';

// lets try to change the first character of the string contant
*str_p = 'G'; // this will result in a segmentation fault. Comment it out to work.


/*-----------------------------------------------------------------------------
 *  String constants can't be modified. A segmentation fault is the result,
 *  because most operating systems will not allow a write
 *  operation on read only memory.
 *-----------------------------------------------------------------------------*/

//print both strings to see if they have changed
printf("%s\n", str_p); //print the string without a variable
printf("%s\n", arr_p); //print the string, which is in an array. 

假设字符串是,

char a[] = "string literal copied to stack";
char *p  = "string literal referenced by p";

在第一种情况下,当'a'进入作用域时,文字将被复制。这里'a'是定义在stack上的数组。这意味着字符串将在堆栈上创建,其数据从代码(文本)内存中复制,通常是只读的(这是特定于实现的,编译器也可以将这种只读的程序数据放在可读写内存中)。

在第二种情况下,p是定义在堆栈(本地作用域)上的指针,并引用存储在其他位置的字符串字面量(程序数据或文本)。通常,修改这样的内存不是好的实践,也不鼓励。