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
在第一个代码中，"string"是一个字符串常量，字符串常量永远不应该被修改，因为它们通常被放置在只读内存中。"str"是一个用来修改常量的指针。
在第二段代码中，"string"是一个数组初始化器，类似于
char str[7] = { 's', 't', 'r', 'i', 'n', 'g', '\0' };
"str"是堆栈上分配的数组，可以自由修改。
其他回答
参见C常见问题，问题1.32
Q: What is the difference between these initializations? char a[] = "string literal"; char *p = "string literal"; My program crashes if I try to assign a new value to p[i]. A: A string literal (the formal term for a double-quoted string in C source) can be used in two slightly different ways: As the initializer for an array of char, as in the declaration of char a[] , it specifies the initial values of the characters in that array (and, if necessary, its size). Anywhere else, it turns into an unnamed, static array of characters, and this unnamed array may be stored in read-only memory, and which therefore cannot necessarily be modified. In an expression context, the array is converted at once to a pointer, as usual (see section 6), so the second declaration initializes p to point to the unnamed array's first element. Some compilers have a switch controlling whether string literals are writable or not (for compiling old code), and some may have options to cause string literals to be formally treated as arrays of const char (for better error catching).
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。
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。
5.5节K&R的字符指针和功能也讨论了这个主题:
There is an important difference between these definitions: char amessage[] = "now is the time"; /* an array */ char *pmessage = "now is the time"; /* a pointer */ amessage is an array, just big enough to hold the sequence of characters and '\0' that initializes it. Individual characters within the array may be changed but amessage will always refer to the same storage. On the other hand, pmessage is a pointer, initialized to point to a string constant; the pointer may subsequently be modified to point elsewhere, but the result is undefined if you try to modify the string contents.
通常，当程序运行时，字符串字面值存储在只读内存中。这是为了防止您意外地更改字符串常量。在第一个例子中，"string"存储在只读内存中，*str指向第一个字符。当您试图将第一个字符更改为'z'时，会发生段错误。
在第二个例子中，字符串"string"被编译器从其只读母数组复制到str[]数组中。然后允许更改第一个字符。你可以通过打印每个地址来检查:
printf("%p", str);
同样，在第二个例子中打印str的大小会显示编译器已经为它分配了7个字节:
printf("%d", sizeof(str));
