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。
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
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.
其他回答
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。
char *str = "string";
分配一个指向字符串字面量的指针,编译器将其放入可执行文件中不可修改的部分;
char str[] = "string";
分配并初始化一个可修改的本地数组
这些答案大部分都是正确的,但为了更清楚一点……
人们所说的“只读内存”是ASM术语中的文本段。它是内存中加载指令的同一个地方。出于安全等明显的原因,这是只读的。当创建一个初始化为字符串的char*时,字符串数据被编译到文本段中,程序初始化指向文本段的指针。所以如果你想改变它,就死定了。段错误。
当作为数组编写时,编译器将初始化的字符串数据放在数据段中,这与全局变量等存在的位置相同。这个内存是可变的,因为数据段中没有指令。这一次,当编译器初始化字符数组(仍然只是一个char*)时,它指向的是数据段而不是文本段,您可以在运行时安全地更改文本段。
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.
首先,str是一个指向"string"的指针。编译器允许将字符串字面量放在内存中不能写入,但只能读取的地方。(这真的应该触发一个警告,因为你将一个const char *分配给一个char *。你是禁用了警告,还是忽略了它们?)
第二,你在创建一个数组,它是你可以完全访问的内存,并用"string"初始化它。您正在创建一个字符[7](六个用于字母,一个用于结尾的'\0'),您可以对它做任何您喜欢的事情。