strtok - char array versus char pointer [duplicate]

Question

Possible Duplicate:
strtok wont accept: char *str

When using the strtok function, using a char * instead of a char [] results in a segmentation fault.

This runs properly:

char string[] = "hello world";
char *result = strtok(string, " ");

This causes a segmentation fault:

char *string = "hello world";
char *result = strtok(string, " ");

Can anyone explain what causes this difference in behaviour?

Answer 1

char string[] = "hello world";

This line initializes string to be a big-enough array of characters (in this case char[12]). It copies those characters into your local array as though you had written out

char string[] = { 'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '\0' };

The other line:

char* string = "hello world";

does not initialize a local array, it just initializes a local pointer. The compiler is allowed to set it to a pointer to an array which you're not allowed to change, as though the code were

const char literal_string[] = "hello world";
char* string = (char*) literal_string;

The reason C allows this without a cast is mainly to let ancient code continue compiling. You should pretend that the type of a string literal in your source code is const char[], which can convert to const char*, but never convert it to a char*.

Answer 2

In the second example:

char *string = "hello world";
char *result = strtok(string, " ");

the pointer string is pointing to a string literal, which cannot be modified (as strtok() would like to do).

You could do something along the lines of:

char *string = strdup("hello world");
char *result = strtok(string, " ");

so that string is pointing to a modifiable copy of the literal.