Why the content of a pointer to char can be a string but I can't scanf a pointer to char withou-CodePudding

Why is the first option OK but the second option should use dynamic memory and the first doesn't?

//first option
char *p = "hello";
//second option
char *p;
scanf("%s",p);
//and introduce hello on the terminal

CodePudding user response：

The pointer p in this declaration

//first option
char *p = "hello";

points to the first character of the string literal "hello" that is stored in memory as a character array of the type char[6].

You can imagine it the following way

char string_literal[] = "hello";
char *p = string_literal;

Only pay attention to that you may not change a string literal pointed to by a pointer. Any attempt to change a string literal results in undefined behavior. For example you may not write

char *p = "hello";
p[0] = 'H';

In this code snippet

//second option
char *p;
scanf("%s",p);

the pointer p is not initialized and has an indeterminate value. So the call of scanf invokes undefined behavior.

At least you could write for example

char s[6];
char *p = s;

scanf( "%s, p ); // or scanf( "%5s", p );

to enter the string "hello";

Or if to use sscanf referred in your comment to the question you could write

char s[6];
char *p = s;

sscanf( "hello", "%s", p );

CodePudding user response：

Because in the first case the compiler allocates (usually) memory for the string in "static" memory. So p is pointing to storage that contains the string constant.

The second case, p is uninitialized, and points to nothing. So you have to allocate memory for it. The alternative to using heap-based allocation is to define like

char p[100];
scanf("0s",p);

you are still allocating memory in this case, but on the stack. Either way, C strings (and pointers) need some valid memory area to point to, whether it is on the stack, heap, or static.

CodePudding user response：

To do what you want in the second case, you would need to do it like this:

char *p;

printf("Enter a word: ");
scanf("%ms",&p);
printf("User entered '%s'\n",p);
free(p);

The m option to %s (in scanf) tells scanf to allocate whatever memory is needed for the string. Read the scanf man-page for more details.