用char * p定义字符串是正确的； 以后再分配价值？

Question

I want to create a string without knowing the exact dimension of it, is this correct or it will have unpredictable behavior?

char *p;
p="unknow string size";

If this is wrong, how i can create something similar, and modify it with string.h fucntion?

[edit]I read again the answer and it was not completly clear, my first doubt is: are those two codes equals?

char *p="unknow string size"

and

char *p;
p="unknow string size";

Answer 1

The only solution in C is to use realloc function

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    char *s = malloc(1);
    printf("Enter a string: \t"); // It can be of any length
    int c;
    int i = 0;
    while((c = getchar()) != '\n' && c != EOF)
    {
        s[i++] = c;
        s = realloc(s, i+1);
    }
    s[i] = '\0';
    printf("Entered string: \t%s", s);
    free(s);
    return 0;
}  

are those two codes equals?

 char *p="unknown string size" 

and

 char *p; p="unknown string size"; 

No. First snippet is declaring p as a pointer to char and initializing it to point to a string literal "unknown string size" . In second snippet, p is defined as a pointer to char and then an assignment is done to it.

Answer 2

It depends, you don't assign the value, y point to the address where "unknow string size" is stored, so you should know that for example you can't modify the string.

If you don't want to modify the string then it's ok, but you should also protect the pointer for trying to write to that memory location cause that would be undefined behavior, you can do this

const char *p;
p = "unknow string size";

this will not prevent you from modifying the string, but then you would need to explicitly cast the const qualifier away to do so.

If you intend to modify the string later, then this is not the way to do it, in that case you should do this

char *p;
size_t length;

length = strlen("unknow string size");
p = malloc(1 + length);
if (p == NULL)
    memoryAllocationProblemDoNotContinue();
strcpy(p, "unknow string size");
 .
 .
 .
 /* use p here */
 .
 .
 .
 free(p);

Answer 3

I would write the function like this:

#include <stdio.h> // printf(), getline()
#include <stdlib.h>
#include <string.h> // strlen()

int main(void)
{
    char *s = NULL;

    printf("Enter a string: \t"); // It can be of any length
    fflush(stdout);

    getline(&s, 0, stdin); // note: first param is **char

    if( NULL != s )
    { // then getline successful

        // trim trailing newline
        if( '\n' == s[strlen(s)-1] ) s[strlen(s)-1] = '\0'; 

        printf("Entered string: \t%s", s);
        free(s);
    }  // end if
    return 0;
} // end function: main