Thread: Strings and Dynamic Memory

Hi, I'm in trouble with strings and pointers. It's a long message, but if anyone could read and say something, I'd be pleased.

1)

Code:

main() {
char *a;
printf("%d", strlen(a));
}

This code prints "0". But if I declare another char b[10] next to it, it begins printing 11. In this case, if I declare a as char *a="", again it prints "0". I used Codeblocs with Mingw here. And if I move the char line with or without b out of main(), it crashes in the beginning of runtime.

It acts different in DevC++. If "char *a;" is in main, it simply crashes in runtime. If I add b[10], it prints "0". If I move the char line with or without b out of main(), it still crashes. The problem is solved if a is initialized as ="".

Why do these compilers act different? And what causes this problem? I think it's because of the lack of initialization; but I saw somebody saying "You don't need". Which is true, and can i initialize a string or a "pointer to string" as a=""? Does it cost 0 byte?

2) A second problem, with dynamic memory allocation. I want to get some text from the user, and assign it to an array of characters. It has to be done dynamically. I want to get all the characters one by one, until it is '\n', and allocate place for one more char every time and assign. Here's a code I tried to do this, but I think it's far away from doing the job.

Code:

main() {
char *a = "\0", *b = "\0";
char c;
int i = 0;
while ((c = getchar()) != '\n') {
b = (char *)malloc((strlen(a) + 1) * sizeof(char));
strcpy(b, a);
b[strlen(a)] = c;
b[strlen(b)] = '\0';
free(a);
a = (char *)malloc(strlen(b) * sizeof(char));
strcpy(a, b);
free(b);
}
a[strlen(a)] = '\0';
printf("%s, %d, %d", a, strlen(a), strlen(b));
}

1. "You don't need" initialization IF the first time you use *a you assign something meaningful to it. Since that's not true in your case, you need initialization.

In case you were wondering, Dev-C++ also uses MinGW, so they're both using the same compiler. It would have to come down to the libraries -- I guess you have Dev-C++ set to use the "debug runtime" which silently initializes things to 0 so that if you try to follow a pointer that you haven't defined, it crashes; while you have Code::Blocks set to use "release runtime", which just uses whatever was in that piece of memory when the program started.

Now, when you make a variable global, it is automatically initialized to 0 (that's what C requires), so then your pointer really is NULL, and following a NULL pointer is guaranteed to crash.

You can initialize a pointer with a string literal, yes, BUT you have to remember that you don't own the memory that is pointed to. So this won't work:

Code:

char *a = "bob";
a[1] = 'b'; //crash! a does not point to writable memory

To resize allocated memory, you need to use realloc.