Strings and Dynamic Memory

grooveordie · 10-19-2008, 04:39 PM

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));
}

tabstop · 10-19-2008, 04:47 PM

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.

Elysia · 10-20-2008, 09:19 AM

It's undefined behavior, simply put. It means anything can happen.
And you need to indent your code. You can't possibly expect us to read that code mess!
You also need to drop the implicit main: http://cpwiki.sourceforge.net/Implicit_main

IceDane · 10-20-2008, 02:08 PM

I didn't manage to read all of the replies - it's late and my eyes are tired.

However, you can use getchar, as you are doing, with a pointer and realloc, and reallocate memory for each char.

However, this would be very inefficient. You should really start by allocating some set size, say.. 128 bytes or some such. You then use a counter to keep track of the number of characters that have been input, and if it reaches something like SET_SIZE-1, you reallocate and increase the size by another, say, 128 bytes. The increment can be smaller, if you want to preserve memory, but a few hundred bytes is nothing now that computers are hardly ever seen with less than two gigabytes of memory.

10-19-2008, 04:39 PM	#1
grooveordie Registered User Join Date: Oct 2008 Location: Turkey Posts: 1	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)); }
grooveordie is offline

10-19-2008, 04:47 PM	#2
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	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.
tabstop is offline

10-20-2008, 02:08 PM	#4
IceDane Ex scientia vera Join Date: Sep 2007 Posts: 426	I didn't manage to read all of the replies - it's late and my eyes are tired. However, you can use getchar, as you are doing, with a pointer and realloc, and reallocate memory for each char. However, this would be very inefficient. You should really start by allocating some set size, say.. 128 bytes or some such. You then use a counter to keep track of the number of characters that have been input, and if it reaches something like SET_SIZE-1, you reallocate and increase the size by another, say, 128 bytes. The increment can be smaller, if you want to preserve memory, but a few hundred bytes is nothing now that computers are hardly ever seen with less than two gigabytes of memory. __________________ "What's up, Doc?" "'Up' is a relative concept. It has no intrinsic value."
IceDane is offline

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