Difference between array and pointer

Question

Yesterday I had a little trouble with a homemade "strcpy" function. It works now though but I'm a little confused!

char* a = "Hello, World!"; //Works
char b[] = "Hello, World!"; //Works also

strcpy(a, "Hello!"); //Segmentation fault
strcpy(b, "Haha!!"); //Works..

Where is the difference? Why does the char pointer cause a "Segmentation fault"?

Why does this even work? :

char* a = "Haha"; //works
a = "LOL"; //works..

I removed the C++ tag since this is about the C subset (and even there you wouldn't implement `strcpy` yourself). — Benjamin Bannier, May 22 '13 at 13:33
Read this section of the C FAQ : http://c-faq.com/aryptr/aryptr2.html — Claudio, May 22 '13 at 13:23
Read: [Difference between `char *str` and `char str[]` and how both stores in memory?](http://stackoverflow.com/questions/15177420/what-does-sizeofarray-return/15177499#15177499) — Grijesh Chauhan, Oct 12 '13 at 10:54

score 13 · Accepted Answer · answered May 22 '13 at 13:21

13

char* a = "Hello, World!";

gives you a pointer to a string literal. A string literal may exist in read only memory so its contents cannot be changed.

char* a = "Haha"; //works
a = "LOL"; //works..

changes the pointer a to point to a different string literal. It doesn't attempt to modify the contents of either string literal so is safe/correct.

char b[] = "Hello, World!"

declares an array on the stack and initialises it with the contents of a string literal. Stack memory is writeable so its perfectly safe to change the contents of this memory.

answered May 22 '13 at 13:21

simonc

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so I cannot modify even a single character in the char pointer, right? – Normal People Scare Me May 22 '13 at 13:22
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@NormalPeopleScareMe That's right. It's pointing to a string *constant*. – May 22 '13 at 13:23
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Well, it's undefined behavior, so it might work or it might not. My advice is to always assume it won't. – Imre Kerr May 22 '13 at 13:24
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@NormalPeopleScareMe It depends what it points to. You can't modify the contents of a pointer to a string literal but you can modify the contents of a pointer to a stack or heap allocated string. – simonc May 22 '13 at 13:24
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Thank you guys. Simonc, you helped me really a lot! – Normal People Scare Me May 22 '13 at 13:24
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Compiling with `-Wwrite-strings` will tell gcc to remember how constant string literals are: It will give you a warning on this code. – Medinoc May 22 '13 at 13:40

score 1 · Answer 2 · answered May 22 '13 at 13:27

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In your first example since you are trying to write to a read only memory pointed by a,you will get the segmentation fault.If you want to use pointers then allocate the memory on heap,use and delete it after its use. Where as b is an array of chars initialized with "Hello, World!".

In the second example you are making the pointer to point to different string literal which should be fine.

answered May 22 '13 at 13:27

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I find this answer very helpful. I didn't know that the string literal is located in the read only memory. Also I found a tutorial how to use char pointers in combination with strcpy which caused a segmentation fault here and they were allocating memory and i couldn't retrace why they did it. Thanks. – Normal People Scare Me May 22 '13 at 17:59

Difference between array and pointer

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