What is difference between the following code?
1.
char *p;
strcpy(p,"String");
2.
char *p;
p = "String";
The pointer points to the same string, but is there any difference?
Asked
Active
Viewed 3,213 times
3 Answers
6
In order for the first operation to work, p must point to a writable block of memory at least 7 bytes in size. The second operation does not require it.
After the first operation the string remains writable: you can do this on the first string, but not the second:
*p= 's'; // Make the value all lowercase
The second pointer assignment points p to a memory of a string literal; writing to that memory is undefined behavior.
Sergey Kalinichenko
- 714,442
- 84
- 1,110
- 1,523
-
@MitchWheat You're right :) – Sergey Kalinichenko Sep 14 '13 at 10:19
-
plus yours is the better answer. – Mitch Wheat Sep 14 '13 at 10:30
-
Does this apply if you say p = ""? Is the empty string considered a string literal in read-only memory? – rasen58 Dec 06 '16 at 02:27
-
@rasen58 Yes, `""` is a string literal in read-only memory. It occupies one byte, i.e. `'\0'`. Writing to it is UB. – Sergey Kalinichenko Dec 06 '16 at 02:30
1
(1) is a memory scribble and possibly a runtime error.
You cannot copy into memory you don't own (haven't allocated in some way).
Mitch Wheat
- 295,962
- 43
- 465
- 541
1
In the first point you say you want to copy the string to the memblock p is pointing to
(so you have to make sure, there is enough space where the string can be copied to)
In the second case you just make p pointing to the read only address of "String".
p -> [S][t][r][i][n][g][0]
But you should get compiler warnings as far you don't declare p as p const *
dhein
- 6,431
- 4
- 42
- 74
-
hence in first String is copied to another location and then address is assigned to pointer. But in second direct address of String is assigned to p. – Devraj Jaiman Sep 14 '13 at 11:32