Strings: char* or char vectors ?

**dancp** · 07-31-2011

When I write some lines as

Code:

int main(int argc, char **argv) {
	char *string = "Bla0 Bla1 Bla2 Bla3";
	char *temp;
	temp = strtok(string, " ");
}

I've an invalid address error on the strtok call.

But if I write

Code:

int main(int argc, char **argv) {
	char string[128] = "Bla0 Bla1 Bla2 Bla3";
	char *temp;
	temp = strtok(string, " ");
}

the program runs correctly.

What's the problem? Can a string be indifferently represented either by a char pointer either by a char vector?

(I'm sorry for the English ... )

**laserlight** · 07-31-2011

The problem is that in the first example, you attempt to modify a string literal, and that is not allowed.

**stahta01** · 07-31-2011

Code:

char *string = "Bla0 Bla1 Bla2 Bla3";

Is a pointer to literal constant.

strtok modifies the string passed to it; if it is a constant it can not modify it.
Note: Some compilers will allow a literal constant to be modified; but, I think the standard changed and the newer compilers do NOT allow it or give a warning if it is allowed.

Tim S.

**grumpy** · 07-31-2011

Originally Posted by stahta01

Note: Some compilers will allow a literal constant to be modified; but, I think the standard changed and the newer compilers do NOT allow it or give a warning if it is allowed.

Modifying a literal gives undefined behaviour in every C standard since 1989.

Some compilers (whether predating ISO standards or not) do allow modification of string literals. Technically, that is an acceptable outcome, given that the result is actually undefined ("undefined" in the standard means "anything goes").

**dancp** · 07-31-2011

Thank you for the replies!

I thought it was a memory access problem caused by the pointer (in the first case).

In the second case, I put the string into a chars vector. Isn't a literal, in this case?

**ssharish2005** · 07-31-2011

>In the second case, I put the string into a chars vector. Isn't a literal, in this case?
In your first case, you use pointer to point to literal string which is nothing but constant saved in Read-only section of data segment in the process memory. To which you can only read but cannot alter or modify the original string.

Where as in the second case, the literal string will be stored in the Read-Write section of data segment, as you declare string buffer to which you copy the data to. Since it lives in the read-write location of the process memory, you can call strtok which allows modifying the string.

ssharish

**Linux Trojan** · 07-31-2011

I am kinda new to this too; but, in the first case, the pointer only points to the first letter in that array. The memory for the entire array is seen by the pointer, it knows its there, but it just points to the first element of the array. In the second case, its not a pointer, so the entire array is "seen" by strtok, not just the first letter.

**laserlight** · 07-31-2011

Originally Posted by Linux Trojan

In the second case, its not a pointer, so the entire array is "seen" by strtok, not just the first letter.

Actually, when the array is passed as an argument to strtok, it is converted to a pointer to its first element, so there is no real difference in this sense.

**AndrewHunter** · 07-31-2011

Originally Posted by Linux Trojan

I am kinda new to this too; but, in the first case, the pointer only points to the first letter in that array. The memory for the entire array is seen by the pointer, it knows its there, but it just points to the first element of the array. In the second case, its not a pointer, so the entire array is "seen" by strtok, not just the first letter.

Are you not reading the replies before making these comments? You are wrong on all accounts. In both cases it is evaluated as a pointer by the compiler. The problem, as was stated multiple times above is that you cannot modify a string literal. May I suggest your review CProgramming.com's C tutorials.

**~~CommonTater~~** · 07-31-2011

Originally Posted by Linux Trojan

I am kinda new to this too; but, in the first case, the pointer only points to the first letter in that array. The memory for the entire array is seen by the pointer, it knows its there, but it just points to the first element of the array. In the second case, its not a pointer, so the entire array is "seen" by strtok, not just the first letter.

Nope... in the first case his char* is actually a pointer to read-only memory in the PE image. He's not allowed to modify it and strtok() inserts nulls, modifying the string.

In the second case his char[] is an array of characters in read-write memory on the stack. Now he can modify it.

**Linux Trojan** · 07-31-2011

Tater: this is very helpful info, ty.

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