c string allocation differences?

Question

I have a functions which takes a char * as its only argument. I then perform some strtok operations on it. Sometimes it works and sometimes it doesent. It working depends upon how the string was constructed. For instance here are the two cases.

int main()
{
   char glob[] = "/abc/def/ghi";
   char *glob2 = "/abc/def/ghi";

   func(glob);  //this one works
   func(glob2); //this one doesnt work

   return 0;
}

What is the difference between the two allocation methods and why does strtok blow up on the second one?

Read litb's excellent post for the difference between char[] and char* - stackoverflow.com/questions/308279/c-strings-vs/308724#308724 — Falaina
– Falaina, Commented Aug 4, 2009 at 14:11
Assigning a string constant to a char* (instead of a const char*) is only legal for historical reasons. It is considered deprecated, and some compilers will issue you a warning about it. — Tyler McHenry
– Tyler McHenry, Commented Aug 4, 2009 at 14:13

Jonathan Leffler · Accepted Answer · 2009-08-04 14:38:21Z

12

strtok() basically modifies the input string.

char *glob2 = "/abc/def/ghi";

In above case the glob2 points to read-only data and hence it fails, whereas with 'char glob[] = "/abc/def/ghi";' the data is not read-only, it's available in char array. Hence it allows the modifications.

edited Aug 4, 2009 at 14:38

Jonathan Leffler

760k145 gold badges961 silver badges1.3k bronze badges

answered Aug 4, 2009 at 14:10

aJ.

35.7k22 gold badges89 silver badges132 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Cristian Ciupitu Over a year ago

Quoting from the man page: Be cautious when using these functions. If you do use them, note that: - These functions modify their first argument. - These functions cannot be used on constant strings. - The identity of the delimiting character is lost. - The strtok() function uses a static buffer while parsing, so it’s not thread safe. Use strtok_r() if this matters to you.

sepp2k · Accepted Answer · 2009-08-04 14:14:59Z

7

char[] str1 = "foo" allocates an array of the chars on the stack (assuming this is inside a function). That array can be modified without a problem.

const char *str = "foo" gives you a pointer to the string foo, which will usually reside in read-only memory.

char *str = "foo" will do the same thing but implicitly remove the const (without actually changing the fact that the pointer likely points to read-only memory).

answered Aug 4, 2009 at 14:14

sepp2k

372k56 gold badges687 silver badges687 bronze badges

Comments

kmarsh · Accepted Answer · 2009-08-04 14:10:07Z

6

Strtok writes to the memory allocated to the string.

You cannot write to statically allocated string memory on most compilers/runtimes/hardware. You can write to the stack.

answered Aug 4, 2009 at 14:10

kmarsh

1,3988 silver badges22 bronze badges

Comments

James Youngman · Accepted Answer · 2009-08-04 22:25:48Z

-1

The other comments are correct; you should use strtok_r() instead.

answered Aug 4, 2009 at 22:25

James Youngman

1821 silver badge3 bronze badges

1 Comment

John Carter Over a year ago

-1, since strtok_r doesn't actually relate directly to the problem here: Both strtok_r and strtok modify the input buffer

Collectives™ on Stack Overflow

c string allocation differences?

4 Answers 4

1 Comment

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related