Array of characters initialization produces segmentation fault

Question

The following piece of code produces a segmentation fault during compilation:

(gdb) run
Starting program: /home/anna/Desktop/a.out
Program received signal SIGSEGV, Segmentation fault.
0xb7e97845 in strtok () from /lib/i386-linux-gnu/libc.so.6

#include <string.h>
#include <stdio.h>

main () {
char * sentence = "This is a sentence.";
char * words[200] ;
words[0] = strtok(sentence," ");
}

After changing the 5th line, no error is thrown.

#include <string.h>
#include <stdio.h>

main () {
char  sentence[] = "This is a sentence.";
char * words[200] ;
words[0] = strtok(sentence," ");
}

Why is this happening?

Note: You're executing the program, the compilation has already successfully created the a.out file. It does not segfault "during compilation". — unwind
– unwind, Commented Dec 20, 2012 at 11:44
@WhozCraig This is not a duplicate of that question - the code in the linked question is not actually writing to a string literal, despite the title. — interjay
– interjay, Commented Dec 20, 2012 at 12:00
@interjay good point. this question is asked at least a couple of times a day. you would think it would be easily findable among the dupes. i'll keep searching. thanks. — WhozCraig
– WhozCraig, Commented Dec 20, 2012 at 12:07
Welcome to SO. It should be int main(void). Generally I have the idea that first reading a bit about C could help you a lot. — Jens Gustedt
– Jens Gustedt, Commented Dec 20, 2012 at 13:31

Community · Accepted Answer · 2017-05-23 10:24:45Z

6

char * sentence = "This is a sentence.";

sentence is a pointer pointing to a string literal "This is a sentence." is stored in a read only memory and you should not be modifying it.
Modifying a string literal in any way results in Undefined Behavior and in your case it manifests in a segmentation fault.

Good Read:
What is the difference between char a[] = ?string?; and char *p = ?string?;?

edited May 23, 2017 at 10:24

CommunityBot

11 silver badge

answered Dec 20, 2012 at 11:39

Alok Save

208k56 gold badges449 silver badges540 bronze badges

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

1 Comment

Alok Save Over a year ago

And the downvote is for? Bother telling only if there is a technical reasoning.

Adeel Ahmed · Accepted Answer · 2012-12-20 12:02:08Z

2

Read the man page of strtok (BUGS section),

These functions modify their first argument.
These functions cannot be used on constant strings.

and char *sentence = "This is a sentence"; is allocated in read-only context, so treated as contant.

answered Dec 20, 2012 at 12:02

Adeel Ahmed

1,6019 silver badges10 bronze badges

Collectives™ on Stack Overflow

Array of characters initialization produces segmentation fault

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related