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, Dec 20 '12 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, Dec 20 '12 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, Dec 20 '12 at 12:07
@interjay how's [this one](http://stackoverflow.com/a/8302326/1322972) instead ? — WhozCraig, Dec 20 '12 at 12:10
@WhozCraig Yeah that seems better. I'm still not sure if this one should be closed though since the fact that `strtok` modifies the string may not be obvious. — interjay, Dec 20 '12 at 12:20
@interjay I hope it is, since the highest up-voted answer doesn't mention it at all (`strtok()`, i mean). — WhozCraig, Dec 20 '12 at 12:23
That's why initializing a pointer to non-`const` `char` with a string literal is poor and dangerous practice, qualify it as `const char *`. Compilers could have many warnings that could help you avoid this problem, `gcc` has `-Wwrite-string` for example. — effeffe, Dec 20 '12 at 13:05
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, Dec 20 '12 at 13:31

score 6 · Answer 1 · edited May 23 '17 at 10:24

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 '17 at 10:24

Community

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answered Dec 20 '12 at 11:39

Alok Save

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And the downvote is for? Bother telling only if there is a technical reasoning. – Alok Save Dec 20 '12 at 11:44

score 2 · Accepted Answer · answered Dec 20 '12 at 12:02

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 '12 at 12:02

Adeel Ahmed

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Array of characters initialization produces segmentation fault

2 Answers2