Segmentation Fault related to Pointer in c

Question

int main()
{
        char *s="Hello";
        *s="World";
        printf("%s\n",s);
}

Why does the above program result in a segmentation fault?

Try compiling with your warning flags on, such as `-Wall` if you're in GCC. That will give you clues. — Andy Lester, Aug 22 '13 at 19:39
In addition to trying to overwrite a string literal, it is also an error to call a variadic function without a variadic declaration for it. — jxh, Aug 22 '13 at 19:43
Don't ask duplicates like this again. Actually, you can't now.... — Andrew Barber, Aug 22 '13 at 21:03

score 4 · Accepted Answer · answered Aug 22 '13 at 19:41

int main()
{
    char *s="Hello"; // makes 's' point to a constant
    *s="World";      // modifies what 's' points to
    printf("%s\n",s);
}

The first line of code makes s point to a constant. The second line tries to modify what s points to. So you are trying to modify a constant, which you can't do because a constant is ... well ... constant.

score 2 · Answer 2 · answered Aug 22 '13 at 19:38

2

because *s is a char not a char*(string)

answered Aug 22 '13 at 19:38

Sam I am says Reinstate Monica

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is this related to dangling pointers?? – user2076561 Aug 22 '13 at 19:39

score 2 · Answer 3 · answered Aug 22 '13 at 19:39

2

char *s="Hello";

declares a pointer to a string literal "Hello". This may exist in read-only memory so the line

*s="World";

is results in undefined behaviour. A crash is a valid (and useful) form of undefined behaviour.

Either of the following alternatives would work

const char* s = "Hello";
s="World";
printf("%s\n",s);

char s[16] = "Hello";
strcpy(s, "World";)
printf("%s\n",s);

answered Aug 22 '13 at 19:39

simonc

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Isn't the issue here that `*s` is a `char`, not a string? – verbose Aug 22 '13 at 19:50
1

That'd cause a warning/error at compile time. An segfault at run time will be coming from the attempt to write (the address of a different string literal) to the read-only location pointer to by `s`. – simonc Aug 22 '13 at 19:58

score 1 · Answer 4 · answered Aug 22 '13 at 19:39

1

s points to static (global) memory when it is created. You cannot reassign to that memory at run-time, hence, the crash.

answered Aug 22 '13 at 19:39

edtheprogrammerguy

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can u explain little more about memory???? – user2076561 Aug 22 '13 at 19:44
The compiler sets aside special memory locations for this type of statically allocated string. When you call `*s = "another string"` you are trying to access that memory which is likely read-only. Behavior is undefined in this case. – edtheprogrammerguy Aug 22 '13 at 19:47
@edtheprogrammerguy we can re-assign but we can't rewrite. – sujin Aug 22 '13 at 19:47
You could legitimately say `s = "another string"`, however. – edtheprogrammerguy Aug 22 '13 at 19:48
u mean static memory as read only memory??? – user2076561 Aug 22 '13 at 19:52
Yes. Allocated by the compiler. – edtheprogrammerguy Aug 22 '13 at 19:53

sujin · Answer 5 · 2013-08-22T19:46:51.427

1

*s is the first char of the string, so assigning string to character makes error.

If you want to assing string use s = "world"

int main()
{
    char *s="Hello";
    s="World";
    printf("%s\n",s);
}

now try it will work.

char *s="hello"; Here s is in readonly location. So we can assign another string, but cannot rewrite new string.

s = "hello"; //work
strcpy(s, "hello"); //segmentation fault

edited Aug 22 '13 at 19:46

answered Aug 22 '13 at 19:40

sujin

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score 1 · Answer 6 · answered Aug 22 '13 at 19:49

There are two problems here.

The statement

*s = "World";

dereferences s, which gives you the first character of the string "Hello", or 'H'. So you're trying to assign a pointer value (the address of the string "World") to a single char object (the first character of the "Hello" string literal).

But...

On some systems (such as yours, apparently), string literals are stored in a read-only data segment, and attempting to modify read-only memory will lead to a runtime error on some systems. Hence the crash.

To change s to point to the "World" string literal, simply drop the dereference:

s = "World";

score 1 · Answer 7 · answered Aug 22 '13 at 19:53

1

*s is the same as s[0]. s[0] has room to store a single character; in this case a 'W'.

There's not room to store the location of "World" in that character.

That's why you're getting a segmentation fault.

answered Aug 22 '13 at 19:53

dcaswell

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Segmentation Fault related to Pointer in c

7 Answers7