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C Discussion Thread
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Jan 15th, 2008

Difference between char* and char[]

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Hi. Below are two pieces of code. This first one works fine:

C Syntax (Toggle Plain Text)
    char str[] = "Hello World";
    char *b;
    b = &(str[3]);
    *b = '3';
    printf("%s\n", str);
    char str[] = "Hello World";
    char *b;
    b = &(str[3]);
    *b = '3';
    printf("%s\n", str);

However this second one results in a segmentation fault.

C Syntax (Toggle Plain Text)
    char *str = "Hello World";
    char *b;
    b = &(str[3]);
    *b = '3';
    printf("%s\n", str);
    char *str = "Hello World";
    char *b;
    b = &(str[3]);
    *b = '3';
    printf("%s\n", str);

The difference is in the definition of the variable str. Why does one method work fine and the other result in a segmentation fault? When is it best to use a str[] definition and the *str definition?

Last edited by Dark_Knight; Jan 15th, 2008 at 12:13 am.

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Dark_Knight

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Jan 15th, 2008

Re: Difference between char* and char[]

The first is local data you can edit (since the array is local data).

The second is a local pointer to global, static (constant) data. You are not allowed to modify constant data.

If you have GNU C, you can compile with -fwritable-strings to keep the global string from being made constant, but this is not recommended.

Duoas

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Re: Difference between char* and char[]

C FAQ.

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