Question

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Why does this Seg Fault?

I receive a segmentation fault when using ++ operator on a char *

#include<stdio.h>

int main()
{
    char *s = "hello";
    printf("%c ", ++(*s));
    return 0;
}

But if I do the following:

#include<stdio.h>

int main()
{
    char *s = "hello";
    char c = *s;
    printf("%c ", ++c);
    return 0;
}

Then the code compiles perfectly, what is the problem with the above code?

Answer 1

The first code snippet is attempting to modify a character in a string literal as:

++(*s)

is attempting to increment the first character in s. String literals are (commonly) read-only and an attempt to modify will cause the segmentation fault (the C standard states If the program attempts to modify such an array, the behavior is undefined.).

The second snippet is modifying a char variable, which is not read-only as after:

char c = *s;

c is a copy of the first character in s and c can be safely incremented.