Why do I get a segmentation fault when writing to a “char *s” initialized with a string literal, but not “char s[]”?

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See the C FAQ, Question 1.32

Q: What is the difference between these initializations?
char a[] = "string literal";
char *p = "string literal";
My program crashes if I try to assign a new value to p[i].

A: A string literal (the formal term
for a double-quoted string in C
source) can be used in two slightly
different ways:

  1. As the initializer for an array of char, as in the declaration of char a[] , it specifies the initial values
    of the characters in that array (and,
    if necessary, its size).
  2. Anywhere else, it turns into an unnamed, static array of characters,
    and this unnamed array may be stored
    in read-only memory, and which
    therefore cannot necessarily be
    modified. In an expression context,
    the array is converted at once to a
    pointer, as usual (see section 6), so
    the second declaration initializes p
    to point to the unnamed array’s first
    element.

Some compilers have a switch
controlling whether string literals
are writable or not (for compiling old
code), and some may have options to
cause string literals to be formally
treated as arrays of const char (for
better error catching).

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