The function signature of strstr is

char *strstr(const char *haystack, const char *needle)

Since you have declared needle a const char*, you are violating the contract. I recommend further reading in the doc, specifically

These functions return a pointer to the beginning of the substring, or NULL if the substring is not found.

in the context of your if (needle == 0) statement.

The function signature of strstr is

char *strstr(const char *haystack, const char *needle)

Since you have declared needle a const char*, you are violating the contract. I recommend further reading in the doc, specifically

These functions return a pointer to the beginning of the substring, or NULL if the substring is not found.

in the context of your if (needle == 0) statement.

PLEASE NOTE: This answer no longer makes contextual sense because of edits to the orginal question.