D_FORTIFY_SOURCE 和 gcc

Question

This function is for generating md5hash:

out = malloc(32+1);
void md5sum( u_char *secret_data, int secret_len, char *in,char *out ) {
        ngx_md5_t       md5;
        u_char hash[16];
        ngx_md5_init(&md5);
        ngx_md5_update(&md5, in, strlen(in));
        ngx_md5_update(&md5, secret_data, secret_len);
        ngx_md5_final(hash, &md5);
        int ii;
        for (ii = 0; ii &lqt; 16; ii++) {
                char tt[2];
                sprintf(tt, "%02x", hash[ii] );
                strcat(out,tt);

        }
}

It works, but if I use option D_FORTIFY_SOURCE with gcc compiler, I get a segmentation fault. If I change type of tt to: char tt[3] , all is ok. Why?

Answer 1

sprintf is putting in a null character to terminate the string. So you need a three-character array to hold a two-character string; it's really 'a' 'b' '\\0'.

Answer 2

You're getting a segmentation fault because you are trying to write 3 characters into a 2 character array. (NUL is a character too)