c - mmap allocation returns 0xfffffffffffffff4 (not MAP_FAILED) -

i trying allocate memory using mmap, here code:

long long *copy; copy = (long long*)mmap(null,                        (size_t)1024,                         prot_read | prot_write | prot_exec,                         map_private | map_anon, -1, 0);    if (copy == map_failed) {     fprintf(stderr, "memory allocation failed (process aborted)\n");     exit(1);   }   printf("pointer: %p\n", copy); 

obviously, check if allocation fails. when happens should -1 gather man pages. thing -12, 0xfffffffffffffff4, error isn't caught , program goes on. thought maybe because of (long long*) cast, cast shouldn't change pointer value. curious why happens , how prevent it.

more strange behavior:

i tried print errno. if use printf("%d\n", errno); prints 0 , pointer still set 0xfffffffffffffff4. if use err(errno, "%p", copy); prints:

program.exe: 0x7f8130981000: success 

and pointer valid, can't use because err terminated execution.

when compiling warnings (-wmissing-prototypes -wstrict-prototypes -werror -wextra), suggested in comments, realized <err.h> wasn't included. when included it, "strange behavior" described @ end of question disappeared; program behaved still bad pointer, didn't know @ point program have worked if didn't check errno.

in other parts of code, have assembly-written functions have been working great (that's why thought mistake wasn't related them). remember of them use %rbx pass information between them, happens %rbx register used errno in gcc.

so looking @ generated assembly code function, realized %rbx wasn't "protected" compiler. adding headers <errno.h> fixed that, , working fine.

even though seems work without proper includes, definetly lesson more careful , use maximum warning options.