I noticed (the hard way sadly) that -Wl,--nmagic aka the linker -n option makes my app segfault when I use global variables. Reading the man page doesn't really answer why. I was using it because it brought my binary from 14K to 1K (and less than 1K if I use a few more commands). Is there anything I can do to restore proper behavior? I thread thread local but Ican't figure out how to set thread local memory. c compilers say FS isn't a valid register and mov fs, rax creates an invalid instruction (or something)

Things I noticed:

• If I don't assign a value it works (which is why I have the if statement).
• If I make it const int test=6 it works (you'll have to delete the if)
• Without the nmagic option it works with no changes

Source:

//clang or gcc -static -Wl,--nmagic -nostdlib test.c 
typedef unsigned long long int size_t;
typedef long long int ssize_t;
typedef long long int int64_t;

ssize_t my_write(int fd, const void *buf, size_t size) {
    register int64_t rax __asm__ ("rax") = 1;
    register int rdi __asm__ ("rdi") = fd;
    register const void *rsi __asm__ ("rsi") = buf;
    register size_t rdx __asm__ ("rdx") = size;
    __asm__ __volatile__ (
        "syscall"
        : "+r" (rax)
        : "r" (rdi), "r" (rsi), "r" (rdx)
        : "cc", "rcx", "r11", "memory"
    );
    return rax;
}

void my_exit(int exit_status) {
    register int64_t rax __asm__ ("rax") = 60;
    register int rdi __asm__ ("rdi") = exit_status;
    __asm__ __volatile__ (
        "syscall"
        : "+r" (rax)
        : "r" (rdi)
        : "cc", "rcx", "r11", "memory"
    );
}

int test=6; //if this isn't assigned it works fine

int _start(){
    if (test == 0)
        test=6;
    my_write(1, "Hello\n", test);
    my_exit(0);
    return 0;
}

size -A ./a.out

section              size      addr
.note.gnu.property     48   4194816
.note.gnu.build-id     36   4194864
.text                 127   4198400
.rodata                 7   4202496
.eh_frame             120   4202504
.data                   4   4210688
.comment               18         0
Total                 360