MontaukOS/programs/lib/libc/crt/crt1.c

/*
    * crt1.c
    * Minimal C runtime startup for MontaukOS userspace programs
    * Copyright (c) 2026 Daniel Hammer
    *
    * This startup shim is intentionally small: it fetches the raw
    * command-line buffer from the kernel, tokenizes it into argc/argv,
    * calls main(), then exits with main()'s return code.
*/

static inline long _sys1(long nr, long a1) {
    long ret;
    __asm__ volatile(
        "mov %[a1], %%rdi\n\t"
        "syscall"
        : "=a"(ret)
        : "a"(nr), [a1] "r"(a1)
        : "rcx", "r11", "rdi", "rsi", "rdx", "r8", "r9", "r10", "memory");
    return ret;
}

static inline long _sys2(long nr, long a1, long a2) {
    long ret;
    __asm__ volatile(
        "mov %[a1], %%rdi\n\t"
        "mov %[a2], %%rsi\n\t"
        "syscall"
        : "=a"(ret)
        : "a"(nr), [a1] "r"(a1), [a2] "r"(a2)
        : "rcx", "r11", "rdi", "rsi", "rdx", "r8", "r9", "r10", "memory");
    return ret;
}

#define SYS_EXIT        0
#define SYS_GETARGS     25
#define SYS_GETEXECPATH 151

extern int main(int argc, char** argv);

/* Global constructor/destructor tables. The linker provides these
   boundary symbols whenever .preinit_array/.init_array/.fini_array
   sections exist; the weak references make them all optional so plain
   C programs keep working unchanged. Frame registration for C++
   exceptions (crtbegin's frame_dummy) also arrives via .init_array. */
extern void (*__preinit_array_start[])(void) __attribute__((weak));
extern void (*__preinit_array_end[])(void)   __attribute__((weak));
extern void (*__init_array_start[])(void)    __attribute__((weak));
extern void (*__init_array_end[])(void)      __attribute__((weak));
extern void (*__fini_array_start[])(void)    __attribute__((weak));
extern void (*__fini_array_end[])(void)      __attribute__((weak));

extern int atexit(void (*fn)(void));
extern void exit(int status);

static void _run_init_arrays(void) {
    for (void (**f)(void) = __preinit_array_start; f < __preinit_array_end; f++) {
        (*f)();
    }
    for (void (**f)(void) = __init_array_start; f < __init_array_end; f++) {
        (*f)();
    }
}

/* Destructors run in reverse registration order, hooked through
   atexit so they also run when the program calls exit() directly. */
static void _run_fini_array(void) {
    for (void (**f)(void) = __fini_array_end; f > __fini_array_start; f--) {
        (*(f - 1))();
    }
}

void _start(void) {
    /* Static: 4 KiB args + 256 argv slots would crowd a 32 KiB stack. */
    static char argbuf[4096];
    static char pathbuf[256];
    static char* argv[256];
    int len = (int)_sys2(SYS_GETARGS, (long)argbuf, (long)sizeof(argbuf));

    int argc = 0;

    /* Real argv[0]: tools like the GCC driver relocate their install
       prefix from it, so "prog" placeholders send them to garbage
       paths. Fall back to "prog" on kernels without the syscall. */
    int plen = (int)_sys2(SYS_GETEXECPATH, (long)pathbuf, (long)sizeof(pathbuf));
    argv[argc++] = (plen > 0) ? pathbuf : (char*)"prog";

    if (len > 0) {
        if (len > (int)sizeof(argbuf) - 1) {
            len = (int)sizeof(argbuf) - 1;
        }

        argbuf[len] = '\0';

        char* p = argbuf;
        while (*p != '\0' && argc < 255) {
            while (*p == ' ') {
                p++;
            }

            if (*p == '\0') {
                break;
            }

            argv[argc++] = p;

            while (*p != '\0' && *p != ' ') {
                p++;
            }

            if (*p != '\0') {
                *p++ = '\0';
            }
        }
    }

    argv[argc] = 0;

    if (__fini_array_start != __fini_array_end) {
        atexit(_run_fini_array);
    }
    _run_init_arrays();

    /* exit() (not raw SYS_EXIT) so the atexit chain - including the
       fini-array walker registered above - runs on return from main. */
    exit(main(argc, argv));
    __builtin_unreachable();
}