Is there a way to check if process is 64 bit or 32 bit?

Okay so I did a lot of research and figured out a better solution. Although the sysctl approach works, the documentation states it should be avoided. The method below uses libproc.h's proc_pidinfo function and works similarly to sysctl. This is obviously for Apple's platforms.

bool Is64Bit (int pid)
{
    proc_bsdshortinfo info;
    if (proc_pidinfo (pid, PROC_PIDT_SHORTBSDINFO,
        0, &info, PROC_PIDT_SHORTBSDINFO_SIZE))
        return info.pbsi_flags & PROC_FLAG_LP64;

    return false;
}

GetBSDProcessList returns a kinfo_proc. The kinfo_proc has a kp_proc member which is of type extern_proc. The extern_proc has a p_flag member, which one of the flags is P_LP64, indicating "Process is LP64"). So you should be able to check with:

int is64bit = proc->kp_proc.p_flags & P_LP64;

---

(Note: As shown in the comment, you need to use the B_get_process_info found in Link:

static int
B_get_process_info(pid_t pid, struct kinfo_proc *kp)
{
    size_t bufsize      = 0;
    size_t orig_bufsize = 0;
    int    retry_count  = 0;
    int    local_error  = 0;
    int    mib[4]       = { CTL_KERN, KERN_PROC, KERN_PROC_PID, 0 };

    mib[3] = pid;
    orig_bufsize = bufsize = sizeof(struct kinfo_proc);

    for (retry_count = 0; ; retry_count++) {
        local_error = 0;
        bufsize = orig_bufsize;
        if ((local_error = sysctl(mib, 4, kp, &bufsize, NULL, 0)) < 0) {
            if (retry_count < 1000) {
                sleep(1);
                continue;
            }
            return local_error;
        } else if (local_error == 0) {
            break;
        }
        sleep(1);
    }

    return local_error;
}

)