Descendents of class Exception are used to communicate between raise methods and rescue statements in begin/end blocks. Exception objects carry information about the exception—its type (the exception’s class name), an optional descriptive string, and optional traceback information. Programs may subclass Exception to add additional information.

Methods
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E
N
Class Public methods
system_call_error === other => true or false

Return true if the receiver is a generic SystemCallError, or if the error numbers self and other are the same.

Source: show 

static VALUE
syserr_eqq(self, exc)
    VALUE self, exc;
{
    VALUE num, e;
    ID en = rb_intern("errno");

    if (!rb_obj_is_kind_of(exc, rb_eSystemCallError)) {
        if (!rb_respond_to(exc, en)) return Qfalse;
    }
    else if (self == rb_eSystemCallError) return Qtrue;

    num = rb_attr_get(exc, en);
    if (NIL_P(num)) {
        num = rb_funcall(exc, en, 0, 0);
    }
    e = rb_const_get(self, rb_intern("Errno"));
    if (FIXNUM_P(num) ? num == e : rb_equal(num, e))
        return Qtrue;
    return Qfalse;
}
SystemCallError.new(msg, errno) => system_call_error_subclass

If errno corresponds to a known system error code, constructs the appropriate Errno class for that error, otherwise constructs a generic SystemCallError object. The error number is subsequently available via the errno method.

Source: show 

static VALUE
syserr_initialize(argc, argv, self)
    int argc;
    VALUE *argv;
    VALUE self;
{
#if !defined(_WIN32) && !defined(__VMS)
    char *strerror();
#endif
    const char *err;
    VALUE mesg, error;
    VALUE klass = rb_obj_class(self);

    if (klass == rb_eSystemCallError) {
        rb_scan_args(argc, argv, "11", &mesg, &error);
        if (argc == 1 && FIXNUM_P(mesg)) {
            error = mesg; mesg = Qnil;
        }
        if (!NIL_P(error) && st_lookup(syserr_tbl, NUM2LONG(error), &klass)) {
            /* change class */
            if (TYPE(self) != T_OBJECT) { /* insurance to avoid type crash */
                rb_raise(rb_eTypeError, "invalid instance type");
            }
            RBASIC(self)->klass = klass;
        }
    }
    else {
        rb_scan_args(argc, argv, "01", &mesg);
        error = rb_const_get(klass, rb_intern("Errno"));
    }
    if (!NIL_P(error)) err = strerror(NUM2LONG(error));
    else err = "unknown error";
    if (!NIL_P(mesg)) {
        VALUE str = mesg;
        size_t len;

        StringValue(str);
        len = strlen(err)+RSTRING(str)->len+3;
        mesg = rb_str_new(0, len);
        snprintf(RSTRING(mesg)->ptr, len+1, "%s - %.*s", err,
                (int)RSTRING(str)->len, RSTRING(str)->ptr);
        rb_str_resize(mesg, strlen(RSTRING(mesg)->ptr));
    }
    else {
        mesg = rb_str_new2(err);
    }
    rb_call_super(1, &mesg);
    rb_iv_set(self, "errno", error);
    return self;
}
Instance Public methods
system_call_error.errno => fixnum

Return this SystemCallError’s error number.

Source: show 

static VALUE
syserr_errno(self)
    VALUE self;
{
    return rb_attr_get(self, rb_intern("errno"));
}