this is a very good blog post՝ back to basics about strings in C and Pascal. i suggest you to read it, and now i’ll tell you something else:

in Oberon, Wirth decided to give up on Pascal strings, and use zero terminated strings.

however, there is no need to run by the whole array to find out the length of the string. we don’t even need to have a separate field that holds the length՝ compiler knows the length of the static string.

thus it can do compile time tests. for example we have the following Oberon source՝

MODULE tst;
IMPORT Out;

VAR i: SHORTINT;
str: ARRAY 1024 OF CHAR;

BEGIN

FOR i := 0 TO LEN(str) DO
Out.Int(i, 0); Out.Ln
END;

END tst.

lets try to compile it՝

[2020-09-02 16:00:20] $ voc -m tst.Mod 
tst.Mod  Compiling tst.

9:   FOR i := 0 TO LEN(str) DO
^
pos   102  err 113  incompatible assignment

Module compilation failed.
[2020-09-02 16:00:21] $

voc compiles Oberon source to C, which is very good to illustrate what happens. if we have՝

str: ARRAY 16 OF CHAR;

then the output C code will be՝

static CHAR tst_str[16];

nothing more։ no field for the length.

still, this՝

FOR i := 0 TO LEN(str) DO

will translate to՝

tst_i = 0;
while (tst_i <= 16) {

as i said we know the length at compile time. doesn’t matter if you generate C or assembly, you already know the number, you can put the number to the output assembly code as well.

when we write a function which receives strings, we should not knowt the length of the received string, we just use ARRAY OF CHAR as an argument՝

MODULE tst2;

PROCEDURE addStrs(VAR a, b: ARRAY OF CHAR);
BEGIN
(* do smth useful here *)
END addStrs;

PROCEDURE test*;
VAR
s0: ARRAY 32 OF CHAR;
s1: ARRAY 64 OF CHAR;
BEGIN
addStrs(s0, s1);
END test;

END tst2.

therefore C code will be՝

static void tst2_addStrs (CHAR *a, ADDRESS a__len, CHAR *b, ADDRESS b__len)
{
/* here we think we can do smth useful */
}

void tst2_test (void)
{
CHAR s0[32];
CHAR s1[64];
tst2_addStrs((void*)s0, 32, (void*)s1, 64);
}

as we see՝ the function also gets the length of strings. and if we do LEN(a) we get the length without any calculations.

now let’s see how dynamic strings work՝

MODULE tst3;

PROCEDURE addStrs(VAR a, b: ARRAY OF CHAR);
BEGIN
(* do smth useful here *)
END addStrs;

PROCEDURE test*(i: INTEGER);
VAR
s: ARRAY 32 OF CHAR;
ps: POINTER TO ARRAY OF CHAR;
BEGIN
NEW(ps, i);
addStrs(s, ps^);
END test;

END tst3.

now we hase a static string՝ s and we allocate a dynamic string with its pointer ps. lets assume we don’t know the size of ps^ string (^ means dereference) and we will receive the length of the allocated string as a function argument. it is not known at compile time.

first function remains unchanged, second function gets translated like this՝

static void tst3_addStrs (CHAR *a, ADDRESS a__len, CHAR *b, ADDRESS b__len)
{
/* do smth useful here */
}

void tst3_test (INT16 i)
{
CHAR s[32];
struct {
ADDRESS len[1];
CHAR data[1];
} *ps = NIL;
ps = __NEWARR(NIL, 1, 1, 1, 1, ((ADDRESS)(i)));
tst3_addStrs((void*)s, 32, (void*)ps->data, ps->len[0]);
}

the _NEWARR is a bit more complicated function, which is a part of the runtime. but we can understand what it does՝ it allocates a space in the heap, and the pointer ps we get now points to the struct, which has a data field and len field.

this is a runtime information, and in this case we have to keep a separate field for the length of the string.

that’s it.

#oberon #c #pascal #wirth #programming #programming-languages #programming_languages #design #implementation #vishap #voc #compiler #compilation #strings #string #heap #stack #storage #storage-management #storage_management #length

բնօրինակ սփիւռքում(եւ մեկնաբանութիւննե՞ր)

this is a very good blog post՝ back to basics about strings in C and Pascal. i suggest you to read it, and now i’ll tell you something else:

in Oberon, Wirth decided to give up on Pascal strings, and use zero terminated strings.

however, there is no need to run by the whole array to find out the length of the string. we don’t even need to have a separate field that holds the length՝ compiler knows the length of the static string.

thus it can do compile time tests. for example we have the following Oberon source՝

MODULE tst;
IMPORT Out;

VAR i: SHORTINT;
str: ARRAY 1024 OF CHAR;

BEGIN

FOR i := 0 TO LEN(str) DO
Out.Int(i, 0); Out.Ln
END;

END tst.

lets try to compile it՝

[2020-09-02 16:00:20] $ voc -m tst.Mod 
tst.Mod  Compiling tst.

9:   FOR i := 0 TO LEN(str) DO
^
pos   102  err 113  incompatible assignment

Module compilation failed.
[2020-09-02 16:00:21] $

voc compiles Oberon source to C, which is very good to illustrate what happens. if we have՝

str: ARRAY 16 OF CHAR;

then the output C code will be՝

static CHAR tst_str[16];

nothing more։ no field for the length.

still, this՝

FOR i := 0 TO LEN(str) DO

will translate to՝

tst_i = 0;
while (tst_i <= 16) {

as i said we know the length at compile time. doesn’t matter if you generate C or assembly, you already know the number, you can put the number to the output assembly code as well.

when we write a function which receives strings, we should not knowt the length of the received string, we just use ARRAY OF CHAR as an argument՝

MODULE tst2;

PROCEDURE addStrs(VAR a, b: ARRAY OF CHAR);
BEGIN
(* do smth useful here *)
END addStrs;

PROCEDURE test*;
VAR
s0: ARRAY 32 OF CHAR;
s1: ARRAY 64 OF CHAR;
BEGIN
addStrs(s0, s1);
END test;

END tst2.

therefore C code will be՝

static void tst2_addStrs (CHAR *a, ADDRESS a__len, CHAR *b, ADDRESS b__len)
{
/* here we think we can do smth useful */
}

void tst2_test (void)
{
CHAR s0[32];
CHAR s1[64];
tst2_addStrs((void*)s0, 32, (void*)s1, 64);
}

as we see՝ the function also gets the length of strings. and if we do LEN(a) we get the length without any calculations.

now let’s see how dynamic strings work՝

MODULE tst3;

PROCEDURE addStrs(VAR a, b: ARRAY OF CHAR);
BEGIN
(* do smth useful here *)
END addStrs;

PROCEDURE test*(i: INTEGER);
VAR
s: ARRAY 32 OF CHAR;
ps: POINTER TO ARRAY OF CHAR;
BEGIN
NEW(ps, i);
addStrs(s, ps^);
END test;

END tst3.

now we hase a static string՝ s and we allocate a dynamic string with its pointer ps. lets assume we don’t know the size of ps^ string (^ means dereference) and we will receive the length of the allocated string as a function argument. it is not known at compile time.

first function remains unchanged, second function gets translated like this՝

static void tst3_addStrs (CHAR *a, ADDRESS a__len, CHAR *b, ADDRESS b__len)
{
/* do smth useful here */
}

void tst3_test (INT16 i)
{
CHAR s[32];
struct {
ADDRESS len[1];
CHAR data[1];
} *ps = NIL;
ps = __NEWARR(NIL, 1, 1, 1, 1, ((ADDRESS)(i)));
tst3_addStrs((void*)s, 32, (void*)ps->data, ps->len[0]);
}

the _NEWARR is a bit more complicated function, which is a part of the runtime. but we can understand what it does՝ it allocates a space in the heap, and the pointer ps we get now points to the struct, which has a data field and len field.

this is a runtime information, and in this case we have to keep a separate field for the length of the string.

that’s it.

#oberon #c #pascal #wirth #programming #programming-languages #programming_languages #design #implementation #vishap #voc #compiler #compilation #strings #string #heap #stack #storage #storage-management #storage_management #length

բնօրինակ սփիւռքում(եւ մեկնաբանութիւննե՞ր)

Modula and Modula-2 went nowhere not because they’re particularly high level languages (they’re not much higher than C!) but because they didn’t have a quasi-free operating system spreading like a virus to bolster their popularity. At the time Unix was almost unique in being a semi-capable operating system written in a portable language that wasn’t wrapped up in billions of proprietary licensing arrangements (because AT&T didn’t notice there was a potential market for it until after the cat had escaped the bag).

source

#modula #modula-2 #modula2 #c #programming-languages #programming_languages #unix

բնօրինակ սփիւռքում(եւ մեկնաբանութիւննե՞ր)

First things first: printf is expecting a valid (i.e. non-NULL) pointer for its %s argument so passing it a NULL is officially undefined. It may print “(null)” or it may delete all files on your hard drive–either is correct behavior as far as ANSI is concerned (at least, that’s what Harbison and Steele tells me.)

https://stackoverflow.com/questions/11589342/what-is-the-behavior-of-printing-null-with-printfs-s-specifier/11589443#11589443

#programming #c #null

բնօրինակ սփիւռքում(եւ մեկնաբանութիւննե՞ր)

today we were discussing different spacecraft accidents caused by/related to software, discussed Arian-5 case, and I remembered a Soviet case, where there was a problem with decimal separator and Fortran compiler. Decimal separator in USSR was comma, but in Fortran code it should be the dot.

Also, Fortran compiler did not consider using comma instead of the dot as an error, but as different code. And my friend said - the same is possible to do in C++ - and sent me the illustration.

#include <iostream>
using namespace std;
int main()
{
cout << "Hello World" << endl;
if (1,2!=1.2)
{
cout << (1,2) << endl;
}
return 0;
}

if you run it, you’ll get

Hello World
2

(:

#safety #programming #mistakes #safe #c++ #code #example #programming-languages #programming_languages #arian #fortran #source

բնօրինակ սփիւռքում(եւ մեկնաբանութիւննե՞ր)

C++ is a horrible language. It’s made more horrible by the fact that a lot of substandard programmers use it, to the point where it’s much much easier to generate total and utter crap with it. Quite frankly, even if the choice of C were to do nothing but keep the C++ programmers out, that in itself would be a huge reason to use C.

Linus Torvalds, here.

#linus #linus_torvalds #git #programming_languages #programming-languages #c++ #c #programming

բնօրինակ սփիւռքում(եւ մեկնաբանութիւննե՞ր)