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Full Discussion: Why does this example C code run and yet SHOULD either not compile or give a segmentation fault?
Top Forums Programming Why does this example C code run and yet SHOULD either not compile or give a segmentation fault? Post 303032991 by wisecracker on Thursday 28th of March 2019 05:05:27 AM
Old 03-28-2019
Thanks Jim...
I only attempted it to see if it was possible, and it DOES give a segmentation fault IF and only IF the 'jmp' goes directly into main() .
BUT it still compiles...

I created this absolute meaningless garbage and it compiles without warnings and errors and look what happens:
Code:
#include <stdio.h>
#include <stdlib.h>

int NUMBER1 = 255;
float NUMBER2 = 3.1415926;
char CHARACTER = 'X';

int test1(int NUMBER1)
{
    __asm__ volatile(
        "nop;"
        "jmp    jump_test2;"
        "nop;");
    NUMBER1 = 123;
    NUMBER2 = 1.414;
    CHARACTER = '!';
    printf("This will never be seen!\n");
    return NUMBER1;
}

float test2(float NUMBER2)
{
    /* The 'test2:' label sits in here. */
    asm(
        "nop;"
        "jump_test2:"
        "nop;");
    NUMBER1 = -65;
    NUMBER2 = 2.718;
    CHARACTER = '?';
    printf("This will be printed.\n");
    printf("%.03f%c\n", NUMBER2, CHARACTER);
    return NUMBER2;
}

int main(int NUMBER1, char **CHARACTER)
{
    NUMBER1 = test1(NUMBER1);
    printf("%d %s\n", NUMBER1, *CHARACTER);
    printf("Hello World!\n");
    return(0);
}

Results on OSX 10.14.3, default bash terminal, gcc 4.2.1.
(IMPORTANT! NOT checked on gcc 2.95.2 or 7.3.0.)
Code:
Last login: Thu Mar 28 08:36:48 on ttys000
AMIGA:amiga~> cd Desktop/Code/C
AMIGA:amiga~/Desktop/Code/C> gcc cross_function_jump.c
AMIGA:amiga~/Desktop/Code/C> cp a.out cross_function_jump
AMIGA:amiga~/Desktop/Code/C> ./cross_function_jump
This will be printed.
2.718?
7 ./cross_function_jump
Hello World!
AMIGA:amiga~/Desktop/Code/C> _

7 ./cross_function_jump is obviously wrong but I have successfully got 'argv[0]'.
I don't care what is happening but compiling AND running without a segmentation fault is is not a fault of the programmer but of the compiler.

Any 'asm()' whether part of compliance or not code should never be allowed to jump out of its own function domain and this was my point entirely.

What I have done I would never use in practice but I would use inline assembly for mission critical stuff inside its own function.

All I wanted to know why these compile and run, garbage results or not.
 

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LEARN ABOUT DEBIAN

cw

CW(7)							 Miscellaneous Information Manual						     CW(7)

NAME

       CW - the international Morse code

DESCRIPTION

       CW  is  an  abbreviation for "continuous wave", the commonly used technical term for Morse code communication.  A basic knowledge or under-
       standing of Morse code is a requirement for Radio Amateurs and Marine Radio Operators in many parts of the world.

   MORSE CODE TIMINGS
       In Morse code, a dot or dash is referred to as an element.  The basic timing unit is the dot period.  This is the time taken to send a dot,
       not  including  any space before or after the dot.  The lengths of all other elements are then derived from this basic unit, using the fol-
       lowing rules:

	      The duration of a dash is three dots.

	      The time between each element (dot or dash) is one dot length.

	      The space between characters is three dot lengths.

	      The space between words is seven dot lengths.

       The following formula calculates the dot period in microseconds from the Morse code speed in words per minute:

	      dot period = ( 1200000 / speed )

       This formula arises from the use of the word PARIS as a 'standard' word for calibrating Morse code speed.  PARIS is 50 units long when sent
       in Morse code.  Analysis of English plain-text indicates that the average word is 50 units, including spaces.

   MORSE CODE CHARACTERS
       The following list shows the IS0 8859-1 (Latin-1) characters that have commonly understood representations in Morse code:

	      ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"$()+-./:;=?_@ and space

       In addition, following ISO 8859-1 and ISO 8859-2 accented characters are also part of the generally accepted international Morse code:

	      UACOEEANS (S with cedilla),  (Z with caron/hacek),

       Finally, libcw adds the following ASCII characters as extensions to single character procedural signals:

	      <>!&^~

   MORSE CODE CHARACTER TABLES
       The  following  table  shows  the Morse code equivalents for the ISO 8859-1, accented ISO 8859-1, and accented ISO 8859-2 characters above.
       The ASCII portion of this table is taken from the ARRL Handbook, and the accented extensions from various other sources:

       Ch   Code     Ch   Code	   Ch	Code	  Ch   Code
       -------------------------------------------------------
       A    .-	     B	  -...	   C	-.-.	  D    -..
       E    .	     F	  ..-.	   G	--.	  H    ....
       I    ..	     J	  .---	   K	-.-	  L    .-..
       M    --	     N	  -.	   O	---	  P    .--.
       Q    --.-     R	  .-.	   S	...	  T    -
       U    ..-      V	  ...-	   W	.--	  X    -..-
       Y    -.--     Z	  --..

       0    -----    1	  .----    2	..---	  3    ...--
       4    ....-    5	  .....    6	-....	  7    --...
       8    ---..    9	  ----.

       "    .-..-.   '	  .----.   $	...-..-   (    -.--.
       )    -.--.-   +	  .-.-.    ,	--..--	  -    -....-
       .    .-.-.-   /	  -..-.    :	---...	  ;    -.-.-.

       =    -...-    ?	  ..--..   _	..--.-

       Ch	       Code    Ch		  Code
       -------------------------------------------------
       U	       ..--    A		  .-.-
       C	       -.-..   O		  ---.
       E	       ..-..   A		  .-..-
       A	       .--.-   N		  --.--
       S (S+cedilla)   ----	(Z+caron/hacek)   --..-

       In addition to the above standard characters, the following characters are conventionally used for punctuation and  procedural  signals	as
       follows:

       Ch   Code     Ch   Code	   Ch	Code	  Ch   Code
       -------------------------------------------------------
       "    .-..-.   '	  .----.   $	...-..-   (    -.--.
       )    -.--.-   +	  .-.-.    ,	--..--	  -    -....-
       .    .-.-.-   /	  -..-.    :	---...	  ;    -.-.-.
       =    -...-    ?	  ..--..   _	..--.-	  @    .--.-.

       and the following are non-conventional extensions implemented by libcw:

       Ch   Code     Ch   Code	    Ch	 Code	 Ch   Code
       -----------------------------------------------------
       <    ...-.-   >	  -...-.-   !	 ...-.	 &    .-...
       ^    -.-.-    ~	  .-.-..

       An alternative view of punctuation and procedural signals is as combination Morse characters:

       Ch   Prosig	Ch   Prosig   Ch   Prosig   Ch	 Prosig
       ---------------------------------------------------------
       "    [AF]	'    [WG]     $    [SX]     (	 [KN]
       )    [KK]	+    [AR]     ,    [MIM]    -	 [DU]
       .    [AAA]	/    [DN]     :    [OS]     ;	 [KR]
       =    [BT]	?    [IMI]    _    [IQ]     @	 [AC]
       <    [VA],[SK]	>    [BK]     !    [SN]     &	 [AS]
       ^    [KA]	~    [AL]

NOTES

       Despite	the fact that this manual page constantly and consistently refers to Morse code elements as dots and dashes, DO NOT think in these
       terms when trying to learn Morse code.  Always think of them as 'dit's and 'dah's.

SEE ALSO

       Man pages for libcw(3,LOCAL), cw(1,LOCAL), cwgen(1,LOCAL), cwcp(1,LOCAL), and xcwcp(1,LOCAL).

								 CW Tutor Package							     CW(7)
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