Why does this example C code run and yet SHOULD either not compile or give a segmentation fault? Post: 303032991

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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

03-28-2019

Registered User

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.

wisecracker

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LEARN ABOUT DEBIAN

djvu2hocr

DJVU2HOCR(1)							 djvu2hocr manual						      DJVU2HOCR(1)

NAME

       djvu2hocr - DjVu to hOCR converter

SYNOPSIS

       djvu2hocr [option...] djvu-file

       djvu2hocr {--version | --help | -h}

DESCRIPTION

       djvu2hocr converts hidden text from a DjVu file to the hOCR[1] format.

OPTIONS

   Text segmentation options
       --word-segmentation=simple
	   Use the same word segmentation as found in the DjVu file.

	   This is the default.

       --word-segmentation=uax29
	   Use the Unicode Text Segmentation[2] algorithm to break lines into words, possibly fixing word segmentation found in the DjVu file.

   Other options
       --version
	   Output version information and exit.

       -h, --help
	   Display help and exit.

PORTABILITY

       djvu2hocr uses a custom extension to hOCR to retain characters which cannot be directly represented in an HTML/XML document. For example,
       control character BEL (^G, U+0007), is converted into the following HTML chunk: <span class="djvu_char" title="#x07"> </span>

SEE ALSO

       djvu(1)

AUTHOR

       Jakub Wilk <jwilk@jwilk.net>
	   Author.

NOTES

	1. hOCR
	   http://docs.google.com/View?docid=dfxcv4vc_67g844kf

	2. Unicode Text Segmentation
	   http://unicode.org/reports/tr29/

djvu2hocr 0.7.9 						    03/10/2012							      DJVU2HOCR(1)

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