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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explain_printf_or_die

explain_printf_or_die(3)				     Library Functions Manual					  explain_printf_or_die(3)

NAME

       explain_printf_or_die - formatted output conversion and report errors

SYNOPSIS

       #include <libexplain/printf.h>
       int explain_printf_or_die(const char *format);
       int explain_printf_on_error(const char *format);

DESCRIPTION

       The explain_printf_or_die function is used to call the printf(3) system call. On failure an explanation will be printed to stderr, obtained
       from the explain_printf(3) function, and then the process terminates by calling exit(EXIT_FAILURE).

       The explain_printf_on_error function is used to call the printf(3) system call. On failure  an  explanation  will  be  printed  to  stderr,
       obtained from the explain_printf(3) function, but still returns to the caller.

       format  The format, exactly as to be passed to the printf(3) system call.

RETURN VALUE

       The  explain_printf_or_die  function  only  returns  on	success, see printf(3) for more information. On failure, prints an explanation and
       exits, it does not return.

       The explain_printf_on_error function always returns the value return by the wrapped printf(3) system call.

EXAMPLE

       The explain_printf_or_die function is intended to be used in a fashion similar to the following example:
	      int result = explain_printf_or_die(format);

SEE ALSO

       printf(3)
	       formatted output conversion

       explain_printf(3)
	       explain printf(3) errors

       exit(2) terminate the calling process

COPYRIGHT

       libexplain version 0.52
       Copyright (C) 2010 Peter Miller

															  explain_printf_or_die(3)

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