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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
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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FTWHICH(1)						      General Commands Manual							FTWHICH(1)

NAME
ftwhich - fault tolerant search for a command name SYNOPSIS
ftwhich [-#hIp][-t#] program_name DESCRIPTION
ftwhich is a fault tolerant version of the which(1) command. ftwhich searches for a given program in all directories included in your PATH environment variable and reports all files with a name that approximately matches the given program_name. ftwhich achieves fault tolerance by calculating the so called Weighted Levenshtein Distance. The Levenshtein Distance is defined as the minimum number of character insertions, deletions and replacements that transform a string A into a string B. ftwhich is similar to the which command with the following differences: - ftwhich is by default NOT case sensitive - ftwhich is fault tolerant - Some shells have a build in which command that will also search aliases. ftwhich can naturally not search for aliases as it does not know about alias definitions. - ftwhich lists all files that approximately match. The files first shown take preference over files of the same name printed later as they are from directories listed earlier in the PATH. - The level of fault tolerance can be adjusted by specifying the optional parameter tolerance. A tolerance of 0 specifies exact match. OPTIONS
-h Prints help/usage information. -I Do case sensitive search (default is case in-sensitive) -p print the actual distance value in front of the found filename. This value is equal to the number of insertions, deletions and replacements necessary to transform the name of the found program into the search key. -# or -t# Set the fault tolerance level to #. The fault tolerance level is an integer in the range 0-255. It specifies the maximum number of errors permitted in finding the approximate match. The default tolerance is (strlen(searchpattern) - number of wildcards)/6 + 1 program_name The program file to search for. '*' and '?' can be used as wildcards. '?' denotes one single character. '*' denotes an arbitrary number of characters. The last argument to ftwhich is not parsed for options as the program needs at least one program_name argument. This means that ftwhich -x will not complain about a wrong option but search for the program named -x. EXAMPLE
Search for all programs like gcc in your PATH: ftwhich gcc This will e.g. find gcc or cc or CC ... To find all files that start with any prefix and end in config and differ in 2 letters from the word config: ftwhich -2 '*config' To find all files that exactly start with the prefix if: ftwhich -0 'if*' To find all clock programs: ftwhich -0 '*clock*' BUGS
The wildcards '?' and '*' can not be escaped. These characters function always as wildcards. This is however not a big problem since there is normally hardly any command that has these characters in its name. AUTHOR
Guido Socher (guido@linuxfocus.org) SEE ALSO
whichman(1), ftff(1) Search utilities January 1999 FTWHICH(1)
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