Segmentation fault when compiled binary is executed

[nae zot bba la]

Hi, very short code here. Basically I want to open a file called
instruction in a++ mode and then write data of %s type to it, then read
from the same file and print to screen. The code compiles but when I
execute the binary and enter some string to expect the program to print
them for me to screen, segmentation fault occurs. Any ideas anyone?

#include <stdio.h>
#include <stdlib.h>

char *add;
char *show;
FILE *instruction;

main()
{
scanf("%s",&add);
instruction=fopen("instruction","a++");
fwrite(add,sizeof(add),1,instruction);
fscanf(instruction,"%s",&show);
printf("%s",show);
fcloseall();
}

 

[sundeyu]

You should allocate memory for add and show first. BTW, you should
close the file and then open it with read mode to get the string
printed out.

 

[Richard Heathfield]

nae zot bba la said:

Hi, very short code here. Basically I want to open a file called
instruction in a++ mode

There is no such mode. You may be thinking of "a+" or "a+b".

and then write data of %s type to it,

But there is no such type. You may be thinking of "sequence of char
terminated by the first null character", which is also known as "string" -
and this is not a data type but a data format.

then read
from the same file and print to screen. The code compiles but when I
execute the binary and enter some string to expect the program to print
them for me to screen, segmentation fault occurs. Any ideas anyone?

#include <stdio.h>
#include <stdlib.h>

char *add;
char *show;
FILE *instruction;

main()

Aside: the preferred form of main(), when taking no arguments, is:

int main(void)

but this is not the cause of your seg fault.

{
scanf("%s",&add);

Quite apart from the hazards involved in using %s with scanf, and quite
apart from the fact that you appear to be ignoring an extremely useful
value returned by scanf, there is also the problem that scanf not only
requires char * rather than char ** as its argument but also requires that
pointer to be pointing to sufficient storage to receive the data in
question. This is almost certainly the cause of your seg fault. Well, it's
the cause of /this/ seg fault, anyway. Once you've fixed it, I fully expect
you to get others.

 

[nae zot bba la]

thx. so basically somethign like

add=malloc(sizeof(a));
show=malloc(sizeof(s));

 

[Richard Heathfield]

nae zot bba la said:

thx. so basically somethign like

add=malloc(sizeof(a));
show=malloc(sizeof(s));

Whilst the above changes will indeed stop you from getting a segmentation
fault, this is only because they will instead give you compilation errors.

Please find a good C book, one that explains about storage, and read it very
closely until you get the hang of - at least - the following facts:

1) Everything has to be Somewhere. Every single character you need to store
requires a byte of storage in which to be stored.
2) Types *matter*.
3) The malloc function can return NULL.

 

[Mike Wahler]

Hi, very short code here. Basically I want to open a file called
instruction in a++ mode and then write data of %s type to it, then read
from the same file and print to screen. The code compiles but when I
execute the binary and enter some string to expect the program to print
them for me to screen, segmentation fault occurs. Any ideas anyone?

#include <stdio.h>
#include <stdlib.h>

char *add;
char *show;
FILE *instruction;

There's no reason for these objects to be defined at file scope.

main()

int main(void)

{
scanf("%s",&add);

Undefined behavior.

instruction=fopen("instruction","a++");

Undefined behavior.

fwrite(add,sizeof(add),1,instruction);

Undefined behavior.

fscanf(instruction,"%s",&show);

Undefined behavior.

printf("%s",show);

Undefined behavior.

fcloseall();

No such function in standard C.

return 0;

}

You don't have a C text, do you? (If you do, you need much more study. If
you
don't, get one. See the reviews at www.accu.org for suggestions).

-Mike

 

[nae zot bba la]

You don't have a C text, do you? (If you do, you need much more study. If

you
don't, get one. See the reviews at www.accu.org for suggestions).

-Mike

MKW, i will go and read this from chapter 1 to the end.
http://publications.gbdirect.co.uk/c_book/chapter1/form_of_a_c_program.html

 

[Christopher Benson-Manica]

MKW, i will go and read this from chapter 1 to the end.
http://publications.gbdirect.co.uk/c_book/chapter1/form_of_a_c_program.html

That seems like a much better resource than a lot of the supposed
tutorials to be found online, but it still is inferior to the K&R2
text (which you should purchase if you have the means to do so).
I spotted a few instances (I imagine more exist) where a C guru
(I am not one) would have cause to quibble:

1) int main() isn't incorrect, but int main( void ) is generally
preferable.

2) Ending main() with a call to exit( EXIT_SUCCESS ) isn't incorrect
either, but it isn't great style. return EXIT_SUCCESS is much more
conventional.

3) The language in 5.4.1 fails to describe "FOO" as a string literal
and therefore leaves the distinction between them and "ordinary"
strings unclear.

4) "The rules for assignment of pointers show that there is no need to
use a cast on the return value from malloc, but it is often done in
practice."

This statement might have been accurate in 1991, but it really
isn't these days. Casting the return value of malloc() is
generally discouraged.

In short, you might read the C FAQ along with the resource you cited:

http://c-faq.com

and fill in some of the holes.

 

[Richard Bos]

That seems like a much better resource than a lot of the supposed
tutorials to be found online, but it still is inferior to the K&R2
text (which you should purchase if you have the means to do so).
I spotted a few instances (I imagine more exist) where a C guru
(I am not one) would have cause to quibble:

4) "The rules for assignment of pointers show that there is no need to
use a cast on the return value from malloc, but it is often done in
practice."

This statement might have been accurate in 1991, but it really
isn't these days. Casting the return value of malloc() is
generally discouraged.

Counter-quibble: yes, it's accurate. Casting malloc() _is_ often done.
Shouldn't be, but it is.

Richard

 

[Keith Thompson]

2) Ending main() with a call to exit( EXIT_SUCCESS ) isn't incorrect
either, but it isn't great style. return EXIT_SUCCESS is much more
conventional.

I don't think I agree. Using exit() is fairly common. I tend to use
0 rather than EXIT_SUCCESS myself, unless I'm also using EXIT_FAILURE.

For no particularly good reason, I tend to think of EXIT_SUCCESS and
EXIT_FAILURE as arguments to exit() rather than as values to be
return'ed from main().

"exit(n);" and "return n;" (in main()) are almost exactly equivalent
anyway, so I don't worry much about which one anyone chooses to use.
(The only difference is the lifetime of variables local to main()
referred to indirectly from an atexit()-registered function; any
program for which this matters is far too obscure anyway.)

 

[Christopher Benson-Manica]

I don't think I agree. Using exit() is fairly common.

At the end of main()? I accept that your experience is much broader
than mine, but FWIW I have seen many more return's than exit()'s at
the end of main().

"exit(n);" and "return n;" (in main()) are almost exactly equivalent
anyway, so I don't worry much about which one anyone chooses to use.

I suppose 1991 conventions may have been different; I was 11 at the
time, so I'm not qualified to comment. In any case, I admit that it
isn't much more than a question of style religion. Isn't there some
(negligible) overhead associated with the call to exit() as opposed to
a straight return, though?

Clearly, if God had intended exit() to be used instead of a return from
main(), He would have allowed void main( void ) as a valid prototype

 

[Gordon Burditt]

"exit(n);" and "return n;" (in main()) are almost exactly equivalent

I suppose 1991 conventions may have been different; I was 11 at the
time, so I'm not qualified to comment. In any case, I admit that it
isn't much more than a question of style religion. Isn't there some
(negligible) overhead associated with the call to exit() as opposed to
a straight return, though?

Since many implementations use startup code like:

_start() { exit(main(argc, argv)); }
(where getting the values of argc and argv is implementation-dependent).

isn't there some (negligible) overhead associated with the return
which doesn't happen if you call exit() directly?

Gordon L. Burditt

 

[Christopher Benson-Manica]

isn't there some (negligible) overhead associated with the return
which doesn't happen if you call exit() directly?

Sounds like six on one system, half a dozen on another. I was not
aware of

_start() { exit(main(argc, argv)); }

, so I appreciate the insight.