Labels and pointers

[Jean-Guillaume Pyraksos]

Is a label something like a pointer i can store in a variable ?

#include <stdio.h>

void foo(void *p)
{
printf("foo\n");
goto *p;
}

int main()
{
foo(&&L1); // ???????? Explain...
printf("bar\n"); // should not be executed ?
L1:
printf("quit\n");
return 0;
}

Can somebody elaborate on this code i found on the net ?...
gcc compiles it but the result is strange (MacIntel Leopard, gcc 4.0.0)

$ gcc prog.c -o prog
$ ./prog
foo
r is not present to run this program
bar
quit
$

Thanks,

-JG

[Mark Bluemel]

Jean-Guillaume Pyraksos wrote:

Is a label something like a pointer i can store in a variable ?

#include <stdio.h>

void foo(void *p)
{
printf("foo\n");
goto *p;
}

int main()
{
foo(&&L1); // ???????? Explain...
printf("bar\n"); // should not be executed ?
L1:
printf("quit\n");
return 0;
}

Can somebody elaborate on this code i found on the net ?...

It appears to use GCC specific extensions - try building it with the
options "-ansi -Wall -pedantic"

gcc compiles it but the result is strange (MacIntel Leopard, gcc 4.0.0)

$ gcc prog.c -o prog
$ ./prog
foo
r is not present to run this program
bar
quit

The reference manual page I found
(http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html) says "You may
not use this mechanism to jump to code in a different function. If you
do that, totally unpredictable things will happen. The best way to avoid
this is to store the label address only in automatic variables and never
pass it as an argument."

So the code you quote is a) non-standard and b) broken even when && is
supported.

[Richard Tobin]

In article <wi**************************@malibu.unice.fr>,
Jean-Guillaume Pyraksos <wi****@hotmail.comwrote:

>Is a label something like a pointer i can store in a variable ?

As others have pointed out, only when using an extension like the
one that gcc provides.

>void foo(void *p)
{
printf("foo\n");
goto *p;
}

int main()
{
foo(&&L1); // ???????? Explain...
printf("bar\n"); // should not be executed ?
L1:
printf("quit\n");
return 0;
}

>Can somebody elaborate on this code i found on the net ?...

Again, as others have pointed out, this code is using the gcc
extension wrongly. You should only use it for jumps where a
normal jump to the location would be legal.

>gcc compiles it but the result is strange (MacIntel Leopard, gcc 4.0.0)

$ gcc prog.c -o prog
$ ./prog
foo
r is not present to run this program
bar
quit

My guess would be that the goto is "working", but because the function
foo is still active the format string "quit" is not in the right place
relative to the stack pointer, and instead it gets the end of some
other string (you can find what it probably is by googling for "is not
present to run this program"). It then executes main's return, but
because foo's stack frame is still there it returns to main, and
continues executing normally.

-- Richard
--
"Consideration shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

[Richard Tobin]

In article <8_******************************@comcast.com>,
Eric Sosman <es*****@ieee-dot-org.invalidwrote:

There are at least two reasons not to use this feature:

And there are also reasons to use it, though most people probably
won't encounter them. It seems quite in accord with the spirit of
C: no worse, for example, than setjmp()/longjmp().

-- Richard
--
"Consideration shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.