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On Mon, 24 Jun 2019, Paul Flint wrote:
Greetings List Lurkers,
Marvelous Marco submits this for our consternation:
<snip>
I was recently messing around with embedding files into ELF executables and
came across this strange behavior:
#include <stdio.h>
// extern unsigned char *_binary_data_txt_start; /* doesn't work */
extern unsigned char _binary_data_txt_start[]; /* does work */
int main() {
printf("%p\n", _binary_data_txt_start);
}
This file segfaults when using the pointer (*) notation, but runs correctly
with the array notation ([]).
I'm compiling with the command:
gcc test.c -o test -Wl,--format=binary,data.txt,--format=default -g
With the above in a file called test.c and some data in a file called
data.txt.
I asked for some explanation but I kept getting back "That makes sense" with
no explanation....
I'm not 100% certain, but I think that what's happening is that
using the * notation only declares the pointer, while using the [] notation
declares the pointer _and initializes it to point at a (zero-size) array_.
I don't get segfaults when I try *_binary_data_txt_start; it runs
correctly and reports the pointer as (nil), where _binary_data_txt_start[]
reports an actual address, which I believe supports this.
- --
John Campbell
[email protected]
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