Re: what am I missing?
- Posted by mattlewis (admin) Jun 19, 2009
- 1165 views
here C code from termbits.h
typedef unsigned char cc_t;
typedef unsigned int speed_t;
typedef unsigned int tcflag_t;
#define NCCS 19
struct termios2 {
tcflag_t c_iflag; /* input mode flags */
tcflag_t c_oflag; /* output mode flags */
tcflag_t c_cflag; /* control mode flags */
tcflag_t c_lflag; /* local mode flags */
cc_t c_line; /* line discipline */
cc_t c_cc[NCCS]; /* control characters */
speed_t c_ispeed; /* input speed */
speed_t c_ospeed; /* output speed */
};
from this I compute 44 bytes for termios2 struct:
4*4 + 1 + 19 + 2*4 = 44
But if I allocate 44 bytes my program crash. I must allocate 45 bytes for it to works. Where is the missing byte?
Jacques
The C language leaves the packaging of data within a struct up to the compiler. Most compliers add padding bytes in order to ensure alignment of data on certain boundaries. In this case, it would appear that it tries to make sure that each new field begins on an even address, so I'm guessing there is a hidden padding byte between c_line and c_cc fields. However, that would also mean that you need to allocate 46 bytes because there should also be a padding byte after c_cc field.
Yes, it's very compiler dependent. I added the following to your struct definition:
#include <stdio.h>
int main(){
struct termios2 t;
printf("termios2: %d\n", sizeof( struct termios2 ) );
printf("c_iflag: %p\n", &t.c_iflag );
printf("c_oflag: %p\n", &t.c_oflag );
printf("c_cflag: %p\n", &t.c_cflag );
printf("c_lflag: %p\n", &t.c_lflag );
printf("c_line: %p\n", &t.c_line );
printf("c_cc: %p\n", &t.c_cc );
printf("c_ispeed: %p\n", &t.c_ispeed );
printf("c_ospeed: %p\n", &t.c_ospeed );
return 0;
}
And with OpenWatcom v1.8, here's what I got:
termios2: 44 c_iflag: 0006fe68 c_oflag: 0006fe6c c_cflag: 0006fe70 c_lflag: 0006fe74 c_line: 0006fe78 c_cc: 0006fe79 c_ispeed: 0006fe8c c_ospeed: 0006fe90So OW1.8 at least doesn't pad between byte sized members.
Matt
