jga@jt.dk (Jens Groth Andreasen) (12/10/90)
I'm developing an application for a PC running INTERACTIVE UNIX System V/386
Release 3.2 using the standard C compiler ('cc').
For reasons beyond my control, some of the structures I use must be
byte aligned (i.e. no padding with unused bytes). I was hoping that
'#pragma pack(1)' could do the job for me, but as the following example show
the directive doesn't work 100% correctly.
/* EXAMPLE BEGINS HERE */
#include<stdio.h>
#pragma pack(1)
typedef struct {
char e1;
int e2;
} S1;
typedef struct {
char e1;
int e2;
char e3;
} S2;
typedef struct {
S1 s1;
S2 s2;
} S3;
static S3 s3 = {
'c', 4711,
'q', 117, 'w'
};
main()
{
printf("%c %i\n%c %i %c\n", s3.s1.e1, s3.s1.e2,
s3.s2.e1, s3.s2.e2, s3.s2.e3 );
}
/* END OF EXAMPLE */
When I compile this with 'cc example.c', the output of the program will be:
g 1962934290
1902313472 w
/* END OF OUTPUT */
If I remove the #pragma pack(1), the output will be the expected, but if I
now compile the (modified) program with 'cc example.c -Zp1' ( the -Zp[1|2|4]
option will byte-, two-byte- or word-align members of structures resp.) I get
the strange output from before.
Can anyone running on a similar system recreate this?
Can anyone explain this?
If I initialize the structure by assignments (like 's3.s1.e2 = 4711') I get
the right output (with or without the #pragma pack(1)).
Please mail me even if you respond by posting, I'll post a summary of the
responds.
Regards, Jens.
----------------------------------------------------------------------
email: jga@jt.dk
jgroth@daimi.aau.dk (this address is the most reliable one)
FAX: +45 86 29 39 77
Phone: +45 86 29 33 66 ext. 6925 (open 7.15 - 15.15 GMT)
Letter post: Jens Groth Andreasen
Jutland Telephone
Sletvej 30
DK-8310 Tranbjerg J.
Denmark (Europe)
----------------------------------------------------------------------danabu@scorpio.uucp (Daniel N. Abushanab) (12/14/90)
I'm very new to shell programming, so maybe this is a stupid question, but I'll ask away anyway. I'm trying to write a script that will look in a directory, see how many files are in there, then randomly select one to be displayed as a background picture. Currently I'm using the time as a selection criterion, where a different picture is displayed depending on what time it is when the request is made. Not very elegant, but like I said, I'm new at this. Any help will be greatly appreciated. Thanks. ---------------------------------------------------------------------------- Daniel N. Abushanab University of California, Berkeley E-mail: danabu@scorpio.berkeley.edu Mechanical Engineering Department phone: (415) 642-5109