name@portiaStanford.EDU (tony cooper) (06/06/90)
nlist is declared in A/UX starting as:
struct nlist { /* symbol table entry */
char n_name[8]; /* symbol name */
Well n_name is restricted to 8 bytes so names must be 7 bytes long or less.
So how do you read sysmbols that are longer than 7 characters? There are
plenty of symbols that are longer. Most other UNIXs declare char *n_name so
they don't have this problem.
The only source code example I have is xload. But it reads the symbol avenrun
which is 7 characters. Other A/UX programs can read longer sysmbols eg netstat
reads symbols longer than 7 such as icmpstat and rthashsize. (But most
symbol reading programs read short symbols only).
Any ideas?
Tony Cooper
tony@popserver.stanford.eduliam@cs.qmw.ac.uk (William Roberts) (06/08/90)
In <1990Jun6.003947.27323@portia.Stanford.EDU> name@portiaStanford.EDU (tony cooper) writes: >nlist is declared in A/UX starting as: >struct nlist { /* symbol table entry */ > char n_name[8]; /* symbol name */ >Well n_name is restricted to 8 bytes so names must be 7 bytes long or less. >So how do you read sysmbols that are longer than 7 characters? Close but no cigar. nlist is actually defined as #define nlist syment in both A/UX 1.1.1 and A/UX 2.0b10, so what you are getting is a struct syment. In order to allow backwards compatibility with the Stone Age, the first 8 bytes of the structure are defined in <syms.h> as union { char _n_name[SYMNMLEN]; /* old COFF version */ struct { long _n_zeroes; /* new == 0 */ long _n_offset; /* offset into string table */ } _n_n; char *_n_nptr[2]; /* allows for overlaying */ } _n; Symbols of up to 8 bytes are placed in _n_name by old-style compilers. New style compilers (e.g. the A/UX ones) set _n_zeroes to zero as a marker, and then put an offset into the string table in _n_offset. Naturally there are #defines to hide the _n union and the _n_n structure. Old and new styles don't mix (I don't think) and the syment() library routine returns the value associated with the symbol (normally an address) in n_value. >There are >plenty of symbols that are longer. Most other UNIXs declare char *n_name so >they don't have this problem. >The only source code example I have is xload. But it reads the symbol avenrun >which is 7 characters. Other A/UX programs can read longer sysmbols eg netstat >reads symbols longer than 7 such as icmpstat and rthashsize. (But most >symbol reading programs read short symbols only). **** WARNING: I am now speculating - see confession below **** As explained above, you can still do this, but you need 4 zero bytes first to show that this is what you are doing. This is a snag for the standard BSD cliche, namely struct nlist Nl[] = { { "icmpstat" }, #define NL_ICMPSTAT 0 { "rthashsize" }, #define NL_RTHASHSIZE 1 0 }; Why? Because you can't declare a static initialiser for a union. I've been think about this today (and running adb on the nlist.o file from libc.a, which is where I gleaned the above details) and my considered solution is as follows: Replace the existing nlist.h with the one from the shar file given below. Edit your code so that it converts anything of the form shown above into the the following form struct nlist_syment Nl[] = { NLIST_ENTRY("icmpstat"), #define NL_ICMPSTAT 0 NLIST_ENTRY("rthashsize"), #define NL_RTHASHSIZE 1 NLIST_ENTRY(0) }; You haven't got real source code compatibility anyway, and I think that the above change is small enough to be OK for the simple things: obviously anything which uses fields other than n_name and n_value are going to need extensive rewriting. In the longer term, perhaps Apple could simply abandon the defunct 8 character limit on symbol names, though this still won't help make BSD and SysV completely interchangeable. Here's my replacement for nlist.h and a little demo program. I have to confess that the test program seems unable to read quite a variety of symbols which definitely DO exist - anyone know what I haven't got quite right? # cc -o nlist_test nlist_test.c; nlist_test symbol icmpstat has value 0x0 (type 0) symbol rthashsize has value 0x110087c0 (type 0) symbol icmp_input has value 0x1004990e (type 0) symbol avenrun has value 0x0 (type 0) symbol icmproto has value 0x0 (type 0) # nm -x /unix | egrep "icmpstat|rthashsize|icmp_input|avenrun|icmproto" icmproto |0x11008d20|static| | | |.data avenrun |0x12011440|extern| | | |.bss rthashsize |0x110087c0|extern| | | |.data icmp_input |0x1004990e|extern| | | |.text icmpstat |0x12017c34|extern| | | |.bss # Just as well I decided to test it :-( # This is a shar archive. # Remove everything above and including the cut line. # Then run the rest of the file through sh. #--------cut--------cut--------cut--------cut--------cut-------- #! /bin/sh # shar: Shell Archiver # Run the following with /bin/sh to create: # nlist.h # nlist_test.c # This archive created: Thu Jun 7 19:47:30 WET DST 1990 echo shar: extracting "nlist.h" '('1498 chars')' if test -f nlist.h then echo shar: will not overwrite existing file "nlist.h" else cat << \SHAR_EOF > nlist.h /* Imitation nlist.h file for rough and ready compatibility between * the BSD style of doing things and SysV * * Use this file in preference to a.out.h for BSD files, though * you might want to convert over entirely to the SysV scheme. * * William Roberts QMW, University of London 7th June 1990 */ #ifndef __NLIST__ #define __NLIST__ struct nlist_syment { long n_zeroes; /* must be 0 */ char *n_name; /* offset into string table */ long n_value; /* value of symbol */ short n_scnum; /* section number */ unsigned short n_type; /* type and derived type */ char n_sclass; /* storage class */ char n_numaux; /* number of aux. entries */ }; #define nlist syment #define NLIST_ENTRY(s) {0, (s)} /* The tradition BSD cliche of * * struct nlist nl[] = { * { "name1" }, * { "name2" }, * { "" } * } * * should be converted to * * struct nlist_syment nl[] = { * NLIST_ENTRY("name1"), * NLIST_ENTRY("name2"), * NLIST_ENTRY(0) * }; * * Note that you probably won't get away with using an empty string as the * end marker rather than NULL. * * You need the struct nlist_syment as defined above otherwise you will get * the struct syment which includes a union and so can't be given a * static initialiser. */ #endif SHAR_EOF if test 1498 -ne `wc -c < nlist.h` then echo shar: error transmitting "nlist.h" '('should be 1498 chars')' else echo nlist.h fi fi echo shar: extracting "nlist_test.c" '('494 chars')' if test -f nlist_test.c then echo shar: will not overwrite existing file "nlist_test.c" else cat << \SHAR_EOF > nlist_test.c #include <stdio.h> #include "nlist.h" struct nlist_syment Nl[] = { NLIST_ENTRY("icmpstat"), NLIST_ENTRY("rthashsize"), NLIST_ENTRY("icmp_input"), NLIST_ENTRY("avenrun"), NLIST_ENTRY("icmproto"), NLIST_ENTRY(0) }; main() { int i; i = nlist("/unix", Nl); if (i < 0) { perror("nlist"); } else { for (i=0; Nl[i].n_name != 0; i++) { printf("symbol %s has value 0x%x (type %d)\n", Nl[i].n_name, Nl[i].n_value, Nl[i].n_type); } } } SHAR_EOF if test 494 -ne `wc -c < nlist_test.c` then echo shar: error transmitting "nlist_test.c" '('should be 494 chars')' else echo nlist_test.c fi fi # End of shar archive exit 0 -- William Roberts ARPA: liam@cs.qmw.ac.uk Queen Mary & Westfield College UUCP: liam@qmw-cs.UUCP Mile End Road AppleLink: UK0087 LONDON, E1 4NS, UK Tel: 071-975 5250 (Fax: 081-980 6533)
liam@cs.qmw.ac.uk (William Roberts) (06/08/90)
Got it! The common factor between the things that didn't work
with my version of nlist.h (some of which have worked for me in
other A/UX programs after I had converted to dynamic
initialisation into the n_name array) is
THEY ARE LESS THAN 9 CHARACTERS LONG
You guessed it - the compiler puts the symbol directly into the
entry if it is small enough to fit, only using the string table
if the name is too long. Damn!
Apple, please can you fix nlist.c (aka syment()) so that it
checks the length of new-style labels passed in as a parameter
and is prepared to be flexible about matching oldstyle with
newstyle.
I've reworked my nlist.h file but it is now terribly ugly. The
revised nlist.h and nlist_test.c files are appended to this
message, and the compilation and test run go as follows:
# cc -o nlist_test nlist_test.c
"nlist_test.c", line 25: warning: non-null byte ignored in string initializer
"nlist_test.c", line 26: warning: non-null byte ignored in string initializer
# nlist_test
First, the long names...
symbol rthashsize has value 0x110087c0 (type 0)
symbol icmp_input has value 0x1004990e (type 0)
symbol icmproto has value 0x0 (type 0)
symbol avenrun has value 0x0 (type 0)
symbol clstat has value 0x0 (type 0)
symbol icmpstat has value 0x0 (type 0)
Then the short names...
symbol icmproto has value 0x11008d20 (type 0)
symbol avenrun has value 0x12011440 (type 0)
symbol clstat has value 0x1201c444 (type 0)
symbol icmpstat has value 0x12017c34 (type 0)
symbol rthashsi has value 0x0 (type 0)
symbol icmp_inp has value 0x0 (type 0)
#
You guessed, it doesn't try to do anything clever about
matching just the first 8 characters either (probably a good
thing).
Hope this helps you fix your problems with xload...
# This is a shar archive.
# Remove everything above and including the cut line.
# Then run the rest of the file through sh.
#--------cut--------cut--------cut--------cut--------cut--------
#! /bin/sh
# shar: Shell Archiver
# Run the following with /bin/sh to create:
# nlist.h
# nlist_test.c
# This archive created: Thu Jun 7 20:42:47 WET DST 1990
echo shar: extracting "nlist.h" '('2365 chars')'
if test -f nlist.h
then
echo shar: will not overwrite existing file "nlist.h"
else
cat << \SHAR_EOF > nlist.h
/* Imitation nlist.h file for rough and ready compatibility between
* the BSD style of doing things and SysV
*
* Use this file in preference to a.out.h for BSD files, though
* you might want to convert over entirely to the SysV scheme.
*
* William Roberts QMW, University of London 7th June 1990
*/
#ifndef __NLIST__
#define __NLIST__
struct nlist_syment_long
{
long n_zeroes; /* must be 0 */
char *n_name; /* offset into string table */
long n_value; /* value of symbol */
short n_scnum; /* section number */
unsigned short n_type; /* type and derived type */
char n_sclass; /* storage class */
char n_numaux; /* number of aux. entries */
};
#define SYMNMLEN 8
struct nlist_syment_short
{
char n_name[SYMNMLEN]; /* the symbol name */
long n_value; /* value of symbol */
short n_scnum; /* section number */
unsigned short n_type; /* type and derived type */
char n_sclass; /* storage class */
char n_numaux; /* number of aux. entries */
};
#define nlist syment
#define NLIST_LONG_ENTRY(s) {0, (s)}
#define NLIST_SHORT_ENTRY(s) {(s)}
/* The tradition BSD cliche of
*
* struct nlist nl[] = {
* { "longname1" },
* { "name2" },
* { 0 }
* }
*
* should be converted to
*
* struct nlist_syment_long nl_long[] = {
* NLIST_LONG_ENTRY("longname1"),
* NLIST_LONG_ENTRY(0)
* };
*
* struct nlist_syment_short nl_short[] = {
* NLIST_SHORT_ENTRY("name2"),
* NLIST_SHORT_ENTRY(0)
* };
*
* The reason for having two different structures is that the brain-damaged
* linker insists on saving a few bytes for short symbols by using the
* other variant, but the syment routine (equivalent to nlist) will
* only compare like with like.
*
* Note that you probably won't get away with using an empty string as the
* end marker rather than NULL.
*
* You need the struct nlist_syments as defined above otherwise you will get
* the struct syment which includes a union and so can't be given a
* static initialiser.
*/
#endif
SHAR_EOF
if test 2365 -ne `wc -c < nlist.h`
then
echo shar: error transmitting "nlist.h" '('should be 2365 chars')'
else
echo nlist.h
fi
fi
echo shar: extracting "nlist_test.c" '('1683 chars')'
if test -f nlist_test.c
then
echo shar: will not overwrite existing file "nlist_test.c"
else
cat << \SHAR_EOF > nlist_test.c
#include <stdio.h>
#include "nlist.h"
struct nlist_syment_long Nl_long[] = {
NLIST_LONG_ENTRY("rthashsize"),
NLIST_LONG_ENTRY("icmp_input"),
/* the rest won't work because the COFF file will have them as
* short form entries...
*/
NLIST_LONG_ENTRY("icmproto"),
NLIST_LONG_ENTRY("avenrun"),
NLIST_LONG_ENTRY("clstat"),
NLIST_LONG_ENTRY("icmpstat"),
NLIST_LONG_ENTRY(0)
};
struct nlist_syment_short Nl_short[] = {
NLIST_SHORT_ENTRY("icmproto"),
NLIST_SHORT_ENTRY("avenrun"),
NLIST_SHORT_ENTRY("clstat"),
NLIST_SHORT_ENTRY("icmpstat"),
/* These are long entries - the compiler will issue a warning when
* it truncates the initialiser and then they won't match with the
* long form entries in the COFF file.
*/
NLIST_SHORT_ENTRY("rthashsize"),
NLIST_SHORT_ENTRY("icmp_input"),
NLIST_SHORT_ENTRY(0)
};
main()
{
int i;
printf("First, the long names...\n");
i = nlist("/unix", Nl_long);
if (i < 0) {
perror("nlist");
} else {
for (i=0; Nl_long[i].n_name != 0; i++) {
printf("symbol %s has value 0x%x (type %d)\n",
Nl_long[i].n_name, Nl_long[i].n_value, Nl_long[i].n_type);
}
}
/* Note that the "run off end" test for Nl_short is
* different, and that we need to restrict the string length
* to a maximum of 8 characters when printing it out
*/
printf("\nThen the short names...\n");
i = nlist("/unix", Nl_short);
if (i < 0) {
perror("nlist");
} else {
for (i=0; Nl_short[i].n_name[0] != 0; i++) {
printf("symbol %.8s has value 0x%x (type %d)\n",
Nl_short[i].n_name, Nl_short[i].n_value, Nl_short[i].n_type);
}
}
}
SHAR_EOF
if test 1683 -ne `wc -c < nlist_test.c`
then
echo shar: error transmitting "nlist_test.c" '('should be 1683 chars')'
else
echo nlist_test.c
fi
fi
# End of shar archive
exit 0
--
William Roberts ARPA: liam@cs.qmw.ac.uk
Queen Mary & Westfield College UUCP: liam@qmw-cs.UUCP
Mile End Road AppleLink: UK0087
LONDON, E1 4NS, UK Tel: 071-975 5250 (Fax: 081-980 6533)