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1 This document explains a couple of things that are specific to VMS.
2 There are currently two "chapters", the first deals with cross-assembly
3 issues, and the second deals with the VMS debugger and GNU-CC.
6 ***********************************************************************
7 ****************** Notes for Cross Assembly with VMS ******************
8 ***********************************************************************
10 If you wish to build gas on a non-VMS system to cross-assemble,
11 you should use:
13 configure ${hosttype} -target=vms
15 and then follow the usual procedure. The object files generated on
16 Unix will be correct from a binary point of view, but the real trick is
17 getting them to the VMS machine. The format of the object file is
18 a variable-length record, but each record contains binary data. gas
19 writes the records in the same format that VMS would expect,
20 namely a two-byte count followed by that number of bytes.
22 If you try to copy the file to a VMS system using ftp, the ftp
23 protocol will screw up the file by looking for nulls (record terminator for
24 unix) and it will insert it's own record terminators at that point. This
25 will obviously corrupt the file.
27 If you try to transfer the file with ftp in binary mode, the
28 file itself will not be corrupt, but VMS will think that the file contains
29 fixed-length records of 512 bytes. You can use the public-domain FILE
30 utility to change this with a command like:
32 $FILE foo.o/type=variable
34 If you do not have this utility available, the following program can be
35 used to perform this task:
37 #include <fab.h>
39 #define RME$C_SETRFM 1
41 struct FAB * fab;
43 main(int argc, char * argv[]){
44 int i, status;
45 fab = (struct FAB*) malloc(sizeof(struct FAB));
46 *fab = cc$rms_fab; /* initialize FAB*/
47 fab->fab$b_fac = FAB$M_PUT;
48 fab->fab$l_fop |= FAB$M_ESC;
49 fab->fab$l_ctx = RME$C_SETRFM;
50 fab->fab$w_ifi = 0;
51 for(i=1;i<argc;i++){
52 printf("Setting %s to variable length records.\n",argv[i]);
53 fab->fab$l_fna = argv[i];
54 fab->fab$b_fns = strlen(argv[i]);
55 status = sys$open(fab,0,0);
56 if((status & 7) != 1) lib$signal(status);
57 fab->fab$b_rfm = FAB$C_VAR;
58 status = sys$modify(fab,0,0);
59 if((status & 7) != 1) lib$signal(status);
60 status = sys$close(fab,0,0);
61 if((status & 7) != 1) lib$signal(status);
62 };
63 }
65 If you have NFS running on the VMS system, what you need to do
66 depends upon which NFS software you are running on the VMS system. There
67 are a number of different TCP/IP packages for VMS available, and only very
68 limited testing has been performed. In the tests that has been done so
69 far, the contents of the file will always be correct when transferring the
70 file via NFS, but the record attributes may or may not be correct.
72 One proprietary TCP/IP/NFS package for VMS is known to
73 automatically fix the record attributes of the object file if you NFS mount
74 a unix disk from the VMS system, and if the file has a ".obj" extension on
75 the unix system. Other TCP/IP packages might do this for you as well, but
76 they have not been checked.
78 No matter what method you use to get the file to the VMS system, it is
79 always a good idea to check to make sure that it is the correct type by
80 doing a "$dir/full" on the object file. The desired record attributes will
81 be "None". Undesirable record attributes will be "Stream-LF" or anything
82 else.
84 Once you get the files on the VMS system, you can check their integrity
85 with the "$anal/obj" command. (Naturally at some point you should rename
86 the .o files to .obj). As far as the debugger is concerned, the records
87 will be correct, but the debugger will not be able to find the source files,
88 since it only has the file name, and not the full directory specification.
89 You must give the debugger some help by telling it which directories to
90 search for the individual files - once you have done this you should be
91 able to proceed normally.
93 It is a good idea to use names for your files which will be valid
94 under VMS, since otherwise you will have no way of getting the debugger to
95 find the source file when deugging.
97 The reason for this is that the object file normally contins specific
98 information that the debugger can use to positively identify a file, and if
99 you are assembling on a unix system this information simply does not exist
100 in a meaningful way. You must help the debugger by using the "SET FILE="
101 command to tell the debugger where to look for source files. The debugger
102 records will be correct, except that the debugger will not be initially
103 able to find the source files. You can use the "SET FILE" command to tell
104 the debugger where to look for the source files.
106 I have only tested this with a SVr4 i486 machine, and everything seems to
107 work OK, with the limited testing that I have done. Other machines may
108 or may not work. You should read the chapters on cross-compilers in the gcc
109 manual before fooling with this. Since gas does not need to do any floating
110 point arithmetic, the floating point constants that are generated here should
111 be correct - the only concern is with constant folding in the main compiler.
112 The range and precision of floats and doubles are similar on the 486 (with
113 a builtin 80387) and the VAX, although there is a factor of 2 to 4
114 difference in the range. The double, as implemented on the 486, is quite
115 similar to the G_FLOAT on the VAX.
117 ***********************************************************************
118 ****************** Notes for using GNU CC with the VMS debugger********
119 ***********************************************************************
122 1) You should be aware that GNU-C, as with any other decent compiler,
123 will do things when optimization is turned on that you may not expect.
124 Sometimes intermediate results are not written to variables, if they are only
125 used in one place, and sometimes variables that are not used at all will not be
126 written to the symbol table. Also, parameters to inline functions are often
127 inaccessible. You can see the assembly code equivalent by using KP7 in the
128 debugger, and from this you can tell if in fact a variable should have the
129 value that you expect. You can find out if a variable lives withing a register
130 by doing a 'show symbol/addr'.
132 2) Overly complex data types, such as:
134 int (*(*(*(*(*(* sarr6)[1])[1])[2])[3])[4])[5];
136 will not be debugged properly, since the debugging record overflows an internal
137 debugger buffer. gcc-as will convert these to *void as far as the debugger
138 symbol table is concerned, which will avoid any problems, and the assembler
139 will give you a message informing you that this has happened.
141 3) You must, of course, compile and link with /debug. If you link
142 without debug, you still get traceback table in the executable, but there is no
143 symbol table for variables.
145 4) Included in the patches to VMS.C are fixes to two bugs that are
146 unrelated to the changes that I have made. One of these made it impossible to
147 debug small programs sometimes, and the other caused the debugger to become
148 confused about which routine it was in, and give this incorrect info in
149 tracebacks.
151 5) If you are using the GNU-C++ compiler, you should modify the
152 compiler driver file GNU_CC:[000000]GCC.COM (or GXX.COM). If you have a
153 separate GXX.COM, then you need to change one line in GXX.COM to:
154 $ if f$locate("D",p2) .ne. P2_Length then Debug = " ""-G0"""
155 Notice zero---> ^
156 If you are using a GCC.COM that does both C and C++, add the following lines to
157 GCC.COM:
159 $!
160 $! Use old style debugging records for VMS
161 $!
162 $ if (Debug.nes."" ).and. Plus then Debug = " ""-G0"""
164 after the variables Plus and Debug are set. The reason for this, is that C++
165 compiler by default generates debugging records that are more complex,
166 with many new syntactical elements that allow for the new features of the
167 language. The -G0 switch tells the C++ compiler to use the old style debugging
168 records. Until the debugger understands C++ there is not any point to try and
169 use the expanded syntax.
171 6) When you have nested scopes, i.e.:
172 main(){
173 int i;
174 {int i;
175 {int i;
176 };};}
177 and you say "EXAM i" the debugger needs to figure out which variable you
178 actually want to reference. I have arranged things to define a block to the
179 debugger when you use brackets to enter a new scope, so in the example above,
180 the variables would be described as:
181 TEST\main\i
182 TEST\main\$0\i
183 TEST\main\$0\$0\i
184 At each level, the block name is a number with a dollar sign prefix, the
185 numbers start with 0 and count upward. When you say EXAM i, the debugger looks
186 at the current PC, and decides which block it is currently in. It works from
187 the innermost level outward until it finds a block that has the variable "i"
188 defined. You can always specify the scope explicitly.
190 7) With C++, there can be a lot of inline functions, and it would be
191 rather restrictive to force the user to debug the program by converting all of
192 the inline functions to normal functions. What I have done is to essentially
193 "add" (with the debugger) source lines from the include files that contain the
194 inline functions. Thus when you step into an inline function it appears as if
195 you have called the function, and you can examine variables and so forth.
196 There are several *very* important differences, however. First of all, since
197 there is no function call involved, you cannot step over the inline function
198 call - you always step into it. Secondly, since the same source lines are used
199 in many locations, there is a separate copy of the source for *each* usage.
200 Without this, breakpoints do not work, since we must have a 1-to-1 mapping
201 between source lines and PC.
202 Since you cannot step over inline function calls, it can be a real pain
203 if you are not really interested in what is going on for that function call.
204 What I have done is to use the "-D" switch for the assembler to toggle the
205 following behavior. With the "-D" switch, all inline functions are included in
206 the object file, and you can debug everything. Without the "-D" switch
207 (default case with VMS implementation), inline functions are included *only* if
208 they did not come from system header files (i.e. from GNU_CC_INCLUDE: or
209 GNU_GXX_INCLUDE:). Thus, without the switch the user only debugs his/her own
210 inline functions, and not the system ones. (This is especially useful if you do
211 a lot of stream I/O in C++). This probably will not provide enough granularity
212 for many users, but for now this is still somewhat experimental, and I would
213 like to reflect upon it and get some feedback before I go any further.
214 Possible solutions include an interactive prompting, a logical name, or a new
215 command line option in gcc.c (which is then passed through somehow to the guts
216 of the assembler).
217 The inline functions from header files appear after the source code
218 for the source file. This has the advantage that the source file itself is
219 numbered with the same line numbers that you get with an editor. In addition,
220 the entire header file is not included, since the assembler makes a list of
221 the min and max source lines that are used, and only includes those lines from
222 the first to the last actually used. (It is easy to change it to include the
223 whole file).
225 8) When you are debugging C++ objects, the object "this" is refered to
226 as "$this". Actually, the compiler writes it as ".this", but the period is
227 not good for the debugger, so I have a routine to convert it to a $. (It
228 actually converts all periods to $, but only for variables, since this was
229 intended to allow us to access "this".
231 9) If you use the asm("...") keyword for global symbols, you will not
232 be able to see that symbol with the debugger. The reason is that there are two
233 records for the symbol stored in the data structures of the assembler. One
234 contains the info such as psect number and offset, and the other one contains
235 the information having to do with the data type of the variable. In order to
236 debug as symbol, you need to be able to coorelate these records, and the only
237 way to do this is by name. The record with the storage attributes will take
238 the name used in the asm directive, and the record that specifies the data type
239 has the actual variable name, and thus when you use the asm directive to change
240 a variable name, the symbol becomes invisible.
242 10) Older versions of the compiler ( GNU-C 1.37.92 and earlier) place
243 global constants in the text psect. This is unfortunate, since to the linker
244 this appears to be an entry point. I sent a patch to the compiler to RMS,
245 which will generate a .const section for these variables, and patched the
246 assembler to put these variables into a psect just like that for normal
247 variables, except that they are marked NOWRT. static constants are still
248 placed in the text psect, since there is no need for any external access.