Common problems when converting C header files: what changed

Revision 3 to 4. Removed lines have a minus sign; added lines have a plus sign.

Old	New		Line
1	1		# h2pas reports an error
2	2
3	3		Here is a list of common C structures, not recognized by h2pas and how to fix them:
4	4
5	5		## h2pas problem: extern "C"
6	6
7	7		Some header files contain the C++ namespace block:
8	8
9	9		``` c
10	10		#if defined(__cplusplus)
11	11		extern "C" {
12	12		#endif
13	13		...
14	14		#if defined(__cplusplus)
15	15		}
16	16		#endif
17	17		```
18	18
19	19		Fix: Add the Remove C++ 'extern "C"' lines or the Pre H2Pas tool to the before h2pas tools.
20	20
21	21		## h2pas problem: Empty macro
22	22
23	23		Some header files contain empty macros used for further extensions:
24	24
25	25		``` c
26	26		#define MPI_FILE_DEFINED
27	27		```
28	28
29	29		Fix: Add the Remove empty C macros or the Pre H2Pas tool to the before h2pas tools.
30	30
31	31		## h2pas problem: Implicit array types
32	32
33	33		C allows implicit arrays in parameter types. For example:
34	34
35	35		``` c
36	36		int MPI_Group_range_incl(MPI_Group, int, int [][3], MPI_Group *);
37	37		```
38	38
39	39		The int \[\]\[3\] is an implicit type, which is not allowed under Pascal. h2pas supports adding pointer types. So, it enough to replace all \[\] with *.
40	40
41	41		Fix: Add the Replace \[\] with * or the Pre H2Pas tool to the before h2pas tools.
42	42
43	43		## h2pas problem: Makros for 0 pointers
44	44
45	45		Some header files contain typed 0 pointers:
46	46
47	47		``` c
48	48		#define MPI_BOTTOM (void *)0
49	49		```
50	50
51	51		Fix: Add the *Replace macro values 0 pointer like (char )0 with NULL or the Pre H2Pas tool to the before h2pas** tools.
52	52
53	53		## h2pas problem: function types are silently skipped
54	54
55	55		C allows to define function types, while pascal only allows pointers to functions. For example:
56	56
57	57		``` c
58	58		typedef int (MPI_Comm_copy_attr_function)(MPI_Comm, int, void , void ,
59	59		void , int );
60	60		```
61	61
62	62		The C header never uses this definition directly, instead it always uses pointers to such types MPI_Comm_copy_attr_function*, for instace:
63	63
64	64		``` c
65	65		#define MPI_COMM_NULL_COPY_FN ((MPI_Comm_copy_attr_function*)0)
66	66		```
67	67
68	68		The translation can work this way: Make the function type a pointer. That means add a * in front of the definition and prepend a P and remove the * behind all references. For example:
69	69
70	70		``` c
71	71		typedef int (PMPI_Comm_copy_attr_function)(MPI_Comm, int, void , void *,
72	72		void , int );
73	73		#define MPI_COMM_NULL_COPY_FN ((PMPI_Comm_copy_attr_function)0)
74	74		```
75	75
76	76		Solution: Add the tool Convert function types to pointer or Pre H2Pas tool to the before h2pas tools.
77	77
78	78		# Test compilation and add tools to beautify the output
79	79
80	80		When h2pas runs without errors it has created a pascal file. The -i switch defines if it is a unit or an include file. The next step is test compilation. Setup up a test project that uses the new pascal source. Then use the 'Run h2pas and compile' button in the h2pas wizard.
81	81
82	82		### Example: MPICH2
83	83
84	84		Create a new project with Project -\> New Project -\> Program. Save it a mpihelloworld. Change the code to
85	85
86	86		``` pascal
87	87		program MPIHelloWorld;
88	88
89	89		{$mode objfpc}{$H+}
90	90		{$linklib mpich}
91	91		{$linklib rt}
92	92
93	93		uses
94	94		{$IFDEF UNIX}pthreads{$ENDIF}, MPI;
95	95
96	96		begin
97	97		MPI_Init(@argc, @argv);
98	98		writeln('Hello, world.');
99	99		MPI_Finalize;
100	100		end.
101	101		```
102	102
103	103		Add the h2p/mpi.pas to the project. Project -\> Add editor file to project.
104	104
105	105		# Common compiler errors on h2pas output
106	106
107	107		Sometimes h2pas does not create valid pascal code. Here are the common problems and how to fix them.
108	108
109	109		## Unit name contains the file path
110	110
111	111		h2pas sometimes add the whole path to the unitname.
112	112
113	113		Fix: Add Replace "unit filename;" with "unit name;" or the Post H2Pas tool to the After h2pas tools.
114	114
115	115		## Missing include files
116	116
117	117		h2pas converts \#include directives to pascal {\$i } directives. If you create a unit for each header file, then you can remove all include directives with the tool Remove includes or the Post H2Pas.
118	118
119	119		## Forward type not resolved
120	120
121	121		For example: mpi.pas(26,16) Error: Forward type not resolved "PMPI_Aint" The error line is often a pointer type to a record:
122	122
123	123		``` pascal
124	124		PMPI_Aint = ^MPI_Aint;
125	125		```
126	126
127	127		h2pas adds PMPI_Aint to give nameless C pointers like MPI_Aint a name. It adds the pointers to the start of the file. But pascal requires, that the forward definition is in the same type* section. Sometimes h2pas adds a pointer type, although it already exists.
128	128
129	129		Fix: Add the tool Remove redefined pointer types or the Post H2Pas to the After h2pas tools.
130	130
131	131		## Removing system types
132	132
133	133		h2pas adds some system types, like PLongint, which are nowadays part of the system unit.
134	134
135	135		Fix: Add Remove type redefinitons like PLongint or the Post H2Pas tool to the After h2pas tools.
136	136
137	137		## Empty type/var/const sections
138	138
139	139		After the above tools removed some variables, types, constants some setions become empty, which is not allowed in pascal.
140	140
141	141		Fix: Add Remove empty type/var/const sections or the Post H2Pas tool to the After h2pas tools.
142	142
143	143		## h2pas forgets to add IFDEFs for function bodies
144	144
145	145		h2pas converts macros to functions. But if the macro was in an IFDEF block, h2pas does not add IFDEFs around the function body.
146	146
147	147		Fix: Add the tool Add missing h2pas IFDEFs for function bodies or the Post H2Pas. Add this right in front of the tool Reduce compiler directives in pascal file.
148	148
149	149		## Implicit Types
150	150
151	151		C allows implicits types in parameters. For example: int \[\]\[3\]. Pascal does not allow this, so you must give it a name and declare a type. But h2pas does not do this automatically and adds instead the implicit type in a quasi pascal syntax. For example:
152	152
153	153		``` c
154	154		int some_func(int [][3]);
155	155		```
156	156
157	157		becomes
158	158
159	159		``` pascal
160	160		function some_func(_para1:array[0..2] of Plongint):longint;cdecl;external;
161	161		```
162	162
163	163		Fix: Luckily there is now a tool to remove these implicit types and add explicit types, it's called Replace implicit types. It is also part of the Post H2Pas tool. Add it to the after tool list.
164	164
165	165		## Array of nothing
166	166
167	167		Sometimes h2pas converts the C ellipsis parameter type '...' wrongly to 'array of )'. It should be 'array of const)'.
168	168
169	169		Fix: Add the tool Fix open arrays or the Post H2Pas to the after tools.
170	170
171	171		## Identifier not found
172	172
173	173		There are three cases:
174	174
175	175		- The identifier is defined in the unit, but after it is used
176	176		- The identifier is defined in another unit (another .h file)
177	177		- The identifier is a pointer to an existing type, but h2pas forgot to add it
178	178		- The identifier is not defined anywhere
179	179
180	180		### The identifier is defined in the unit, but after it is used
181	181
182	182		In pascal forward defined types are only allowed in a type section. For example:
183	183
184	184		``` pascal
185	185		type
186	186		TMyRecord = record
187	187		Next: PMyRecord; // using the forward definition
188	188		end;
189	189		PMyRecord = ^TMyRecord;
190	190		```
191	191
192	192		The below is not allowed, because the forward definition is in another type section:
193	193
194	194		``` pascal
195	195		type
196	196		TMyRecord = record
197	197		Next: PMyRecord; // using the forward definition
198	198		end;
199	199		type
200	200		PMyRecord = ^TMyRecord;
201	201		```
202	202
203	203		Solution: The code must be reordered. Add the tool fix forward definitions-
204	204
205	205		### The identifier is defined in another unit (another .h file)
206	206
207	207		#### Solution 1: Add the unit to the uses section
208	208
209	209		If the other unit is already using this unit, then you have a circle. A circle dependency is allowed between .h files, but not between pascal units. In this case you must move code between both units or use IFDEFs like the gtk2 bindings or use the below merge function of the h2pas wizard.
210	210
211	211		#### Solution 2: Merge the two include files
212	212
213	213		The h2pas wizard can merge include files into one. For example first.h includes via the c include directive the file second.h. Select the second.h in the wizard and check the 'Merge file' feature. Then the source of second.h' will be appended to first.h before sending the output to h2pas. As always: This does not change the header files.
214	214
215	215		#### Example: MPICH2
216	216
217	217		The type MPI_Request is defined in mpi.h and used in mpio.h. Both c header files heavily use each other, so you can not put them into two separated units. Select mpio.h and enable the 'Merge file' function.
218	218
219	219		### The identifier is a pointer to an existing type, but h2pas forgot to add it
220	220
221	221		Solution: Add the tool Add missing pointer types like PPPChar or the Post H2Pas tool to the after H2Pas tools.
222	222
223	223		### The identifier is not defined anywhere
224	224
225	225		Probably you are missing a header file or you are using one with the wrong version or it is private identifier.
226	226
227	227		Solution: Search the header file and convert it too. If you can not find it or you don't want to translate this file, you can comment the identifier or replace it.
228	228
229	229		### Example: MPICH2
230	230
231	231		Note: this is just a demonstration. The real solution for the MPICH2 headers is to setup the Defines correct, so that the below example vanishes automatically.
232	232
233	233		The mpi.h file defines MPI_File as pointer of ADIOI_FileD, but ADIOI_FileD is not defined public:
234	234
235	235		``` c
236	236		typedef struct ADIOI_FileD *MPI_File;
237	237		```
238	238
239	239		The h2pas tool translated this to:
240	240
241	241		``` pascal
242	242		MPI_File = ^ADIOI_FileD;
243	243		```
244	244
245	245		Because ADIOI_FileD is not defined public, it is a private structure, so you can simply replace it with a Pointer: Add to the 'After h2pas' tools a new tool of type 'Search and replace' with the following properties:
246	246
247	247		\| Property \| Value \|
248	248		\|-------------\|----------------------------------\|
249	249		\| Caption \| Replace ADIOI_FileD with Pointer \|
250	250		\| Name \| ReplaceADIOI_FileDwithPointer \|
251	251		\| SearchFor \| ADIOI_FileD \|
252	252		\| ReplaceWith \| Pointer \|
253	253		\| Options \| \[trtMatchCase,trtWholeWord\] \|
254	254
255	255		## Illegal expression
256	256
257	257		### Example: MPICH2
258	258
259	259		Compiler gives Illegal expression on the following statement
260	260
261	261		``` pascal
262	262		const
263	263		MPI_LONG_LONG = MPI_LONG_LONG_INT;
264	264		```
265	265
266	266		The reason is, that h2pas translated the MPI_LONG_LONG_INT constant to a function.
267	267
268	268		Solution: Fix h2pas or use a trick:
269	269
270	270		Replace the
271	271
272	272		``` c
273	273		#define MPI_LONG_LONG MPI_LONG_LONG_INT
274	274		```
275	275
276	276		with
277	277
278	278		``` c
279	279		#define MPI_LONG_LONG ((MPI_Datatype)0x4c000809)
280	280		```
281	281
282	282		by adding a Search and replace tool before h2pas.
283	283
284	284		# Common other issues
285	285
286	286		## The pascal file contains a lot of unneccessary ifdef
287	287
288	288		h2pas translates even the C \#ifdef directives. Many of them are C specific and not needed under FPC and can even make it impossible many tools on this side to explore the code. That's why there is a tool to clean up and disable or remove many unneeded IFDEFs: Reduce compiler directives in pascal file. Do not forget to insert the tool Add missing h2pas IFDEFs for function bodies in front of it.
289	289
290	290		### Hints about unneeded directives (\$IFDEFs, \$DEFINEs, ...)
291	291
292	292		You can see the directives in the [[IDE Window: Code Explorer\|Code Explorer]].
293	293
294	294		The tool will only remove the \$IFDEFs and \$DEFINEs, that are unneeded on all platforms. That means for example a {\$IFDEF CPU386} will not be removed - even if you are currently working on an intel 32 bit compatible machine it will not be removed. This allows to create platform independent bindings. Of course C headers often contain a lot of IFDEFs not needed under FPC (Delphi, Kylix, gpc, ...).
295	295
296	296		For example: Many C headers are enclosed in
297	297
298	298		``` pascal
299	299		{$IFNDEF FOO}
300	300		{$DEFINE FOO}
301	301		...
302	302		{$ENDIF}
303	303		```
304	304
305	305		This trick allows to include a C header file multiple times - like the units under pascal. So, pascal does not need this. Therefore the tool Reduce compiler directives in pascal file and the Post H2Pas tool have two properties: Defines and Undefines. Click on the ... button of the property to edit them. Each line can contain a macroname, that should be (un)defined. Add the line FOO to the Undefines. This way the \$IFNDEF FOO is always true and the tool will remove it. The \$DEFINE FOO is now unneeded too and will be removed too.
306	306
307	307		#### Example: MPICH2
308	308
309	309		Add the following to the Undefines property:
310	310
311		-	~~` MPI_INCLUDED`\~~
312		-	~~` MPIO_INCLUDE`\~~
313		-	~~` NEEDS_MPI_FINT`\~~
314		-	~~` MPI_BUILD_PROFILING`\~~
315		-	~~` MPICH_SUPPRESS_PROTOTYPES`\~~
316		-	~~` MPI_Wtime`\~~
317		-	~~` HAVE_MPICH2G2`\~~
318		-	~~` FOO`\~~
319		-	~~` HAVE_PRAGMA_HP_SEC_DEF`~~
	311	+	```text
	312	+	MPI_INCLUDED
	313	+	MPIO_INCLUDE
	314	+	NEEDS_MPI_FINT
	315	+	MPI_BUILD_PROFILING
	316	+	MPICH_SUPPRESS_PROTOTYPES
	317	+	MPI_Wtime
	318	+	HAVE_MPICH2G2
	319	+	FOO
	320	+	HAVE_PRAGMA_HP_SEC_DEF
	321	+	```
320	322
321	323		Add the following to the Defines property:
322	324
323		-	~~` MPICH2`\~~
324		-	~~` MPICH_NEW_MPIIO`~~
	325	+	```text
	326	+	MPICH2
	327	+	MPICH_NEW_MPIIO
	328	+	```
325	329
326	330		## The C header files contain redefinitions
327	331
328	332		This is ok for C compilers, because a type is the same if its definition is the same. But for Pascal each type is a different type.
329	333
330	334		Solution: Add the tool Remove redefinitions in pascal unit or the Post H2Pas to the after h2pas tools. As the name implies, it only works on units, not on include files. And it expect at least a valid pascal syntax. That's why it is important to add this tool after the tools that fix the pascal syntax like Replace "unit filename;" with "unit name;", Remove empty type/var/const sections, Remove includes, Replace implicit types and Fix open arrays.
331	335
332	336		### Example: MPICH2
333	337
334	338		The mpi.h contains several redefinitions like MPIIMPL_HAVE_MPI_TYPE_CREATE_DARRAY. They can all be fixed with this tool.
335	339
336	340		## Alias macros are always translated by h2pas to constants
337	341
338	342		h2pas converts macros like
339	343
340	344		``` c
341	345		#define A B
342	346		```
343	347
344	348		to
345	349
346	350		``` pascal
347	351		const A = B;
348	352		```
349	353
350	354		which is almost always correct. But some c header contain aliases for types, variables and functions too.
351	355
352	356		### Example: MPICH2
353	357
354	358		The mpi.h contains the macro
355	359
356	360		``` c
357	361		#define MPI_LONG_LONG MPI_LONG_LONG_INT
358	362		```
359	363
360	364		h2pas converts this simply to
361	365
362	366		``` pascal
363	367		const
364	368		MPI_LONG_LONG = MPI_LONG_LONG_INT;
365	369		```
366	370
367	371		but MPI_LONG_LONG_INT is a function.
368	372
369	373		Solution: Add the tool Fixes section type of alias definitions in pascal unit or the Pre H2Pas tool to the Before H2Pas tools.
370	374
371	375		Note: This tool can also fix alias to external functions. For example:
372	376
373	377		``` pascal
374	378		function ExternalFunc1(_para1:longint; _para2:pointer):longint;cdecl;external name 'ExternalFunc1';
375	379		const
376	380		ExternalFuncAlias1 = ExternalFunc1;// should be replaced with full declaration
377	381		```
378	382
379	383		## h2pas creates functions for constants
380	384
381	385		For example:
382	386
383	387		``` pascal
384	388		function MPI_2COMPLEX : MPI_Datatype;
385	389		begin
386	390		MPI_2COMPLEX:=MPI_Datatype(MPI_DATATYPE_NULL);
387	391		end;
388	392		```
389	393
390	394		All used identifiers are types and constants, so this function can be replaced with a simple constant. This is checked and done by the tool Replace simple functions with constants or the Post H2Pas.
391	395
392	396		## h2pas creates functions for type casts
393	397
394	398		For example:
395	399
396	400		``` pascal
397	401		function PMPI_Win_f2c(win : longint) : MPI_Win;
398	402		begin
399	403		PMPI_Win_f2c:=MPI_Win(win);
400	404		end;
401	405		```
402	406
403	407		This function can be replaced with a simple type, resulting in a type cast. This is checked and done by the tool Replace simple functions with type casts or the Post H2Pas tool.
404	408
405	409		## Some identifiers are used before they are defined
406	410
407	411		C allows much more forward definitions than pascal, so the definitions must be topologically sorted. For example:
408	412
409	413		``` pascal
410	414		type
411	415		PMPI_Datatype = ^MPI_Datatype;
412	416		MPI_Datatype = longint;
413	417		```
414	418
415	419		The pointer PMPI_Datatype must be moved to the same type section as MPI_Datatype.
416	420
417	421		Solution: Add the Fix forward definitions by reordering or the Pre H2Pas tool to the Before H2Pas tools. The tool should be put after the tools fixing redefinitions and types. At the moment the tool only moves pointer types. ToDo: reorder some more definitions like constants.
418	422
419	423		## The C header files do not contain parameter names
420	424
421	425		For example there is an ugly function without parameter names:
422	426
423	427		``` c
424	428		int MPI_Intercomm_create(MPI_Comm, int, MPI_Comm, int, int, MPI_Comm * );
425	429		```
426	430
427	431		The parameter names are defined in the corresponding .c file:
428	432
429	433		``` c
430	434		int MPI_Intercomm_create(MPI_Comm local_comm, int local_leader,
431	435		MPI_Comm peer_comm, int remote_leader, int tag,
432	436		MPI_Comm *newintercomm)
433	437		```
434	438
435	439		Solution: There is no solution yet. Except manual editing or searching for other .h files. Often such .h files were auto generated and there are some nicer header files somewhere on the internet.
436	440
437	441		Proposal: Write a clever tool, that searches functions with missing parameter names and the corresponding functions in the .c files with parameter names and improve the .h file.
438	442
439	443		## See also
440	444
441	445		- [[Creating bindings for C libraries]]
442	446		- [[Pascal for C users]]
443	447		- [[Pascal for C++ users]]
444	448		- [[SWIG]]
445	449
446	450		---
447	451		From the Free Pascal wiki, page "Common problems when converting C header files", as it stood on 2022-05-03 after a change by E-ric, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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