in reply to Re: (OT) How can I do unless(caller) { main; } in C using GCC in Windows?
in thread (OT) How can I do unless(caller) { main; } in C using GCC in Windows?

That's a fair question.

Perlmonks is most likely to understand what unless(caller) { main; } does; that's been the largest barrier to communication so far. I basically want to replicate this snippet in as many programming languages as possible, because I prefer my software to work well in shell script environments.

Strawberry Perl's GCC compiler happens to be the locus of the problem, because it does not recognize the same macros that GCC does in an ordinary Unix environment. Any Perl monks familiar with Strawberry Perl's internals could help with this issue.

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Re^3: (OT) How can I do unless(caller) { main; } in C using GCC in Windows?
by Anonymous Monk on Sep 12, 2012 at 22:57 UTC
    So you're talking about Weak symbol?

    win32 has "entry points", default is main, then WinMain, but if gcc weak symbol thing is hiding it as entry point, they you have to specify via some gcc switches what the entry point is

    what gcc is probably doing is renaming the symbol to something not main ( or _main )

    Find out what it is, then use  gcc ... -e _the_new_main

    To find out , compile the .c file to .o, I don't remember what switch to use, -O/-c, something like that. Then use objdump -x ...o to see the symbols

    Strawberry Perl's GCC compiler happens to be the locus of the problem, because it does not recognize the same macros that GCC does in an ordinary Unix environment. Any Perl monks familiar with Strawberry Perl's internals could help with this issue.

    There are no strawberry perl internals, they're the same as regular perl

[reply]
[d/l]

      Thanks for your help! I'm able to create the .o objects and objdump their contents, but I don't know how to look in the dumps to find entry points.

      C:\> gcc -c scriptedmain.c scriptedmain.h
C:\> gcc -c test.c scriptedmain.c scriptedmain.h
C:\> objdump -x scriptedmain.o
scriptedmain.o:     file format pe-i386
scriptedmain.o
architecture: i386, flags 0x00000039:
HAS_RELOC, HAS_DEBUG, HAS_SYMS, HAS_LOCALS
start address 0x00000000

Characteristics 0x104
    line numbers stripped
    32 bit words

Time/Date        Wed Dec 31 19:00:00 1969
Magic            0000
MajorLinkerVersion    0
MinorLinkerVersion    0
SizeOfCode        00000000
SizeOfInitializedData    00000000
SizeOfUninitializedData    00000000
AddressOfEntryPoint    00000000
BaseOfCode        00000000
BaseOfData        00000000
ImageBase        00000000
SectionAlignment    00000000
FileAlignment        00000000
MajorOSystemVersion    0
MinorOSystemVersion    0
MajorImageVersion    0
MinorImageVersion    0
MajorSubsystemVersion    0
MinorSubsystemVersion    0
Win32Version        00000000
SizeOfImage        00000000
SizeOfHeaders        00000000
CheckSum        00000000
Subsystem        00000000    (unspecified)
DllCharacteristics    00000000
SizeOfStackReserve    00000000
SizeOfStackCommit    00000000
SizeOfHeapReserve    00000000
SizeOfHeapCommit    00000000
LoaderFlags        00000000
NumberOfRvaAndSizes    00000000

The Data Directory
Entry 0 00000000 00000000 Export Directory [.edata (or where ever we f
+ound it)]
Entry 1 00000000 00000000 Import Directory [parts of .idata]
Entry 2 00000000 00000000 Resource Directory [.rsrc]
Entry 3 00000000 00000000 Exception Directory [.pdata]
Entry 4 00000000 00000000 Security Directory
Entry 5 00000000 00000000 Base Relocation Directory [.reloc]
Entry 6 00000000 00000000 Debug Directory
Entry 7 00000000 00000000 Description Directory
Entry 8 00000000 00000000 Special Directory
Entry 9 00000000 00000000 Thread Storage Directory [.tls]
Entry a 00000000 00000000 Load Configuration Directory
Entry b 00000000 00000000 Bound Import Directory
Entry c 00000000 00000000 Import Address Table Directory
Entry d 00000000 00000000 Delay Import Directory
Entry e 00000000 00000000 CLR Runtime Header
Entry f 00000000 00000000 Reserved

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00000034  00000000  00000000  000000b4  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000000  00000000  00000000  00000000  2**2
                  ALLOC, LOAD, DATA
  2 .bss          00000000  00000000  00000000  00000000  2**2
                  ALLOC
  3 .rdata        00000024  00000000  00000000  000000e8  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
SYMBOL TABLE:
[  0](sec -2)(fl 0x00)(ty   0)(scl 103) (nx 1) 0x00000000 scriptedmain
+.c
File 
[  2](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 1) 0x00000000 _meaning_of_
+life
AUX tagndx 0 ttlsiz 0x0 lnnos 0 next 0
[  4](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x34 nreloc 3 nlnno 0
[  6](sec  2)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .data
AUX scnlen 0x0 nreloc 0 nlnno 0
[  8](sec  3)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .bss
AUX scnlen 0x0 nreloc 0 nlnno 0
[ 10](sec  4)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .rdata
AUX scnlen 0x21 nreloc 0 nlnno 0
[ 12](sec  1)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x0000000a .weak._main.
+_meaning_of_life
[ 13](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 ___main
[ 14](sec  0)(fl 0x00)(ty  20)(scl 105) (nx 1) 0x00000000 _main
AUX lnno 1 size 0x0 tagndx 12
[ 16](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _printf


RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE 
00000014 DISP32            ___main
00000024 dir32             .rdata
00000029 DISP32            _printf

C:\> objdump -x test.o
test.o:     file format pe-i386
test.o
architecture: i386, flags 0x00000039:
HAS_RELOC, HAS_DEBUG, HAS_SYMS, HAS_LOCALS
start address 0x00000000

Characteristics 0x104
    line numbers stripped
    32 bit words

Time/Date        Wed Dec 31 19:00:00 1969
Magic            0000
MajorLinkerVersion    0
MinorLinkerVersion    0
SizeOfCode        00000000
SizeOfInitializedData    00000000
SizeOfUninitializedData    00000000
AddressOfEntryPoint    00000000
BaseOfCode        00000000
BaseOfData        00000000
ImageBase        00000000
SectionAlignment    00000000
FileAlignment        00000000
MajorOSystemVersion    0
MinorOSystemVersion    0
MajorImageVersion    0
MinorImageVersion    0
MajorSubsystemVersion    0
MinorSubsystemVersion    0
Win32Version        00000000
SizeOfImage        00000000
SizeOfHeaders        00000000
CheckSum        00000000
Subsystem        00000000    (unspecified)
DllCharacteristics    00000000
SizeOfStackReserve    00000000
SizeOfStackCommit    00000000
SizeOfHeapReserve    00000000
SizeOfHeapCommit    00000000
LoaderFlags        00000000
NumberOfRvaAndSizes    00000000

The Data Directory
Entry 0 00000000 00000000 Export Directory [.edata (or where ever we f
+ound it)]
Entry 1 00000000 00000000 Import Directory [parts of .idata]
Entry 2 00000000 00000000 Resource Directory [.rsrc]
Entry 3 00000000 00000000 Exception Directory [.pdata]
Entry 4 00000000 00000000 Security Directory
Entry 5 00000000 00000000 Base Relocation Directory [.reloc]
Entry 6 00000000 00000000 Debug Directory
Entry 7 00000000 00000000 Description Directory
Entry 8 00000000 00000000 Special Directory
Entry 9 00000000 00000000 Thread Storage Directory [.tls]
Entry a 00000000 00000000 Load Configuration Directory
Entry b 00000000 00000000 Bound Import Directory
Entry c 00000000 00000000 Import Address Table Directory
Entry d 00000000 00000000 Delay Import Directory
Entry e 00000000 00000000 CLR Runtime Header
Entry f 00000000 00000000 Reserved

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         0000002c  00000000  00000000  000000b4  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000000  00000000  00000000  00000000  2**2
                  ALLOC, LOAD, DATA
  2 .bss          00000000  00000000  00000000  00000000  2**2
                  ALLOC
  3 .rdata        00000024  00000000  00000000  000000e0  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
SYMBOL TABLE:
[  0](sec -2)(fl 0x00)(ty   0)(scl 103) (nx 1) 0x00000000 test.c
File 
[  2](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _main
[  3](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x2a nreloc 4 nlnno 0
[  5](sec  2)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .data
AUX scnlen 0x0 nreloc 0 nlnno 0
[  7](sec  3)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .bss
AUX scnlen 0x0 nreloc 0 nlnno 0
[  9](sec  4)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .rdata
AUX scnlen 0x21 nreloc 0 nlnno 0
[ 11](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 1) 0x00000000 ___main
AUX tagndx 0 ttlsiz 0x0 lnnos 0 next 0
[ 13](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _meaning_of_
+life
[ 14](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _printf


RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE 
0000000a DISP32            ___main
0000000f DISP32            _meaning_of_life
0000001a dir32             .rdata
0000001f DISP32            _printf

      [download]

      If I'm reading these correctly, both seem to contain the entry points _main and __main. At least the offsets differ, but compiling scriptedmain by referring to the entry point offset doesn't work.

      C:\Users\andrew\Desktop\src\scriptedmain\c>gcc -o scriptedmain -e00000
+014 scriptedmain.c scriptedmain.h
c:/strawberry/c/bin/../lib/gcc/i686-w64-mingw32/4.4.3/../../../../i686
+-w64-mingw32/lib/libmingw32.a(lib32_libmingw32_a-crt0_c.o): In functi
+on `main':
/opt/W64_156151-src.32/build-crt/../mingw-w64-crt/crt/crt0_c.c:18: und
+efined reference to `WinMain@16'
collect2: ld returned 1 exit status

      [download]
[reply]
[d/l]
[select]

        Ok, so whatever that is, its a bug in libmingw32

        Report bugs to mingw folks :)

        I tried 

        gcc -o scriptedmain scriptedmain.c  -s -Wl,-subsystem,console  -nostdl
+ib

        [download]

        Which avoid libmingw32 but this fails with 

        undefined reference to `___main'
undefined reference to `_printf'
undefined reference to `_printf'

        [download]

        Naturally -e ___main doesn't help anything 

        $ gcc -o scriptedmain scriptedmain.c  -s -Wl,-subsystem,console  -nost
+dlib -e ___main
c:/mingw/bin/../lib/gcc/mingw32/4.7.0/../../../../mingw32/bin/ld.exe: 
+warning: cannot find entry symbol ___main; defaul
ing to 00401000

        [download]

        So however you're supposed to work it, ask the mingw folks

[reply]
[d/l]
[select]

      I found a solution that works in Windows and Unix, using simple preprocessor instructions.

      http://stackoverflow.com/a/12397886/350106

[reply]
        :p you didn't know about that? I thought you wanted to exploit the compilers built-in mechanisms
[reply]

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