Re: [FFmpeg-devel] [PATCH] configure: link to libatomic when it's present

[Brad Smith <brad-at-comstyle.com@ffmpeg.org>]

On 1/26/2022 6:49 AM, Anton Khirnov wrote:

> Quoting Brad Smith (2022-01-23 20:40:30)
>> Testing this commit out it does as I had suspected and even with --as-needed
>> causes a false positive on OpenBSD / FreeBSD.
> Why?

Looking at this again and thinking about what it does, the test as is is 
flawed.
With the order being used it doesn't check to see if  the linked binary even
is linked against libatomic. Using --as-needed it drops the libatomic 
dependency
but since the link succeeds then the code as it is and based on the 
ordering, as
I commented about, says go ahead and link in libatomic. Changing the order
so it is "" "-latomic" instead of "-latomic" "" does the right thing.

humpty$ cc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE 
-DPIC -O2 -pipe -I/usr/local/include -I/usr/X11R6/include -std=c11 -fPIC 
-c -o test.o test.c
humpty$ cc -Wl,--as-needed -Wl,-z,noexecstack -o test test.o -latomic 
-L/usr/local/lib -L/usr/X11R6/lib
humpty$ objdump -p test

test:     file format elf64-x86-64

Program Header:
     PHDR off    0x0000000000000040 vaddr 0x0000000000000040 paddr 
0x0000000000000040 align 2**3
          filesz 0x00000000000002a0 memsz 0x00000000000002a0 flags r--
   INTERP off    0x00000000000002e0 vaddr 0x00000000000002e0 paddr 
0x00000000000002e0 align 2**0
          filesz 0x0000000000000013 memsz 0x0000000000000013 flags r--
     LOAD off    0x0000000000000000 vaddr 0x0000000000000000 paddr 
0x0000000000000000 align 2**12
          filesz 0x00000000000005dc memsz 0x00000000000005dc flags r--
     LOAD off    0x00000000000005e0 vaddr 0x00000000000015e0 paddr 
0x00000000000015e0 align 2**12
          filesz 0x0000000000000450 memsz 0x0000000000000450 flags r-x
     LOAD off    0x0000000000000a30 vaddr 0x0000000000002a30 paddr 
0x0000000000002a30 align 2**12
          filesz 0x00000000000001c0 memsz 0x00000000000001c0 flags rw-
     LOAD off    0x0000000000000bf0 vaddr 0x0000000000003bf0 paddr 
0x0000000000003bf0 align 2**12
          filesz 0x0000000000000000 memsz 0x0000000000000055 flags rw-
  DYNAMIC off    0x0000000000000a88 vaddr 0x0000000000002a88 paddr 
0x0000000000002a88 align 2**3
          filesz 0x0000000000000120 memsz 0x0000000000000120 flags rw-
    RELRO off    0x0000000000000a30 vaddr 0x0000000000002a30 paddr 
0x0000000000002a30 align 2**0
          filesz 0x00000000000001c0 memsz 0x00000000000005d0 flags r--
EH_FRAME off    0x00000000000004c8 vaddr 0x00000000000004c8 paddr 
0x00000000000004c8 align 2**2
          filesz 0x0000000000000034 memsz 0x0000000000000034 flags r--
OPENBSD_RANDOMIZE off    0x0000000000000a30 vaddr 0x0000000000002a30 
paddr 0x0000000000002a30 align 2**3
          filesz 0x0000000000000030 memsz 0x0000000000000030 flags rw-
    STACK off    0x0000000000000000 vaddr 0x0000000000000000 paddr 
0x0000000000000000 align 2**0
          filesz 0x0000000000000000 memsz 0x0000000000000000 flags rw-
     NOTE off    0x00000000000002f4 vaddr 0x00000000000002f4 paddr 
0x00000000000002f4 align 2**2
          filesz 0x0000000000000018 memsz 0x0000000000000018 flags r--

Dynamic Section:
   NEEDED      libc.so.96.1
   FLAGS_1     0x8000000
   DEBUG       0x0
   RELA        0x438
   RELASZ      0x30
   RELAENT     0x18
   RELACOUNT   0x1
   JMPREL      0x468
   PLTRELSZ    0x60
   PLTGOT      0x2bb8
   PLTREL      0x7
   SYMTAB      0x310
   SYMENT      0x18
   STRTAB      0x3f8
   STRSZ       0x3f
   GNU_HASH    0x3a0
   HASH        0x3c0

libatomic is dropped by --as-needed since it is not necessary.

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