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linking succeeds with clang but not gcc (-llzma on Debian bullseye amd64)

On Debian bullseye (testing), with these xz/lzma-related packages installed:

ii  liblzma-dev:amd64                     5.2.4-1+b1                           amd64
ii  liblzma5:amd64                        5.2.4-1+b1                           amd64
ii  lzma-dev                              9.22-2.1                             all

Clang generates a working ./a.out without issue:

$>gunzip < /usr/share/doc/liblzma-dev/examples/02_decompress.c.gz | sed -s 's/extern int/int' | clang -std=c18 -Wall -Wextra -x c -llzma -

However, attempting to use gcc fails:

$>gunzip < /usr/share/doc/liblzma-dev/examples/02_decompress.c.gz | sed -s 's/extern int/int' | gcc -std=c18 -Wall -Wextra -x c -llzma -

/usr/bin/ld: /tmp/ccl8uTG0.o: in function `init_decoder':
:(.text+0x20): undefined reference to `lzma_stream_decoder'
/usr/bin/ld: /tmp/ccl8uTG0.o: in function `decompress':
:(.text+0x1e1): undefined reference to `lzma_code'
/usr/bin/ld: /tmp/ccl8uTG0.o: in fuction `main':
:(.text+0x4e4): undefined reference to `lzma_end'
collect2: error: ld returned 1 exit status

g++ --version reports 9.3.0-3

clang++ --version reports 9.0.1-10

(Those compilers are both installed via apt-get.)

/etc/ld.so.conf.d/x86_64-linux-gnu.conf contains

# Multiarch support
/usr/local/lib/x86_64-linux-gnu
/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu

I have checked that /usr/lib/x86_64-linux-gnu/liblzma.so.5.2.4 (and its .so.5 and .so symlinks, and the associated .a file too) exist.

Furthermore, the output of readelf --symbols /usr/lib/x86_64-linux-gnu/liblzma.so.5.2.4 | grep 'lzma_[sec]' shows that those three symbols exist (along with many others):

    67: 000000000000c720   106 FUNC    GLOBAL DEFAULT   13 lzma_stream_decoder@@XZ_5.0
   108: 00000000000039d0   701 FUNC    GLOBAL DEFAULT   13 lzma_code@@XZ_5.0
   117: 0000000000003c90    65 FUNC    GLOBAL DEFAULT   13 lzma_end@@XZ_5.0

What am I doing wrong?

Solution

  • Clang puts the -llzma outside the --as-needed area of the linker command line:

    […] -L/usr/lib -llzma /tmp/--e00a22.o -lgcc --as-needed -lgcc_s […]

    GCC puts it after --as-needed:

    […] --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 […]
  -llzma /tmp/ccAZHRvb.o -lgcc --push-state

    As a result, the linker notices that -llzma is not needed at that point in the command line and drops it. Linking is order-sensitive.

    You can fix this by using - -llzma instead of -llzma -, as HolyBlackCat suggested.