strange behaviour with readlink under cmake

I'm trying to use readlink -f to get the absolute path to a shared library following all symlinks.

>readlink -f /opt/gcc4.9.3/lib64/libstdc++.so.6
/opt/gcc4.9.3/lib64/libstdc++.so.6.0.20

but when I do this in cmake it doesn't expand the full path E.g. set(CPP11_PATH ${CMAKE_CURRENT_BINARY_DIR}/) execute_process(COMMAND ldd ${CPP11_PATH} COMMAND grep libstdc++ COMMAND awk "{ print $3; }" OUTPUT_VARIABLE LIBSTDCPP_PATH) message("LIBSTDCPP_PATH=${LIBSTDCPP_PATH}") execute_process(COMMAND readlink -f ${LIBSTDCPP_PATH} OUTPUT_VARIABLE LIBSTDCPP_ABSPATH) message("LIBSTDCPP_ABSPATH=${LIBSTDCPP_ABSPATH}")

prints:

LIBSTDCPP_PATH=/opt/gcc4.9.3/lib64/libstdc++.so.6
LIBSTDCPP_ABSPATH=/opt/gcc4.9.3/lib64/libstdc++.so.6

This also happens if I wrap it in a shell script: execute_process(COMMAND doreadlink.sh ${LIBSTDCPP_PATH} OUTPUT_VARIABLE LIBSTDCPP_ABSPATH2) message("LIBSTDCPP_ABSPATH2=${LIBSTDCPP_ABSPATH2}")

>LIBSTDCPP_ABSPATH2=/opt/gcc4.9.3/lib64/libstdc++.so.6

It also happens using get_filename_component() with REALPATH which ought to be the canonical cmake way to do this. get_filename_component(LIBSTDCPP_PATH ${LIBSTDCPP_PATH} REALPATH) message("LIBSTDCPP_PATH2=${LIBSTDCPP_PATH}")

Can anyone explain?

I have tried both cmake rebuild_cache and removing the CMakeCache.txt and running with --trace to ensure what I think is being run is really being run. I've also confirmed it happens on rhel5 rhel6 & rhel7 and with both cmake 2.8 & 3.4.

There is a workaround that behaves correctly: execute_process(COMMAND ldd ${CPP11_PATH} COMMAND grep libstdc++ COMMAND awk "{ print $3; COMMAND xargs readlink -f }" OUTPUT_VARIABLE LIBSTDCPP_PATH)

What might cmake be doing under the hood?

I tried running: strace -f cmake pwd

some of the output is:

[pid 23195] execve("/usr/lib64/qt-3.3/bin/readlink", ["readlink", "-f", "/lib64/libstdc++.so.6\n"], [/* 53 vars */]) = -1 ENOENT (No such file or directory)
[pid 23195] execve("/usr/local/bin/readlink", ["readlink", "-f", "/lib64/libstdc++.so.6\n"], [/* 53 vars */]) = -1 ENOENT (No such file or directory)
[pid 23195] execve("/usr/bin/readlink", ["readlink", "-f", "/lib64/libstdc++.so.6\n"], [/* 53 vars */] 
[pid 23174]  "", 1)   = 0
[pid 23195]  )      = 0
[pid 23174] close(11)                   = 0
[pid 23174] rt_sigprocmask(SIG_SETMASK, [],  
[pid 23195] brk(0 
[pid 23174]  NULL, 8) = 0
[pid 23195]  )         = 0x12e5000
[pid 23174] read(9, "", 1024)           = 0
[pid 23174] close(9)                    = 0
[pid 23195] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0 
[pid 23174] close(6)                    = 0
[pid 23195]  )        = 0x7ff41654f000
[pid 23174] close(8)                    = 0
[pid 23195] access("/etc/ld.so.preload", R_OK 
[pid 23174] select(8, [3 5 7], NULL, NULL, NULL 
[pid 23195]  )      = -1 ENOENT (No such file or directory)
[pid 23195] open("tls/x86_64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] open("tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] open("x86_64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] open("libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] open("/usr/local/lib/tls/x86_64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] stat("/usr/local/lib/tls/x86_64", 0x7fff36038350) = -1 ENOENT (No such file or directory)
[pid 23195] open("/usr/local/lib/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] stat("/usr/local/lib/tls", 0x7fff36038350) = -1 ENOENT (No such file or directory)
[pid 23195] open("/usr/local/lib/x86_64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)

snip

O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] stat("/usr/lib/oracle/10.2.0.4/client/lib/tls", 0x7fff36038350) = -1 ENOENT (No such file or directory)
[pid 23195] open("/usr/lib/oracle/10.2.0.4/client/lib/x86_64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
[pid 23195] stat("/usr/lib/oracle/10.2.0.4/client/lib/x86_64", 0x7fff36038350) = -1 ENOENT (No such file or directory)
[pid 23195] open("/usr/lib/oracle/10.2.0.4/client/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
rt_sigprocmask(SIG_BLOCK, [INT TERM CHLD], [], 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
write(2, "LIBSTDCPP_ABSPATH=/usr/lib64/lib"..., 45LIBSTDCPP_ABSPATH=/usr/lib64/libstdc++.so.6

) = 45

So it even looks like its executing the right system command.

Here is a CMakeLists.txt to reproduce the problem:

cmake_minimum_required(VERSION 2.8)
project(TEST CXX) 

set(CPP11_PATH ${CMAKE_CURRENT_BINARY_DIR}/cpp11)
execute_process(COMMAND g++ ${CMAKE_CURRENT_SOURCE_DIR}/cpp11.cpp -o${CPP11_PATH})

#execute_process(COMMAND ldd ${CPP11_PATH} COMMAND grep libstdc++ COMMAND awk "{ print $3; }" COMMAND xargs readlink -f OUTPUT_VARIABLE LIBSTDCPP_PATH)
execute_process(COMMAND ldd ${CPP11_PATH} COMMAND grep libstdc++ COMMAND awk "{ print $3; }" OUTPUT_VARIABLE LIBSTDCPP_PATH)
message("LIBSTDCPP_PATH=${LIBSTDCPP_PATH}")
execute_process(COMMAND readlink -f ${LIBSTDCPP_PATH} OUTPUT_VARIABLE LIBSTDCPP_ABSPATH)
message("LIBSTDCPP_ABSPATH=${LIBSTDCPP_ABSPATH}")

Solution

Many shell utilities which output single line terminates it with newline (\n). This is done for pretty output in the terminal.

Unlike to backtricks operator (`exec-some-command`) in Linux shell, which automatically strips trailing newline, execute_process doesn't do this by default.

The simplest way to strip trailing newline within execute_process output is to use OUTPUT_STRIP_TRAILING_WHITESPACE option for it:

execute_process(COMMAND <...>
    OUTPUT_VARIABLE LIBSTDCPP_PATH
    OUTPUT_STRIP_TRAILING_WHITESPACE
)