From: Masahiro Yamada
Sent: 03 June 2019 12:38 Hi David,
On Mon, Jun 3, 2019 at 8:14 PM David Laight David.Laight@aculab.com wrote:
From: Masahiro Yamada
Sent: 03 June 2019 11:49
To print the pathname that will be used by shell in the current environment, 'command -v' is a standardized way. [1]
'which' is also often used in scripting, but it is not portable.
When I worked on commit bd55f96fa9fc ("kbuild: refactor cc-cross-prefix implementation"), I was eager to use 'command -v' but it did not work. (The reason is explained below.)
I kept 'which' as before but got rid of '> /dev/null 2>&1' as I thought it was no longer needed. Sorry, I was wrong.
It works well on my Ubuntu machine, but Alexey Brodkin reports annoying warnings from the 'which' on CentOS 7 when the given command is not found in the PATH environment.
$ which foo which: no foo in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)
Given that behavior of 'which' is different on environment, I want to try 'command -v' again.
The specification [1] clearly describes the behavior of 'command -v' when the given command is not found:
Otherwise, no output shall be written and the exit status shall reflect that the name was not found.
However, we need a little magic to use 'command -v' from Make.
$(shell ...) passes the argument to a subshell for execution, and returns the standard output of the command.
Here is a trick. GNU Make may optimize this by executing the command directly instead of forking a subshell, if no shell special characters are found in the command line and omitting the subshell will not change the behavior.
In this case, no shell special character is used. So, Make will try to run the command directly. However, 'command' is a shell-builtin command. In fact, Make has a table of shell-builtin commands because it must spawn a subshell to execute them.
Until recently, 'command' was missing in the table.
This issue was fixed by the following commit:
| commit 1af314465e5dfe3e8baa839a32a72e83c04f26ef | Author: Paul Smith psmith@gnu.org | Date: Sun Nov 12 18:10:28 2017 -0500 | | * job.c: Add "command" as a known shell built-in. | | This is not a POSIX shell built-in but it's common in UNIX shells. | Reported by Nick Bowler nbowler@draconx.ca.
This is not included in any released versions of Make yet. (But, some distributions may have back-ported the fix-up.)
To trick Make and let it fork the subshell, I added a shell special character '~'. We may be able to get rid of this workaround someday, but it is very far into the future.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html
Fixes: bd55f96fa9fc ("kbuild: refactor cc-cross-prefix implementation") Cc: linux-stable stable@vger.kernel.org # 5.1 Reported-by: Alexey Brodkin abrodkin@synopsys.com Signed-off-by: Masahiro Yamada yamada.masahiro@socionext.com
scripts/Kbuild.include | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include index 85d758233483..5a32ca80c3f6 100644 --- a/scripts/Kbuild.include +++ b/scripts/Kbuild.include @@ -74,8 +74,11 @@ endef # Usage: CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu- m68k-linux-) # Return first <prefix> where a <prefix>gcc is found in PATH. # If no gcc found in PATH with listed prefixes return nothing +# +# Note: the special character '~' forces Make to invoke a shell. This workaround +# is needed because this issue was only fixed after GNU Make 4.2.1 release. cc-cross-prefix = $(firstword $(foreach c, $(filter-out -%, $(1)), \
$(if $(shell which $(c)gcc), $(c))))
$(if $(shell command -v $(c)gcc ~), $(c))))I see a problem here: command -v foo bar could be deemed to be an error (extra argument).
OK, the specification does not allow to pass arguments with -v.
You could use: $(shell sh -c "command -v $(c)gcc") or maybe: $(shell command$${x:+} -v $(c)gcc)
How about this?
$(shell : ~; command -v $(c)gcc)
Overcomplicated ....
I've not looked at the list of 'special characters' in make, but I suspect any variable expansion is enough. Since ${x:+} always expands to the empty string (whether or not 'x' is defined) it can't have any unfortunate side effects.
I'd comment as: # Note: ${x:+} always expands to the empty string and forces all # versions of make to actually exec $SHELL rather than try to # directly execute the shell builtin 'command'.
David
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