Downloaded g++ 4.8 from the PPA but can't set it as default?

Question

I have downloaded g++ 4.8 on Ubuntu 12.10 by doing:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install g++-4.8

but when I do:

sudo update-alternatives --config g++

to switch g++ versions it says:

update-alternatives: error: no alternatives for g++.

However if I do:

g++ --version

it says:

g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

How do I set my g++ 4.8 installation to be the default g++?

score 18 · Answer 1 · edited Apr 13 '17 at 12:24

18

You need to let update-alternatives to know that you have 2 C++ compilers, create a record for each one, and then configure which one you want to use. This is done with the following:

sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.6.3 60
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 40
sudo update-alternatives --config g++

From this point forward, the only thing required when switching compilers is this command:

sudo update-alternatives --config g++

See man update-alternatives for more info.

^{Source: How to use multiple instances of gcc?}

edited Apr 13 '17 at 12:24

Community

1

answered Nov 07 '13 at 00:06

Radu Rădeanu

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@Braiam It can be done in the same way with any package. – Radu Rădeanu Nov 07 '13 at 00:08
3

Can someone elaborate on the 60 and 40? Ive seen this in lots of places – user997112 Nov 07 '13 at 00:09
@user997112 I suggest you to read the man, but, if you like more 70 and 30, then use 70 and 30... – Radu Rădeanu Nov 07 '13 at 00:10
oh- percentages of some kind – user997112 Nov 07 '13 at 00:12
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@user997112 these are priority numbers, and are only relevant when using --auto. you can see some examples using `update-alternatives --display editor` – Braiam Nov 07 '13 at 00:14
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On 12.04 I get: update-alternatives: error: alternative g++ can't be master: it is a slave of gcc – rickfoosusa Jan 09 '15 at 16:22
This is the correct answer. – Alex Reynolds Feb 27 '17 at 12:12

Braiam · Accepted Answer · 2013-11-07T01:03:22.230

11

First of all, you must discover where are your 4.8 binaries of all the tools:

$ which gcc-4.8
/usr/bin/gcc-4.8
$ which g++-4.8
/usr/bin/g++-4.8
$ ls /usr/bin/*-4.8
/usr/bin/cpp-4.8         /usr/bin/x86_64-linux-gnu-cpp-4.8
/usr/bin/g++-4.8         /usr/bin/x86_64-linux-gnu-g++-4.8
/usr/bin/gcc-4.8         /usr/bin/x86_64-linux-gnu-gcc-4.8
/usr/bin/gcc-ar-4.8      /usr/bin/x86_64-linux-gnu-gcc-ar-4.8
/usr/bin/gcc-nm-4.8      /usr/bin/x86_64-linux-gnu-gcc-nm-4.8
/usr/bin/gcc-ranlib-4.8  /usr/bin/x86_64-linux-gnu-gcc-ranlib-4.8
/usr/bin/gcov-4.8

So, we have all our binaries, now lets see if some symlinks are available for such binaries:

$ cd /usr/bin
$ ls -l gcc* cpp g++
lrwxrwxrwx 1 root root      7 sep 18 14:02 cpp -> cpp-4.7
lrwxrwxrwx 1 root root      7 abr 22  2013 g++ -> g++-4.7
lrwxrwxrwx 1 root root      7 sep 18 14:02 gcc -> gcc-4.7

As we can see, only cpp, g++ and gcc has symbolic links. We have two options here.

Symlinking

We replace the symlinks with ours, removing the actuals first:

sudo rm /usr/bin/cpp /usr/bin/gcc /usr/bin/g++

Then creating ours

sudo ln -s /usr/bin/cpp-4.8 /usr/bin/cpp
sudo ln -s /usr/bin/gcc-4.8 /usr/bin/gcc
sudo ln -s /usr/bin/g++-4.8 /usr/bin/g++

To revert it back use the same commands but with 4.7 or 4.6 instead

sudo rm /usr/bin/cpp /usr/bin/gcc /usr/bin/g++
sudo ln -s /usr/bin/cpp-4.7 /usr/bin/cpp
sudo ln -s /usr/bin/gcc-4.7 /usr/bin/gcc
sudo ln -s /usr/bin/g++-4.7 /usr/bin/g++

edited Nov 07 '13 at 01:03

answered Nov 06 '13 at 23:26

Braiam

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When I did: ls -l gcc* cpp g++ it returned: ls: cannot access gcc*: No such file or directory ls: cannot access cpp: No such file or directory ls: cannot access g++: No such file or directory – user997112 Nov 06 '13 at 23:37
No apology required :) Once I call the remove function it works, but when I go to create "our" symlinks it says: sudo ln -s -T /usr/bin/cpp /usr/bin/cpp-4.8 ln: failed to create symbolic link `/usr/bin/cpp-4.8': File exists – user997112 Nov 07 '13 at 00:01
(I called the rm command twice just so on the second time it confirmed/complained that the existing links were in fact deleted) – user997112 Nov 07 '13 at 00:02
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**This is a terrible idea**. There are many programs who use `gcc` to compile kernel modules and some of them (e.g. Nvidia) will silently crash when the `gcc` they call will not be the one the kernel was compiled with. – nbubis Aug 27 '15 at 11:33
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@nbubis well, the question is "How do I set my g++ 4.8 installation to be the default g++?", so, one should presume whoever does this, they know what they are doing, no? On the other hand, what alternative can you propose? – Braiam Aug 27 '15 at 13:37

Anton K · Answer 3 · 2014-03-13T18:26:58.727

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This is the great description and step-by-step instruction how to create and manage master and slave (gcc and g++) alternatives.

Shortly, it's

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.7 60 --slave /usr/bin/g++ g++ /usr/bin/g++-4.7 
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 40 --slave /usr/bin/g++ g++ /usr/bin/g++-4.8 
sudo update-alternatives --config gcc

edited Mar 13 '14 at 18:26

answered Mar 12 '14 at 23:13

Anton K

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Welcome to Ask Ubuntu! Whilst this may theoretically answer the question, [it would be preferable](http://meta.stackexchange.com/q/8259) to include the essential parts of the answer here, and provide the link for reference. – Stormvirux Mar 13 '14 at 02:52
You're right. I added the essential part – Anton K Mar 13 '14 at 18:28
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Saved my day with that slave stuff. tx – Mr. Developerdude Mar 19 '16 at 23:37

Downloaded g++ 4.8 from the PPA but can't set it as default?

3 Answers3

Symlinking

Linked