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

169,590

@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
1

@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
2

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

67,791
32
179
269

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
3

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. – Nathaniel Bubis Aug 27 '15 at 11:33
1

@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

10

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

201
2
5

Welcome to Ask Ubuntu! Whilst this may theoretically answer the question, it would be preferable 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
1

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