gcc


Enable C++11 standard for GCC on Eclipse CDT

When using Eclipse CDT to write C++, we noticed that it did not enable by default the C++11 standard. Following the steps below, we added the -std=c++11 flag on the GCC C++ Compiler command line arguments enabling the standard for our use.

  1. From the main window of Eclipse, on the list on the left, where your projects are listed, right click on your project and then click Properties from the new menu
  2. In the new window, navigate from the list on the left and expand the C/C++ Build option to view its children and then click the Settings item
  3. In the middle of the window, you will see a new list, expand (if needed) the item GCC C++ Compiler and click on the Miscellaneous child
  4. On the right, a text box named Other Flags will appear, append -std=c++11 to the list of tokens in the box as seen in the image below
  5. Click on the Apply button for the effects to take place and then the OK button to close the properties window

Next time you compile, the -std=c++11 flag will be present on your compiler command line and the C++11 standard will be used.

Advertisements

Downgrade gcc on CentOS 7.0 (64bit) to version 4.8.2

Recently we had to download gcc from version 4.8.5 to 4.8.2.

We were trying to compile some tools and they required using that older version of gcc.
To compile the old version of the gcc we used the version version that was shipped with the distribution.

Methodology

#Making sure we are not missing any 32bit libraries since we are on a 64bit machine
yum install glibc.i686 ncurses-libs.i686;
#Download the source code
wget http://ftp.gnu.org/gnu/gcc/gcc-4.8.2/gcc-4.8.2.tar.gz;
#Extract the files
tar -xvf gcc-4.8.2.tar.gz;
#Navigate to the folder
cd gcc-4.8.2/;
#Make sure we have all dependencies met
./contrib/download_prerequisites;
#Configure the installation and assign the installation folder to be /usr/local/gcc/4.8.2. Finally make all necessary checks before compilation.
./configure --prefix=/usr/local/gcc/4.8.2;
#Build
make;
#Install
sudo make install;

Fedora 24: Solution to gcrypt.h: No such file or directory

Recently we tried to compile cisco-decrypt.c* on Fedora 24 (64bit).
We got the full source code from the website of Maurice Massar.
Download full source code here: cisco-decrypt.c (compressed) (184 downloads)

We needed this tool to configure the Gnome 3 native network client to connect to a specific Cisco VPN network.
During the configuration we setup a “Cisco Compatible VPN (vpnc)” VPN.

* Please note that this tool is NOT a hacking nor cracking tool. In order for you to make any use of it, you need to have a valid PCF file given to you by your system administrator. It is only useful when you need to get the group password to configure a system that does not accept the PCF file with the encrypted password.

To compile the code you can use one of the following two methods:

Compilation method 1

gcc -Wall -o cisco-decrypt cisco-decrypt.c $(libgcrypt-config --libs --cflags)

If the package libgcrypt-devel is not installed you will get a prompt message as follows, which will instruct you to install the missing package.
You need to type y to both questions so that the installation will proceed. Once the installation is done, execute once more the compilation command.

As you can see below, you might get a whole bunch of errors, ignore them and try to compile once more. It seems to be a bug that will not affect the end result (at least in this scenario).

gcc -Wall -o cisco-decrypt cisco-decrypt.c $(libgcrypt-config --libs --cflags)
bash: libgcrypt-config: command not found...
Install package 'libgcrypt-devel' to provide command 'libgcrypt-config'? [N/y] y

Proceed with changes? [N/y] y

gcc: error: Waiting: No such file or directory
gcc: error: in: No such file or directory
gcc: error: queue...: No such file or directory
gcc: error: Loading: No such file or directory
gcc: error: list: No such file or directory
gcc: error: of: No such file or directory
gcc: error: packages....: No such file or directory
gcc: error: The: No such file or directory
gcc: error: following: No such file or directory
gcc: error: packages: No such file or directory
gcc: error: have: No such file or directory
gcc: error: to: No such file or directory
gcc: error: be: No such file or directory
gcc: error: installed:: No such file or directory
gcc: error: libgcrypt-devel-1.6.6-1.fc24.x86_64: No such file or directory
gcc: error: Development: No such file or directory
gcc: error: files: No such file or directory
gcc: error: for: No such file or directory
gcc: error: the: No such file or directory
gcc: error: libgcrypt: No such file or directory
gcc: error: package: No such file or directory
gcc: error: libgpg-error-devel-1.24-1.fc24.x86_64: No such file or directory
gcc: error: Development: No such file or directory
gcc: error: files: No such file or directory
gcc: error: for: No such file or directory
gcc: error: the: No such file or directory
gcc: error: libgpg-error: No such file or directory
gcc: error: package: No such file or directory
gcc: error: Waiting: No such file or directory
gcc: error: in: No such file or directory
gcc: error: queue...: No such file or directory
gcc: error: Waiting: No such file or directory
gcc: error: for: No such file or directory
gcc: error: authentication...: No such file or directory
gcc: error: Waiting: No such file or directory
gcc: error: in: No such file or directory
gcc: error: queue...: No such file or directory
gcc: error: Downloading: No such file or directory
gcc: error: packages...: No such file or directory
gcc: error: Requesting: No such file or directory
gcc: error: data...: No such file or directory
gcc: error: Testing: No such file or directory
gcc: error: changes...: No such file or directory
gcc: error: Installing: No such file or directory
gcc: error: packages...: No such file or directory

Compilation method 2

In case the above method does not work for you for some reason, you can try the following.

gcc -Wall -o cisco-decrypt cisco-decrypt.c -lgcrypt

If the package libgcrypt-devel is not installed you will get an error as follows.

gcc -Wall -o cisco-decrypt cisco-decrypt.c -lgcrypt
cisco-decrypt.c:30:20: fatal error: gcrypt.h: No such file or directory
 #include <gcrypt.h>
                    ^
compilation terminated.

In this case use

sudo dnf install libgcrypt-devel

to install the missing library and try again to compile.

To use

Open your PCF file with a text editor. Find the line that starts with enc_GroupPwd= and copy the characters after that.

Paste the characters as the first command line argument to the newly compiled application. The password will be the line returned right after.

./cisco-decrypt 886E2FC74BFCD8B6FAF47784C386A50D0C1A5D0528D1E682B7EBAB6B2E91E792E389914767193F9114FA26C1E192034754F85FC97ED36509
Th!sIsMyK3y#

Other notes

In the case you get these errors:

/tmp/ccHrH1kZ.o: In function `c_decrypt':
cisco-decrypt.c:(.text+0x243): undefined reference to `gcry_md_hash_buffer'
cisco-decrypt.c:(.text+0x267): undefined reference to `gcry_md_hash_buffer'
cisco-decrypt.c:(.text+0x2b4): undefined reference to `gcry_md_hash_buffer'
cisco-decrypt.c:(.text+0x31d): undefined reference to `gcry_cipher_open'
cisco-decrypt.c:(.text+0x33b): undefined reference to `gcry_cipher_setkey'
cisco-decrypt.c:(.text+0x356): undefined reference to `gcry_cipher_setiv'
cisco-decrypt.c:(.text+0x382): undefined reference to `gcry_cipher_decrypt'
cisco-decrypt.c:(.text+0x391): undefined reference to `gcry_cipher_close'
/tmp/ccHrH1kZ.o: In function `main':
cisco-decrypt.c:(.text+0x41e): undefined reference to `gcry_check_version'
collect2: error: ld returned 1 exit status

It most probably means that you did not add on your compilation command one of the following two parameters

  1. $(libgcrypt-config –libs –cflags)
  2. -lgcrypt

You need one of these two options to be on the command line to compile.


C/C++: How NOT to define multiple pointers

When defining pointers in C/C++ you should be careful on how you use the * characters. If you try to define multiple pointers on the same line and you do not add the * character in front of each variable, then the results will not be what you would expect. In the following examples we added the * to the data type definition, hoping that all variables would become pointers of that data type. Unfortunately, as the compiler points out later on while making the comparisons, only the first variable in each line becomes a pointer of the data type.

Wrong Examples

C Source Code:

#include <stdio.h>

int main()
{
    int* a, b;
    int *c, d;
    int * e, f;

    a == b;
    c == d;
    e == f;

    return 0;
}

Compiler Output:

$ gcc -o main *.c                                                                                          
main.c: In function 'main':                                                                                      
main.c:9:7: warning: comparison between pointer and integer                                                      
     a == b;                                                                                                     
       ^                                                                                                         
main.c:10:7: warning: comparison between pointer and integer                                                     
     c == d;                                                                                                     
       ^                                                                                                         
main.c:11:7: warning: comparison between pointer and integer                                                     
     e == f;                                                                                                     
       ^                                                                                                         

C++ Source Code:

#include <iostream>
using namespace std;

int main()
{
    int* a, b;
    int *c, d;
    int * e, f;

    a == b;
    c == d;
    e == f;

   return 0;
}

Compiler Output:

$ g++ -std=c++11 *.cpp -o main

main.cpp: In function ‘int main()’:
main.cpp:10:10: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
     a == b;
          ^
main.cpp:11:10: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
     c == d;
          ^
main.cpp:12:10: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
     e == f;
          ^

Correct Examples

#include <stdio.h>

int main()
{
    int* a, * b;
    int *c, *d;
    int * e, * f;

    a == b;
    c == d;
    e == f;

    return 0;
}
#include <iostream>
using namespace std;

int main()
{
    int* a, * b;
    int *c, *d;
    int * e, * f;

    a == b;
    c == d;
    e == f;

   return 0;
}