`info functions` prints the names and data types of all defined functions. See [16 Examining the Symbol Table][1].


  [1]: http://sourceware.org/gdb/onlinedocs/gdb/Symbols.html

I am trying to work out how to make bash (force?) expand variables in a string (which was loaded from a file).

I have a file called &quot;something.txt&quot; with the contents: 

    hello $FOO world



I then run

    export FOO=42
    echo $(cat something.txt)

this returns: 

       hello $FOO world

It didn&#39;t expand $FOO even though the variable was set. I can&#39;t eval or source the file - as it will try and execute it (it isn&#39;t executable as it is - I just want the string with the variables interpolated). 

Any ideas? 



Forcing bash to expand variables in a string loaded from a file

I am trying to plot some data using pandas in Ipython Notebook, and while it gives me the object, it doesn&#39;t actually plot the graph itself. So it looks like this:

    In [7]:

    pledge.Amount.plot()

    Out[7]:
    
    &lt;matplotlib.axes.AxesSubplot at 0x9397c6c&gt;

The graph should follow after that, but it simply doesn&#39;t appear. I have imported matplotlib, so that&#39;s not the problem. Is there any other module I need to import?


in Ipython notebook / Jupyter, Pandas is not displaying the graph I try to plot

How can you list all functions in a program with GDB?


Ask GDB to list all functions in a program

<p>How can you list all functions in a program with GDB?</p>


Which of the following techniques is the best option for dividing an integer by 2 and why?

Technique 1:

    x = x &gt;&gt; 1;

Technique 2:

    x = x / 2;

Here `x` is an integer.

Which is better option to use for dividing an integer number by 2?

In a codebase I reviewed, I found the following idiom.

    void notify(struct actor_t act) {
        write(act.pipe, &quot;M&quot;, 1);
    }
    // thread A sending data to thread B
    void send(byte *data) {
        global.data = data;
        notify(threadB);
    }
    // in thread B event loop
    read(this.sock, &amp;cmd, 1);
    switch (cmd) {
        case &#39;M&#39;: use_data(global.data);break;
        ...
    }

&quot;Hold it&quot;, I said to the author, a senior member of my team, &quot;there&#39;s no memory barrier here! You don&#39;t guarantee that `global.data` will be flushed from the cache to main memory. If thread A and thread B will run in two different processors - this scheme might fail&quot;.

The senior programmer grinned, and explained slowly, as if explaining his five years old boy how to tie his shoelaces: &quot;Listen young boy, we&#39;ve seen here many thread related bugs, in high load testing, and in real clients&quot;, he paused to scratch his longish beard, &quot;but we&#39;ve never had a bug with this idiom&quot;.

&quot;But, it says in the book...&quot;

&quot;Quiet!&quot;, he hushed me promptly, &quot;Maybe theoretically, it&#39;s not guaranteed, but in practice, the fact you used a function call is effectively a memory barrier. The compiler will not reorder the instruction `global.data = data`, since it can&#39;t know if anyone using it in the function call, and the x86 architecture will ensure that the other CPUs will see this piece of global data by the time thread B reads the command from the pipe. Rest assured, we have ample real world problems to worry about. We don&#39;t need to invest extra effort in bogus theoretical problems.

&quot;Rest assured my boy, in time you&#39;ll understand to separate the real problem from the I-need-to-get-a-PhD non-problems.&quot;

Is he correct? Is that really a non-issue in practice (say x86, x64 and ARM)?

It&#39;s against everything I learned, but he does have a long beard and a really smart looks!

Extra points if you can show me a piece of code proving him wrong!

Is function call an effective memory barrier for modern platforms?

Does the typical `malloc` (for x86-64 platform and Linux OS) naively lock a mutex at the beginning and release it when done, or does it lock a mutex in a more clever way at a finer level, so that lock contention is reduced? If it indeed does it the second way, how does it do it?

How does malloc work in a multithreaded environment?

I am on osx. I  found this 
http://neugierig.org/software/c-repl/
but the links on that page for code seem to be broken.

Is there a REPL for C programming?

The title of the question might be a bit strange, but the thing is that, as far as I know, there is nothing that speaks against tail call optimization at all. However, while browsing open source projects, I already came across a few functions that actively try to stop the compiler from doing a tail call optimization, for example the implementation of [CFRunLoopRef](http://opensource.apple.com/source/CF/CF-635.21/CFRunLoop.c) which is full of such *hacks*. For example:

    static void __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__() __attribute__((noinline));
    static void __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__(CFRunLoopObserverCallBack func, CFRunLoopObserverRef observer, CFRunLoopActivity activity, void *info) {
        if (func) {
            func(observer, activity, info);
        }
        getpid(); // thwart tail-call optimization
    }

I would love to know why this is seemingly so important, and are there any cases were I as a *normal* developer should keep this is mind too? Eg. are there common pitfalls with tail call optimization?

Why would code actively try to prevent tail-call optimization?

Can I map an IP address like `127.0.0.1` to a domain name *and* a port?

For example, I would like to map `127.0.0.1` to `api.example.com:8000`

Can I map a hostname *and* a port with /etc/hosts?

I&#39;ve booted up a CentOS server on rackspace and executed `yum install httpd`&#39;d. Then `services httpd start`. So, just the barebones.

I can access its IP address remotely over ssh (22) no problem, so there&#39;s no problem with the DNS or anything (I think...), but when I try to connect on port 80 (via a browser or something) I get connection refused. 

From localhost, however, I can use telnet (80), or even lynx on itself and get served with no problem. From outside (my house, my school, a local coffee shop, etc...), telnet connects on 22, but not 80. 

I use `netstat -tulpn` (&lt;- I&#39;m not going to lie, I don&#39;t understand the `-tulpn` part, but that&#39;s what the internet told me to do...) and see 

    tcp    0    0 :::80     :::*    LISTEN    -                   

as I believe I should. The `httpd.conf` says `Listen 80`. 

I have `services httpd restart`&#39;d many a time.

Honestly I have no idea what to do. There is NO way that rackspace has a firewall on incoming port 80 requests. I feel like I&#39;m missing something stupid, but I&#39;ve booted up a barebones server twice now and have done the absolute minimum to get this functioning thinking I had mucked things up with my tinkering, but neither worked. 

Any help is greatly appreciated! (And sorry for the long winded post...)

**Edit**
I was asked to post the output of `iptables -L`. So here it is:

    Chain INPUT (policy ACCEPT)
    target     prot opt source               destination         
    ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED 
    ACCEPT     icmp --  anywhere             anywhere            
    ACCEPT     all  --  anywhere             anywhere            
    ACCEPT     tcp  --  anywhere             anywhere            state NEW tcp dpt:ssh 
    REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited 
    
    Chain FORWARD (policy ACCEPT)
    target     prot opt source               destination         
    REJECT     all  --  anywhere             anywhere            reject-with icmp-host-prohibited 
    
    Chain OUTPUT (policy ACCEPT)
    target     prot opt source               destination   



apache not accepting incoming connections from outside of localhost

How to copy contents from a directory and not symbolic link?

If have some code where I would like to use C++11x extensions as much as possible, but have a fallback if this is not supported. Currently the OSX version of GCC and the VisualC compiler has little to no support for C++11x, so I use:
    
    #if (defined(__APPLE__) || (defined(_WIN32)))
       ...fallback code without C++11x ...
    #else
       ... code using C++11x ...
    #endif

And this works, but is not really the right thing to do, especially since the gcc compiler in MacPorts DOES support c++11x.

Is there a `#define C11X_SUPPORTED` type macro? Perhaps something only GCC has?

Is there a preprocessor directive for detecting C++11x support?

&gt; **TL/DR**: Where can I find more information about building a GCC 4.7.0 cross-compiling toolchain for ARM (gnueabi) platform (intended to run on a Raspberry Pi device)?

I have just got a brand new Raspberry Pi and I am very eager to start programming for it. I&#39;ve managed to install the GCC toolchain (I am using the Arch Linux system image) and compiled some basic programs, all working fine.

I&#39;ve also tried to compile the Boost libraries because I often use them in my projects and everything seemed to work fine by following the instructions (`./bootstrap.sh` + `./b2`) except for the fact that the compilation was painfully slow. I left it on for a few hours but it barely got past the first few source files. After I left it running for the night, I discovered that the build process aborted due to RAM shortage.

So, my guess is that Rasp Pi is simply underpowered for compiling something of such size as Boost. So, cross-compilation comes to my mind. However, even though there is a lot of information about ARM cross compilation available online, I find it confusing. Where does one even start? 

I have a recent GCC version (4.7.0) available on my Raspberry Pi, so I would ideally like to cross-compile with the same version. Where can I get the GCC 4.7.0 toolchain for ARM? (I will be compiling on x86 CentOS 6.2)

**Edit:**

I deallocated unneeded GPU memory and set up a 4GB swap partition on a USB drive, while build files are on a NFS share. Boost is now compiling much much faster, so it is manageable. I would still like to know how can I set up a GCC 4.7 toolchain for cross compilation on my x86 PC though, since I intend to do a lot of compiling and I would like it to be as fast as possible.

**Edit 2:**

Since GCC 4.7.0 is relatively new, there does not seem to be a pre-built cross-compiler (i386-&gt;ARM). I will probably have to build one myself, which seems an non-trivial task (I&#39;ve tried and failed). Does anyone know of a tutorial to follow for building a GCC cross-compiler, hopefully for one of the recent versions?  

I&#39;ve tried with [this great shell script][1] (which worked great for building a same-arch compiler) and I&#39;ve successfully built `binutils` and GCC&#39;s prerequisites, but then GCC build kept failing with many cryptic errors. I am really lost here, so I would greatly appreciate your help.

GCC on Raspberry Pi was configured with

    --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib 
    --mandir=/usr/share/man --infodir=/usr/share/info 
    --with-bugurl=https://bugs.archlinux.org/ 
    --enable-languages=c,c++,fortran,lto,objc,obj-c++ --enable-shared 
    --enable-threads=posix --with-system-zlib --enable-__cxa_atexit 
    --disable-libunwind-exceptions --enable-clocale=gnu 
    --disable-libstdcxx-pch --enable-libstdcxx-time 
    --enable-gnu-unique-object --enable-linker-build-id --with-ppl 
    --enable-cloog-backend=isl --enable-lto --enable-gold 
    --enable-ld=default --enable-plugin --with-plugin-ld=ld.gold 
    --with-linker-hash-style=gnu --disable-multilib --disable-libssp 
    --disable-build-with-cxx --disable-build-poststage1-with-cxx 
    --enable-checking=release --host=arm-unknown-linux-gnueabi 
    --build=arm-unknown-linux-gnueabi 

**Edit 3:**

I managed to build a 4.7 GCC toolchain for ARM (yay!) using [this shell script][2] as suggested by user dwelch in the comments. I also built `newlib` and `libstdc++` using [this article][3] as a guide. The toolchain works fine, but hen I run the executable on my Raspberry Pi, it fails with `Illegal instruction`. What could be the cause of that? 


  [1]: http://joelinoff.com/blog/?p=658
  [2]: http://github.com/dwelch67/raspberrypi
  [3]: http://wiki.osdev.org/OS_Specific_Toolchain

Cross-compilation for Raspberry Pi in GCC. Where to start?

Consider the following scenario:

- Shared Library libA.so ,with no dependencies.
- Shared Library libB.so, with libA.so as its dependency.

I want to compile a binary file that links with the libB.
Should I link the binary with libB only or with libA either?

Is there any way to link only with the direct dependencies, letting the resolution of unresolved symbols from the dependencies for runtime?

I&#39;m worried about the fact that the library libB implementation may change in the future, introducing other dependencies (libC, libD, libE for instance). Am I going to have problems with that?

In other words:

- libA files: a.cpp a.h
- libB files: b.cpp b.h
- main program files: main.cpp

Of course, b.cpp includes a.h and main.cpp includes b.h.

Compilation commands:

    g++ -fPIC a.cpp -c
    g++ -shared -o libA.so a.o
    
    g++ -fPIC b.cpp -c -I.
    g++ -shared -o libB.so b.o -L. -lA

Which of the bellow options should I use?

    g++ main.cpp -o main -I. -L. -lB

or

    g++ main.cpp -o main -I. -L. -lB -lA

I couldn&#39;t use the first option. The linker complains about the unresolved symbols from the library libA. But it sound a little strange to me.

Thanks very much.


**-- Updated comments:**


When I link the binary, the linker will try to resolve all symbols from the main and the libB. However, libB has undefined symbols from the libA. That&#39;s why the linker complains about that.

That&#39;s why I need to link with the libA too.
However I found a way to ignore unresolved symbols from shared libraries.
Looks like I should use the following command line to do that:

    g++ main.cpp -o main -I. -L. -lB -Wl,-unresolved-symbols=ignore-in-shared-libs

Looks like it is still possible to use the `-rpath` option.
However I need to understand it a little better.

Does anyone knows any possible pitfalls when using the `-Wl,-unresolved-symbols=ignore-in-shared-libs` option?


**-- Updated comments 2:**


`-rpath` should not be used for this purpose. It is useful to force a library to be found in a given directory. The `-unresolved-symbol` approach looks much better.


Thanks again.

Linking with dynamic library with dependencies

I&#39;m not sure if the following code can cause redundant calculations, or is it compiler-specific?

    for (int i = 0; i &lt; strlen(ss); ++i)
    {
        // blabla
    }

Will `strlen()` be calculated every time when `i` increases?

Will strlen be calculated multiple times if used in a loop condition?

Does GDB have a built in scripting mechanism, should I code up an expect script, or is there an even better solution out there?  

I&#39;ll be sending the same sequence of commands every time and I&#39;ll be saving the output of each command to a file (most likely using GDB&#39;s built-in logging mechanism, unless someone has a better idea).

What are the best ways to automate a GDB debugging session?

I&#39;m debugging an application, and I get a lot of messages like this:

    [New Thread 0x7fffde152700 (LWP 11142)]
    [Thread 0x7fffde152700 (LWP 11142) exited]
    [New Thread 0x7fff967fa700 (LWP 11143)]
    [Thread 0x7fff967fa700 (LWP 11143) exited]
    [New Thread 0x7fff967fa700 (LWP 11144)]
    [Thread 0x7fff967fa700 (LWP 11144) exited]
    [New Thread 0x7fff967fa700 (LWP 11145)]
    [Thread 0x7fff967fa700 (LWP 11145) exited]
    [New Thread 0x7fffde152700 (LWP 11146)]
    [Thread 0x7fffde152700 (LWP 11146) exited]
    [New Thread 0x7fffde152700 (LWP 11147)]
    [Thread 0x7fffde152700 (LWP 11147) exited]
    [New Thread 0x7fffde152700 (LWP 11148)]
    [Thread 0x7fffde152700 (LWP 11148) exited]
    [New Thread 0x7fffde152700 (LWP 11149)]
    [Thread 0x7fffde152700 (LWP 11149) exited]
    [New Thread 0x7fffde152700 (LWP 11150)]
    [Thread 0x7fffde152700 (LWP 11150) exited]
    [New Thread 0x7fffde152700 (LWP 11151)]
    [Thread 0x7fffde152700 (LWP 11151) exited]
    [New Thread 0x7fffde152700 (LWP 11152)]
    [Thread 0x7fffde152700 (LWP 11152) exited]
    [New Thread 0x7fffde152700 (LWP 11153)]
    [Thread 0x7fffde152700 (LWP 11153) exited]
    [New Thread 0x7fffde152700 (LWP 11154)]
    [Thread 0x7fffde152700 (LWP 11154) exited]
    [New Thread 0x7fff967fa700 (LWP 11155)]
    [Thread 0x7fff967fa700 (LWP 11155) exited]
    [New Thread 0x7fffde152700 (LWP 11156)]
    [Thread 0x7fffde152700 (LWP 11156) exited]
    [New Thread 0x7fffde152700 (LWP 11157)]
    [Thread 0x7fffde152700 (LWP 11157) exited]

I&#39;m almost never interested in this messages. They make it much harder to read the rest of the output. They can change the timing behaviour of the program, making it hard to reproduce and debug race hazards. How can I disable these messages?

How can I disable new thread/thread exited messages in gdb?

My app runs fine but gdb fails to debug it with the following error

    (gdb) run
    Starting program: /path/to/app 
    Unable to find Mach task port for process-id 83767: (os/kern) failure (0x5).

I am on OS X Lion. GDB version is

    $ gdb --version
    GNU gdb 6.3.50-20050815 (Apple version gdb-1752) (Sat Jan 28 03:02:46 UTC 2012)
    Copyright 2004 Free Software Foundation, Inc.
    GDB is free software, covered by the GNU General Public License, and you are
    welcome to change it and/or distribute copies of it under certain conditions.
    Type &quot;show copying&quot; to see the conditions.
    There is absolutely no warranty for GDB.  Type &quot;show warranty&quot; for details.
    This GDB was configured as &quot;x86_64-apple-darwin&quot;.


gdb fails with &quot;Unable to find Mach task port for process-id&quot; error

I&#39;ve followed the instructions [on the GDB wiki][1] to install the python pretty-printers for viewing STL containers. My `~/.gdbinit` now looks like this:

    python 
    import sys 
    sys.path.insert(0, &#39;/opt/gdb_prettyprint/python&#39;) 
    from libstdcxx.v6.printers import register_libstdcxx_printers 
    register_libstdcxx_printers (None) 
    end 

However, when I run GDB and attempt to print an STL type, I get the following:

    print myString
    Python Exception &lt;class &#39;gdb.error&#39;&gt; No type named std::basic_string&lt;char&gt;::_Rep.: 
    $3 = 

  [1]: http://sourceware.org/gdb/wiki/STLSupport

Can anyone shed some light on this? I&#39;m running Ubuntu 12.04, which comes with GDB 7.4.


How to pretty-print STL containers in GDB?

Is it possible to clear the command window of gdb? I mean, is there a command in gdb that do the same (for the command windows) as the `clear` command in a bash terminal?

How clear gdb command screen?

In the [x86-64 Tour of Intel Manuals](http://x86asm.net/articles/x86-64-tour-of-intel-manuals/), I read

&gt; Perhaps the most surprising fact is that an instruction such as `MOV EAX, EBX` automatically zeroes upper 32 bits of `RAX` register.

The Intel documentation (3.4.1.1 General-Purpose Registers in 64-Bit Mode in manual Basic Architecture) quoted at the same source tells us:

&gt; * 64-bit operands generate a 64-bit result in the destination general-purpose register.
&gt; * 32-bit operands generate a 32-bit result, zero-extended to a 64-bit result in the destination general-purpose register.
&gt; * 8-bit and 16-bit operands generate an 8-bit or 16-bit result. The upper 56 bits or 48 bits (respectively) of the destination general-purpose register are not be modified by the operation. If the result of an 8-bit or 16-bit operation is intended for 64-bit address calculation, explicitly sign-extend the register to the full 64-bits.

In x86-32 and x86-64 assembly, 16 bit instructions such as

    mov ax, bx

don&#39;t show this kind of &quot;strange&quot; behaviour that the upper word of eax is zeroed.

Thus: what is the reason why this behaviour was introduced? At a first glance it seems illogical (but the reason might be that I am used to the quirks of x86-32 assembly).

Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?

I was on Microsoft&#39;s website and noticed two different installers, one for x64 and one for IA-64. Reference:[Installing the .NET Framework 4.5, 4.5.1](http://msdn.microsoft.com/en-us/library/5a4x27ek.aspx)

My understanding is that IA-64 is a subclass of x64, so I&#39;m curious why it would have a separate installer.


What is the difference between x64 and IA-64?

I am trying to create a dumb version of a spin lock. Browsing the web, I came across a assembly instruction called &quot;PAUSE&quot; in x86 which is used to give hint to a processor that a spin-lock is currently running on this CPU. The intel manual and other information available state that 

&gt; The processor uses this hint to avoid the memory order violation in
&gt; most situations, which greatly improves processor performance. For
&gt; this reason, it is recommended that a PAUSE instruction be placed in
&gt; all spin-wait loops. The documentation also mentions that  &quot;wait(some
&gt; delay)&quot; is the pseudo implementation of the instruction.

The last line of the above paragraph is intuitive. If I am unsuccessful in grabbing the lock, I must wait for some time before grabbing the lock again.

However, what do we mean by memory order violation in case of a spin lock? 
Does &quot;memory order violation&quot; mean the incorrect *speculative* load/store of the instructions after spin-lock?

The spin-lock question has been asked on Stack overflow before but the memory order violation question remains unanswered (at-least for my understanding).

 

What is the purpose of the &quot;PAUSE&quot; instruction in x86?

I want a 128 bit integer because I want to store results of multiplication of two 64 bit numbers. Is there any such thing in gcc 4.4 and above? 

Content Type	Original Author	Original Content on Stackoverflow
Question	pythonic	View Question on Stackoverflow
Solution 1 - C	ks1322	View Answer on Stackoverflow

Ask GDB to list all functions in a program

C Problem Overview

C Solutions

Solution 1 - C

in Ipython notebook / Jupyter, Pandas is not displaying the graph I try to plot

Forcing bash to expand variables in a string loaded from a file

Attributions