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Communications of the ACM


C Is Not a Low-Level Language

C Is Not a Low-Level Language, illustration

Credit: Bug Fish

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In the wake of the recent Meltdown and Spectre vulnerabilities, it is worth spending some time looking at root causes. Both of these vulnerabilities involved processors speculatively executing instructions past some kind of access check and allowing the attacker to observe the results via a side channel. The features that led to these vulnerabilities, along with several others, were added to let C programmers continue to believe they were programming in a low-level language, when this hasn't been the case for decades.

Processor vendors are not alone in this. Those of us working on C/C++ compilers have also participated.


Martin Sandberg

This article makes a common fundamental mistake - that language are really powerful. They are not. Let me use that analogy of modern machine tools to illustrate my point.

Assembler - a basic hand tool, a file, a chisel, a screwdriver
C - perhaps a drill press
C++ - A table saw with some attachments, such as a mortising jig

We're done. We haven't even reached the level of a multi-axis CNC machine.

Since languages aren't powerful, coders have to do the heavy lifting. Therefor, good and bad code can be written in ANY language. Most GPUs are coded in C with some special operators or library calls, after all. I've written C programs that spawn off threads by the thousands as required by the work load.

In C++ it is even easier. For example it's possible to make thread-safe blocks completely trivial. Here's a complete header file:

// ===========================================================================
// A simple way to get locking and unlocking a pthread mutex correct. Just
// put:
// threadLock lockThis(pointer to your pthread mutex)
// at the beginning of the exclusive section of the code and when it goes out
// of scope at the end of the section, the mutex will be unlocked by the
// destructor.
// ===========================================================================
// Modification History:
// 06/13/2011 - File Created (MCS & WLH)
// ***************************************<< o >>***************************************


class threadLock{
threadLock(pthread_mutex_t *theLock):myLock(theLock){pthread_mutex_lock(myLock);};
threadLock(pthread_mutex_t &theLock):myLock(&theLock){pthread_mutex_lock(myLock);};
threadLock(const threadLock& source):myLock(source.myLock){};
threadLock& operator =(const threadLock &rhs){
if(this == &rhs)
return *this;
myLock = rhs.myLock;
return *this;


pthread_mutex_t *myLock;

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