C Archives - Red Hat Developer

How to debug where a function returns using LLDB from the command line

By Konrad Kleine September 11, 2019September 10, 2019

I often find myself in a situation when I want to know where a function returns. There’s no need to know the return value, as this may be the same for multiple code paths (e.g., nullptr if something went wrong). It is embarrassing, but I sometimes have put fprintf(stderr, "T1"); in my code just to follow which path the execution took. Needless to say, this behavior requires manual editing and recompilation and should be avoided if possible.

Here’s a way to elegantly debug where a function returns using lldb from the command line.

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Efficient string copying and concatenation in C

By Martin Sebor August 12, 2019August 11, 2019

Among the most heavily used string handling functions declared in the standard C <string.h> header are those that copy and concatenate strings. Both sets of functions copy characters from one object to another, and both return their first argument: a pointer to the beginning of the destination object. The choice of the return value is a source of inefficiency that is the subject of this article.

The code examples shown in this article are for illustration only. They should not be viewed as recommended practice and may contain subtle bugs.

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How the GNU C Library handles backward compatibility

By DJ Delorie August 1, 2019July 29, 2019

One of the GNU C Library’s (glibc’s) unwritten rules is that a program built against an old version of glibc will continue to work against newer versions of glibc. But how does this work? What hidden magic lets you call the same function with different results, just based on when you built your program?

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How C array sizes become part of the binary interface of a library

By Florian Weimer May 6, 2019May 2, 2019

Most C compilers allow accessing an array declared extern, which has indeterminate bounds, like this:

extern int external_array[];

int
array_get (long int index)
{
  return external_array[index];
}

The definition of external_array could reside in a different translation unit and look like this:

int external_array[3] = { 1, 2, 3 };

The question is what happens if this separate definition is changed to this:

int external_array[4] = { 1, 2, 3, 4 };

Or this:

int external_array[2] = { 1, 2 };

Does either change preserve the binary interface (assuming that there is a mechanism that allows the application to determine the size of the array at run time)?

Curiously, the answer is that on many architectures, increasing the array size breaks binary interface (ABI) compatibility. Decreasing the array size may also cause compatibility problems. We’ll look more closely at ABI compatibility in this article and explain how to avoid problems.

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A platform interface for the GNU C Library

By Carlos O'Donell March 27, 2019March 27, 2019

Application developers continue to need newer versions of libraries, including core runtimes like GNU C Library (glibc), for their applications. In this article, I’ll look at some issues related to upgrading glibc in an operating system (OS) distribution, and I also encourage you to read Florian Weimer’s excellent blog post on the topic.

The problem

Deciding between a library rebase or continued backporting of commits involves a complex set of risks and rewards. For some customers and users, it is important not to rebase the library (ensuring the lowest risk of impact by change); but for others, the rebase brings valuable bug fixes (lowest risk of impact from known issues). In other cases, the newer library may perform better, even if the interfaces haven’t changed, because it can take advantage of newer hardware or a newer Linux kernel (performance advantage to first mover).

There is no way to simultaneously satisfy all the requirements of slow-moving versus fast-moving development. The recent work in Fedora Modularity is aimed at solving the root of this problem, but there is a limit to this work. The further down the stack you go, the harder the problem becomes. The potential for breakage further up the stack increases. You can’t always arbitrarily change a component’s installed version without consequences, either at build time or at runtime.

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Using .NET PInvoke for Linux system functions

By Tom Deseyn March 25, 2019September 3, 2019

If you’ve developed Windows applications with .NET, you may have found yourself in a situation where the framework did not provide the APIs you needed. When that happens, you first need to identify the system APIs and then make them available using PInvoke. A website like pinvoke.net provides copy-and-pasteable code snippets for many Win32 API functions.

.NET Platform Invoke (PInvoke) makes it easy to consume native libraries. In this article, we’ll take a look at using PInvoke for Linux system functions.

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What’s new in OpenMP 5.0

By Jakub Jelínek March 19, 2019March 18, 2019

A new version of the OpenMP standard, 5.0, was released in November 2018 and brings several new constructs to the users. OpenMP is an API consisting of compiler directives and library routines for high-level parallelism in C, C++, and Fortran programs. The upcoming version of GCC adds support for some parts of this newest version of the standard.

This article highlights some of the latest features, changes, and “gotchas” to look for in the OpenMP standard.

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RPM packaging: A simplified guide to creating your first RPM

By Mohammad Ahmad March 18, 2019March 15, 2019

The concept of RPM packaging can be overwhelming for first-timers because of the impression a steep learning curve is involved. In this article, I will demonstrate that building an RPM with minimal knowledge and experience is possible. Note that this article is meant as a starting point, not a complete guide to RPM packaging.

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A gentle introduction to jump threading optimizations

By Aldy Hernandez March 13, 2019March 11, 2019

As part of the GCC developers‘ on-demand range work for GCC 10, I’ve been playing with improving the backward jump threader so it can thread paths that are range-dependent. This, in turn, had me looking at the jump threader, which is a part of the compiler I’ve been carefully avoiding for years. If, like me, you’re curious about compiler optimizations, but are jump-threading-agnostic, perhaps you’ll be interested in this short introduction.

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Understanding GCC warnings, Part 2

By Martin Sebor March 13, 2019March 13, 2019

In part 1, I shed light on trade-offs involved in the GCC implementation choices for various types of front-end warnings, such as preprocessor warnings, lexical warnings, type-safety warnings, and other warnings.

As useful as front-end warnings are, those based on the flow of control or data through the program have rather inconvenient limitations. To overcome them, flow-based warnings have increasingly been implemented in what GCC calls the “middle end.” Middle-end warnings are the focus of this article.

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The problem