#pragma alloca 是什么意思

Question

The below is a code segment from the GNU Make software.These lines are taken from make.h.Can you please tell me what does #pragma alloca means.Does #if define (_AIX) means that the code #pragma alloca should run only of AIX is defined. The version of make is 3.75

/* AIX requires this to be the first thing in the file.  */

#if defined (_AIX) && !defined (__GNUC__)
 #pragma alloca
#endif

Please give me a detailed explanation.I am trying to read and learn GNU make.Where can can i find a detailed article of how GNU make works internally.

#Extra

It would be helpful if you can point me on how to read codes (of any opensource software) and learn.Its seems really a tough task to learn from code.Any help would be highly appreciated.

Answer 1

Can you please tell me what does #pragma alloca means

It (apparently, I just googled aix pragma alloca ) means that on AIX systems, the system C compiler will provide alloca() .

Does #if define (_AIX) means that the code #pragma alloca should run only of AIX is defined?

Yes, it does. If the _AIX preprocessor symbol is set, and the __GNUC__ symbol isn't, then the code is run.

__GNUC__ would be set when using GCC, and #pragma alloca isn't a thing on GCC.

I am trying to read and learn GNU make.Where can can i find a detailed article of how GNU make works internally. It would be helpful if you can point me on how to read codes (of any opensource software) and learn.Its seems really a tough task to learn from code.Any help would be highly appreciated.

You will want to choose a simpler program than Make to start with. How about busybox's implementation of ls ?

In general, though, you'll probably want to learn how to skim programs – for instance, these three AIX-related lines are just a small detail and you're getting caught up in the weeds, so to speak.