Is my understanding of C header files/gcc options related to these correct?-CodePudding

I am currently taking a class on C and I am baffled by gcc options a lot of the time, because the videos/documentation on the options are sparse and those that eixst are hard to understand(for idiots/non-technical majors like myself). Please consider the following scenario:

Lets say I have a header file, myHeader.h and main.c which would like to include myHeader.h. Assume they are in the same directory.

In main.c, I could write #include "myHeader.h". However, according to my professor the "" is permitted because gcc will check in the current directory for anything in the "". However, where I am lost is when it comes to how I could add myHeader.h to the gcc header file search path such that #include <myHeader.h> would work. I am wondering what gcc commands would work, and why they work in specific. I would love any references(that aren't super nerdy) to better understand this.

So far, I researched on stackoverflow and on google, and it said something about -Idir gcc command, where dir is the directory you would like to add to the header file search path, but I am confused as to why this works or how to actually implement it. Since the "path" to myHeader.h is CStuff/workspace/myHeader.h I attempted to do gcc -I/CStuff/workspace/myHeader.h but this didn't really work out. I really thought it would take that directory and add it to the header file search path, but it just gave me an error.

I am a very confused business major so please take it easy on me! I really would love a dumbed-down explanation or a reference to a source that is more "basic" and has more than 1-2 sentences of explanation(if possible).

CodePudding user response：

However, where I am lost is when it comes to how I could add myHeader.h to the gcc header file search path such that #include <myHeader.h> would work.

If you want to add search path directory for system headers, there is -isystem <path>. For ordinary headers — which your header very likely is — there is -I <path>. This supports relative and absolute path formats.

To see which search paths GCC is using, add -v to the options:

> gcc main.c -v ...
...
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/9/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
...

There are more special options, see Directory-Options in the GCC documentation, if it comes to search order, for instance.

To see the outcome of including, i.e. the C-preprosessed file, use -save-temps and have a look at the preprocessed file *.i for C, *.ii for C , *.s for assembly.

To also see built-in and explicit #define's values in preprocessed code, also add -H -g3.

CodePudding user response：

In main.c, I could write #include "myHeader.h". However, according to my professor the "" is permitted because gcc will check in the current directory for anything in the "". However, where I am lost is when it comes to how I could add myHeader.h to the gcc header file search path such that #include <myHeader.h> would work.

As a preliminary matter, you are already lost when you conceive the idea that you should want to use that form for the private headers associated with your program. The <header.h> form is conventionally and best reserved for use with system headers. That is, exactly those that do not accompany the code being compiled.

That does not moot the question, however. Sometimes one might want to amend the header search path to help the compiler find headers distributed with the source code and referenced via the "header.h" form, too.

I am wondering what gcc commands would work, and why they work in specific. I would love any references(that aren't super nerdy) to better understand this.

You will not earn friends or respect here by eschewing technical references, nor by casting aspersions on those who do read such references. Technical documents can be hard reading at first, but reading and understanding them is a skill that you will need to cultivate if you want to enjoy success as a programmer.

So far, I researched on stackoverflow and on google, and it said something about -Idir gcc command, where dir is the directory you would like to add to the header file search path,

Good start.

but I am confused as to why this works or how to actually implement it. Since the "path" to myHeader.h is CStuff/workspace/myHeader.h I attempted to do gcc -I/CStuff/workspace/myHeader.h but [...] it just gave me an error.

It is important to pay attention to details. You yourself wrote:

where dir is the directory you would like to add to the header file search path

(emphasis added). If /CStuff/workspace/myHeader.h is the header you want gcc to find, then that path is not the path of a directory. The directory is /CStuff/workspace, so -I/CStuff/workspace is a viable option.

Alternatively, for gcc runs in which the working directory is also /CStuff/workspace, you can refer to it by the name . (that is, use -I.). It is a general feature of Unix and Windows paths, not specific to gcc, that the . represents the current working directory.