Using C macro variable in #include

Question

I have C code file containing the following include:

#include MYHEADER

I know such include using variable is a bad practice but it is not my code, I just want to compile it. I want MYHEADER variable to have <some_header.h> value, rather than "some_header.h". I have tried may ways to pass it, so far none of them worked.

E.g. arm-none-eabi-gcc -c -DMYHEADER=<some.h> -o sample.o sample.c does not work, I always get error: #include expects "FILENAME" or \<FILENAME\>. Using ARM GCC compiler on Windows.

Please advise.

Answer 1

The message you get is produced by GCC if MYHEADER is defined to be empty (there are no tokens in its replacement list). If the command-line processor (also called a shell) you are using takes <some.h> to indicate file redirection, with < indicating to take input from some.h and > taking the -o that follows it to indicate writing to -o, then it leaves -DMYHEADER= with an empty replacement list.

If so, a fix may be to quote the switch:

arm-none-eabi-gcc -c "-DMYHEADER=<some.h>" -o sample.o sample.c 

Here are two tests for this, in addition to trying the above command:

• Check whether your directory contains a file named -o. If so, it was created by the indirection, confirming that is what is happening.

• Create a file named x.c, put MYHEADER (and nothing else) in it, and execute arm-none-eabi-gcc -E x.c -DMYHEADER=<some.h>. If the command-line processor complains about no file name following >, that confirms the hypothesis. If it executes the command, or if it executes the command after you add, say, foo after the >, the output will reveal what MYHEADER is replaced with. (The -E switch requests preprocessing, so the single line containing MYHEADER will be replaced by a blank line if -DMYHEADER= is passed to GCC.)

One problem with this hypothesis is the command-line processor should have complained it could not open some.h for the input redirection. But perhaps you have a file of that name in your directory, which would satisfy the command-line processor.

Another possibility is that the C standard does not fully specify how tokens resulting from macro replacement in an #include are processed. When <name.h> appears directly in an #include, it is processed by a special grammar token, an h-char-sequence. When macro replacement is used instead, the standard says, in C 2018 6.10.2 4:

… The method by which a sequence of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.

I do not expect this is the problem, as a test on Compiler Explorer suggests GCC processes a replacement of MYHEADER with <stdio.h> in an ordinary suitable way.

Answer 2

Adding a #define MYHEADER <some_header.h> before the #include MYHEADER might work. or If you have many like this, you could put all the "#define"s in a file.txt and #include that file.txt before using the #inclue MYHEADER