Minor tools and tips for includes, defines, and names

• You can get all of the pre-defined values for your platform using

              gcc -E -dM - < /dev/null
          

  The -E option runs only the C preprocessor and stops. The input file is empty (/dev/null), and the -dM option generates a list of the #define directives. Numbers in the results can be in decimal or hex, and it is not always clear which is being used (e.g., does 1.17549435082228750797e-38F have a 2-digit decimal or 3-digit hex exponent?). The output varies from platform to platform, but should give a complete list of default defines. With gcc version 7.3.0, there are 347 values #defined this way.

• The program whatsincluded.c has nothing but an empty main() program and a #include for time.h. Compile it with the provided makefile and it will list all of the files that end up being included. This does not answer the question "which file defines time_t", but gives a smaller set of candidates to search. On my desktop, the command "make" generates the ouput

              gcc -M whatsincluded.c 
              whatsincluded.o: whatsincluded.c /usr/include/stdc-predef.h \
              /usr/include/time.h /usr/include/features.h \
              /usr/include/x86_64-linux-gnu/sys/cdefs.h \
              /usr/include/x86_64-linux-gnu/bits/wordsize.h \
              /usr/include/x86_64-linux-gnu/bits/long-double.h \
              /usr/include/x86_64-linux-gnu/gnu/stubs.h \
              /usr/include/x86_64-linux-gnu/gnu/stubs-64.h \
              /usr/lib/gcc/x86_64-linux-gnu/7/include/stddef.h \
              /usr/include/x86_64-linux-gnu/bits/time.h \
              /usr/include/x86_64-linux-gnu/bits/types.h \
              /usr/include/x86_64-linux-gnu/bits/typesizes.h \
              /usr/include/x86_64-linux-gnu/bits/types/clock_t.h \
              /usr/include/x86_64-linux-gnu/bits/types/time_t.h \
              /usr/include/x86_64-linux-gnu/bits/types/struct_tm.h \
              /usr/include/x86_64-linux-gnu/bits/types/struct_timespec.h \
              /usr/include/x86_64-linux-gnu/bits/types/clockid_t.h \
              /usr/include/x86_64-linux-gnu/bits/types/timer_t.h \
              /usr/include/x86_64-linux-gnu/bits/types/struct_itimerspec.h \
              /usr/include/x86_64-linux-gnu/bits/types/locale_t.h \
              /usr/include/x86_64-linux-gnu/bits/types/__locale_t.h
          

  Of course, edit the file whatsincluded.c to test other or additional include files.

• The file timetype.c has instructions in its comments on how to run it through the preprocessor alone to find what the type time_t ultimately is. Basically, use the -E option of gcc and then use grep to extract lines that include the string being searched for. As a measure of preprocessor insanity, with gcc 7.3.0 the 4 lines in timetype.c lead to 460 lines generated by the preprocessor. Admittedly only 155 of those lines are non-blank and non-comment, but that is still a lot getting spewed by a four-line code. Of those, 17 lines include the string "time_t".

• Common compilation error messages include things like
  1. "name 'thingy' not found"
  2. "undefined reference to `elapsedtime' "
  3. "function sqrt multiply defined"
  4. "function main not defined"
  In general, you need to track down where a name occurs and make sure the correct one is being linked in or included. Just because you named your function diff() and put it into a file named diff.C does not mean that is the name the compiler actually looks for. In partuclar C++ uses name-mangling, which means a perfectly reputable function named diff() gets turned into something monstrous like _GLOBAL__sub_I__Z4diffidf8timespecS_. This can be turned off by using a declaration like 'extern "C" {diff( ... )}'. To see what names are generated, use the strings command, e.g.,

              strings diff.o
          

  This extracts whatever can be interpreted as a printable string in the binary object file diff.o. A more useful tool is nm command, which will print which symbols are defined and which symbols are used in the file. To find which are defined in a file,

              nm diff.o | grep -w T
          

  The grep command looks for "T" as a word; "T" indicates that the symbol is defined in the file on which nm is run. (Actually, it means "The symbol is in the text (code) section" of the file). The letter "U" means that the file uses but does not define the corresponding name. In that case, some other file (object or library) must be linked in to provide the definition of the name. nm is particularly useful for library (.so) files, which can have many functions defined and you need to track down which .so file defines the function you need.

  When mixing C and Fortran codes, most compilers append an underscore to names to avoid issues from Fortran's case insensitivity, and using nm and strings will help you find where an underscore is needed in calling Fortran from C or vice-versa. So if your diff.c function actually defines diff_, then the code calling it may also need to invoke diff_(), not diff().