What's exactly a source file for the C11 standard

Question

I've a doubt about a sentence in this paragraph of the standard C11:

5.1.1.1 Program structure

A C program need not all be translated at the same time. The text of the program is kept in units called source files, (or preprocessing files) in this International Standard. A source file together with all the headers and source files included via the preprocessing directive #include is known as a preprocessing translation unit.

At first it says that the text of a program is kept in units called source files. After it says that a source file together with headers and source files are said to be preprocessing translation unit.

So is a source file a different entity respect to a header file or a source file include both?

Answer 1

The term source file (or preprocessing file) reflects to both .c source file and .h header. The C Standard does not differentiate them as such.

It is explicitely mentioned in K. N. King's "C Programming: A Modern Approach" book (section 15.2).

Answer 2

A source file is any text file (or other implementation-specific entity, per Basile's answer) containing source code. This specifically includes both .c and .h files. It might also includes files with other suffixes, like preprocessor or generated-code files.

When you take some source file actually passed to the compiler (so usually only .c files), that file together with (the transitive closure of) everything it #includes forms a translation unit.

So: every translation unit is built of source files.

But: not every source file is the starting point of a translation unit.

Answer 3

Pedantically, a source file (in the sense of C99 or C11 standard) is not even necessarily a file (in the operating system sense). A conforming implementation could read the "source file" from a database or some constant string inside some program.

^{A proprietary IBM compiler in the 1990s (VisualAge???) is rumored to have obtained the source to compile from some database. Today, tinycc also provides a libtcc library with a function tcc_compile_string to compile a string. TinyCC is compiling very quickly into slow machine code.}

Practically, there are often several source files (like .h & .c files) which are processed within the same compilation or translation unit.

^{IIUC, the funny thing about the C standards is that they don't suppose a filesystem for the implementation to have. I believe the standard don't even require a computer; you might implement the standard unethically using a bunch of human slaves, but that is immoral and inefficient.}