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| <a name="File-Handling"></a> |
| <h2 class="unnumbered">File Handling</h2> |
| <a name="index-files"></a> |
| |
| <p>Fairly obviously, the file handling code of cpplib resides in the file |
| <samp>files.c</samp>. It takes care of the details of file searching, |
| opening, reading and caching, for both the main source file and all the |
| headers it recursively includes. |
| </p> |
| <p>The basic strategy is to minimize the number of system calls. On many |
| systems, the basic <code>open ()</code> and <code>fstat ()</code> system calls can |
| be quite expensive. For every <code>#include</code>-d file, we need to try |
| all the directories in the search path until we find a match. Some |
| projects, such as glibc, pass twenty or thirty include paths on the |
| command line, so this can rapidly become time consuming. |
| </p> |
| <p>For a header file we have not encountered before we have little choice |
| but to do this. However, it is often the case that the same headers are |
| repeatedly included, and in these cases we try to avoid repeating the |
| filesystem queries whilst searching for the correct file. |
| </p> |
| <p>For each file we try to open, we store the constructed path in a splay |
| tree. This path first undergoes simplification by the function |
| <code>_cpp_simplify_pathname</code>. For example, |
| <samp>/usr/include/bits/../foo.h</samp> is simplified to |
| <samp>/usr/include/foo.h</samp> before we enter it in the splay tree and try |
| to <code>open ()</code> the file. CPP will then find subsequent uses of |
| <samp>foo.h</samp>, even as <samp>/usr/include/foo.h</samp>, in the splay tree and |
| save system calls. |
| </p> |
| <p>Further, it is likely the file contents have also been cached, saving a |
| <code>read ()</code> system call. We don’t bother caching the contents of |
| header files that are re-inclusion protected, and whose re-inclusion |
| macro is defined when we leave the header file for the first time. If |
| the host supports it, we try to map suitably large files into memory, |
| rather than reading them in directly. |
| </p> |
| <p>The include paths are internally stored on a null-terminated |
| singly-linked list, starting with the <code>"header.h"</code> directory search |
| chain, which then links into the <code><header.h></code> directory chain. |
| </p> |
| <p>Files included with the <code><foo.h></code> syntax start the lookup directly |
| in the second half of this chain. However, files included with the |
| <code>"foo.h"</code> syntax start at the beginning of the chain, but with one |
| extra directory prepended. This is the directory of the current file; |
| the one containing the <code>#include</code> directive. Prepending this |
| directory on a per-file basis is handled by the function |
| <code>search_from</code>. |
| </p> |
| <p>Note that a header included with a directory component, such as |
| <code>#include "mydir/foo.h"</code> and opened as |
| <samp>/usr/local/include/mydir/foo.h</samp>, will have the complete path minus |
| the basename ‘<samp>foo.h</samp>’ as the current directory. |
| </p> |
| <p>Enough information is stored in the splay tree that CPP can immediately |
| tell whether it can skip the header file because of the multiple include |
| optimization, whether the file didn’t exist or couldn’t be opened for |
| some reason, or whether the header was flagged not to be re-used, as it |
| is with the obsolete <code>#import</code> directive. |
| </p> |
| <p>For the benefit of MS-DOS filesystems with an 8.3 filename limitation, |
| CPP offers the ability to treat various include file names as aliases |
| for the real header files with shorter names. The map from one to the |
| other is found in a special file called ‘<samp>header.gcc</samp>’, stored in the |
| command line (or system) include directories to which the mapping |
| applies. This may be higher up the directory tree than the full path to |
| the file minus the base name. |
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