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diff --git a/docs/APRDesign.html b/docs/APRDesign.html new file mode 100644 index 000000000..7d1caeb8d --- /dev/null +++ b/docs/APRDesign.html @@ -0,0 +1,399 @@ +<HTML> +<HEAD><TITLE>APR Design Document</TITLE></HEAD> +<BODY> +<h1>Design of APR</h1> + +<p>The Apache Portable Run-time libraries have been designed to provide a common +interface to low level routines across any platform. The original goal of APR +was to combine all code in Apache to one common code base. This is not the +correct approach however, so the goal of APR has changed. There are places +where common code is not a good thing. For example, how to map requests +to either threads or processes should be platform specific. APR's place +is now to combine any code that can be safely combined without sacrificing +performance.</p> + +<p>To this end we have created a set of operations that are required for cross +platform development. There may be other types that are desired and those +will be implemented in the future.</p> + +<p>This document will discuss the structure of APR, and how best to contribute +code to the effort.</p> + +<h2>APR On Windows and Netware</h2> + +<p>APR on Windows and Netware is different from APR on all other systems, +because those platforms don't use autoconf. On Unix, apr_private.h (private to +APR) and apr.h (public, used by applications that use APR) are generated by +autoconf from acconfig.h and apr.h.in respectively. On Windows (and Netware), +apr_private.h and apr.h are created from apr_private.hw (apr_private.hwn) +and apr.hw (apr.hwn) respectively.</p> + +<p> <strong> + If you add code to acconfig.h or tests to configure.in or aclocal.m4, + please give some thought to whether or not Windows and Netware need + these additions as well. A general rule of thumb, is that if it is + a feature macro, such as APR_HAS_THREADS, Windows and Netware need it. + In other words, if the definition is going to be used in a public APR + header file, such as apr_general.h, Windows needs it. + + The only time it is safe to add a macro or test without also adding + the macro to apr*.h[n]w, is if the macro tells APR how to build. For + example, a test for a header file does not need to be added to Windows. +</strong></p> + +<h2>APR Features</h2> + +<p>One of the goals of APR is to provide a common set of features across all +platforms. This is an admirable goal, it is also not realistic. We cannot +expect to be able to implement ALL features on ALL platforms. So we are +going to do the next best thing. Provide a common interface to ALL APR +features on MOST platforms.</p> + +<p>APR developers should create FEATURE MACROS for any feature that is not +available on ALL platforms. This should be a simple definition which has +the form:</p> + +<code>APR_HAS_FEATURE</code> + +<p>This macro should evaluate to true if APR has this feature on this platform. +For example, Linux and Windows have mmap'ed files, and APR is providing an +interface for mmapp'ing a file. On both Linux and Windows, APR_HAS_MMAP +should evaluate to one, and the ap_mmap_* functions should map files into +memory and return the appropriate status codes.</p> + +<p>If your OS of choice does not have mmap'ed files, APR_HAS_MMAP should +evaluate to zero, and all ap_mmap_* functions should not be defined. The +second step is a precaution that will allow us to break at compile time if a +programmer tries to use unsupported functions.</p> + +<h2>APR types</h2> + +<p>The base types in APR</p> + +<ul> +<li>dso<br> + Shared library routines +<li>mmap<br> + Memory-mapped files +<li>poll<br> + Polling I/O +<li>time<br> + Time +<li>user<br> + Users and groups +<li>locks<br> + Process and thread locks (critical sections) +<li>shmem<br> + Shared memory +<li>file_io<br> + File I/O, including pipes +<li>atomic<br> + Atomic integer operations +<li>strings<br> + String handling routines +<li>memory<br> + Pool-based memory allocation +<li>passwd<br> + Reading passwords from the terminal +<li>tables<br> + Tables and hashes +<li>network_io<br> + Network I/O +<li>threadproc<br> + Threads and processes +<li>misc<br> + Any APR type which doesn't have any other place to belong. This + should be used sparingly. +<li>support<br> + Functions meant to be used across multiple APR types. This area + is for internal functions only. If a function is exposed, it should + not be put here. +</ul> + +<h2>Directory Structure</h2> + +<p>Each type has a base directory. Inside this base directory, are +subdirectories, which contain the actual code. These subdirectories are named +after the platforms the are compiled on. Unix is also used as a common +directory. If the code you are writing is POSIX based, you should look at the +code in the unix directory. A good rule of thumb, is that if more than half +your code needs to be ifdef'ed out, and the structures required for your code +are substantively different from the POSIX code, you should create a new +directory.</p> + +<p>Currently, the APR code is written for Unix, BeOS, Windows, and OS/2. An +example of the directory structure is the file I/O directory:</p> + +<pre> +apr + | + -> file_io + | + -> unix The Unix and common base code + | + -> win32 The Windows code + | + -> os2 The OS/2 code +</pre> + +<p>Obviously, BeOS does not have a directory. This is because BeOS is currently +using the Unix directory for it's file_io.</p> + +<p>There are a few special top level directories. These are test and include. +Test is a directory which stores all test programs. It is expected +that if a new type is developed, there will also be a new test program, to +help people port this new type to different platforms. A small document +describing how to create new tests that integrate with the test suite can be +found in the test/ directory. Include is a directory which stores all +required APR header files for external use.</p> + +<h2>Creating an APR Type</h2> + +<p>The current design of APR requires that most APR types be incomplete. +It is not possible to write flexible portable code if programs can access +the internals of APR types. This is because different platforms are +likely to define different native types. There are only two execptions to +this rule:</p> + +<ul> +<li>The first exception to this rule is if the type can only reasonably be +implemented one way. For example, time is a complete type because there +is only one reasonable time implementation. + +<li>The second exception to the incomplete type rule can be found in +apr_portable.h. This file defines the native types for each platform. +Using these types, it is possible to extract native types for any APR type.</p> +</ul> + +<p>For this reason, each platform defines a structure in their own directories. +Those structures are then typedef'ed in an external header file. For example +in file_io/unix/fileio.h:</p> + +<pre> + struct ap_file_t { + apr_pool_t *cntxt; + int filedes; + FILE *filehand; + ... + } +</pre> + +<p>In include/apr_file_io.h:</p> + </pre> + typedef struct ap_file_t ap_file_t; + </pre> + +<p> This will cause a compiler error if somebody tries to access the filedes +field in this structure. Windows does not have a filedes field, so obviously, +it is important that programs not be able to access these.</p> + +<p>You may notice the apr_pool_t field. Most APR types have this field. This +type is used to allocate memory within APR. Because every APR type has a pool, +any APR function can allocate memory if it needs to. This is very important +and it is one of the reasons that APR works. If you create a new type, you +must add a pool to it. If you do not, then all functions that operate on that +type will need a pool argument.</p> + +<h2>New Function</h2> + +<p>When creating a new function, please try to adhere to these rules.</p> + +<ul> +<li> Result arguments should be the first arguments. +<li> If a function needs a pool, it should be the last argument. +<li> These rules are flexible, especially if it makes the code easier + to understand because it mimics a standard function. +</ul> + +<h2>Documentation</h2> + +<p>Whenever a new function is added to APR, it MUST be documented. New +functions will not be committed unless there are docs to go along with them. +The documentation should be a comment block above the function in the header +file.</p> + +<p>The format for the comment block is:</p> + +<pre> + /** + * Brief description of the function + * @param parma_1_name explanation + * @param parma_2_name explanation + * @param parma_n_name explanation + * @tip Any extra information people should know. + * @deffunc function prototype if required + */ +</pre> + +<p>For an actual example, look at any file in the include directory. The +reason the docs are in the header files is to ensure that the docs always +reflect the current code. If you change paramters or return values for a +function, please be sure to update the documentation.</p> + +<h2>APR Error reporting</h2> + +<p>Most APR functions should return an ap_status_t type. The only time an +APR function does not return an ap_status_t is if it absolutely CAN NOT +fail. Examples of this would be filling out an array when you know you are +not beyond the array's range. If it cannot fail on your platform, but it +could conceivably fail on another platform, it should return an ap_status_t. +Unless you are sure, return an ap_status_t.</p> + +<strong> + This includes functions that return TRUE/FALSE values. How that + is handled is discussed below +</strong> + +<p>All platforms return errno values unchanged. Each platform can also have +one system error type, which can be returned after an offset is added. +There are five types of error values in APR, each with it's own offset.</p> + +<!-- This should be turned into a table, but I am lazy today --> +<pre> + Name Purpose +0) This is 0 for all platforms and isn't really defined + anywhere, but it is the offset for errno values. + (This has no name because it isn't actually defined, + but for completeness we are discussing it here). + +1) APR_OS_START_ERROR This is platform dependent, and is the offset at which + APR errors start to be defined. Error values are + defined as anything which caused the APR function to + fail. APR errors in this range should be named + APR_E* (i.e. APR_ENOSOCKET) + +2) APR_OS_START_STATUS This is platform dependent, and is the offset at which + APR status values start. Status values do not indicate + success or failure, and should be returned if + APR_SUCCESS does not make sense. APR status codes in + this range should be name APR_* (i.e. APR_DETACH) + +4) APR_OS_START_USEERR This is platform dependent, and is the offset at which + APR apps can begin to add their own error codes. + +3) APR_OS_START_SYSERR This is platform dependent, and is the offset at which + system error values begin. +</pre> + +<strong>The difference in naming between APR_OS_START_ERROR and +APR_OS_START_STATUS mentioned above allows programmers to easily determine if +the error code indicates an error condition or a status codition.</strong> + +<p>If your function has multiple return codes that all indicate success, but +with different results, or if your function can only return PASS/FAIL, you +should still return an apr_status_t. In the first case, define one +APR status code for each return value, an example of this is +<code>apr_proc_wait</code>, which can only return APR_CHILDDONE, +APR_CHILDNOTDONE, or an error code. In the second case, please return +APR_SUCCESS for PASS, and define a new APR status code for failure, an +example of this is <code>apr_compare_users</code>, which can only return +APR_SUCCESS, APR_EMISMATCH, or an error code.</p> + +<p>All of these definitions can be found in apr_errno.h for all platforms. When +an error occurs in an APR function, the function must return an error code. +If the error occurred in a system call and that system call uses errno to +report an error, then the code is returned unchanged. For example: </p> + +<pre> + if (open(fname, oflags, 0777) < 0) + return errno; +</pre> + +<p>The next place an error can occur is a system call that uses some error value +other than the primary error value on a platform. This can also be handled +by APR applications. For example:</p> + +<pre> + if (CreateFile(fname, oflags, sharemod, NULL, + createflags, attributes, 0) == INVALID_HANDLE_VALUE + return (GetLAstError() + APR_OS_START_SYSERR); +</pre> + +<p>These two examples implement the same function for two different platforms. +Obviously even if the underlying problem is the same on both platforms, this +will result in two different error codes being returned. This is OKAY, and +is correct for APR. APR relies on the fact that most of the time an error +occurs, the program logs the error and continues, it does not try to +programatically solve the problem. This does not mean we have not provided +support for programmatically solving the problem, it just isn't the default +case. We'll get to how this problem is solved in a little while.</p> + +<p>If the error occurs in an APR function but it is not due to a system call, +but it is actually an APR error or just a status code from APR, then the +appropriate code should be returned. These codes are defined in apr_errno.h +and should be self explanatory.</p> + +<p>No APR code should ever return a code between APR_OS_START_USEERR and +APR_OS_START_SYSERR, those codes are reserved for APR applications.</p> + +<p>To programmatically correct an error in a running application, the error +codes need to be consistent across platforms. This should make sense. APR +has provided macros to test for status code equivalency. For example, to +determine if the code that you received from the APR function means EOF, you +would use the macro APR_STATUS_IS_EOF().</p> + +<p>Why did APR take this approach? There are two ways to deal with error +codes portably.</p> + +<ol type=1> +<li> Return the same error code across all platforms. +<li> Return platform specific error codes and convert them when necessary. +</ol> + +<p>The problem with option number one is that it takes time to convert error +codes to a common code, and most of the time programs want to just output +an error string. If we convert all errors to a common subset, we have four +steps to output an error string:</p> + +<p>The seocnd problem with option 1, is that it is a lossy conversion. For +example, Windows and OS/2 have a couple hundred error codes, but POSIX errno +only defines about 50 errno values. This means that if we convert to a +canonical error value immediately, there is no way for the programmer to +get the actual system error.</p> + +<pre> + make syscall that fails + convert to common error code step 1 + return common error code + check for success + call error output function step 2 + convert back to system error step 3 + output error string step 4 +</pre> + +<p>By keeping the errors platform specific, we can output error strings in two +steps.</p> + +<pre> + make syscall that fails + return error code + check for success + call error output function step 1 + output error string step 2 +</pre> + +<p>Less often, programs change their execution based on what error was returned. +This is no more expensive using option 2 than it is using option 1, but we +put the onus of converting the error code on the programmer themselves. +For example, using option 1:</p> + +<pre> + make syscall that fails + convert to common error code + return common error code + decide execution based on common error code +</pre> + +<p>Using option 2:</p> + +<pre> + make syscall that fails + return error code + convert to common error code (using ap_canonical_error) + decide execution based on common error code +</pre> + +<p>Finally, there is one more operation on error codes. You can get a string +that explains in human readable form what has happened. To do this using +APR, call ap_strerror().</p> + |