Design of APR 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. 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. The first version of APR will focus on what Apache 2.0 needs. Of course, anything that is submitted will be considered for inclusion. This document will discuss the structure of APR, and how best to contribute code to the effort. APR On Windows APR on Windows is different from APR on all other systems, because it doesn'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, apr_private.h and apr.h are created from apr_private.hw and apr.hw respectively. !!!*** If you add code to acconfig.h or tests to configure.in or aclocal.m4, please give some thought to whether or not Windows needs this addition as well. A general rule of thumb, is that if it is a feature macro, such as APR_HAS_THREADS, Windows needs it. 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*.hw, 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. ***!!! APR Features 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. 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: APR_HAS_FEATURE 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. 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. APR types The base types in APR file_io File I/O, including pipes lib A portable library originally used in Apache. This contains memory management, tables, and arrays. locks Mutex and reader/writer locks misc Any APR type which doesn't have any other place to belong network_io Network I/O shmem Shared Memory (Not currently implemented) signal Asynchronous Signals threadproc Threads and Processes time Time Directory Structure 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. 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: apr | -> file_io | -> unix The Unix and common base code | -> win32 The Windows code | -> os2 The OS/2 code Obviously, BeOS does not have a directory. This is because BeOS is currently using the Unix directory for it's file_io. In the near future, it will be possible to use individual files from the Unix directory. There are a few special top level directories. These are test, inc, include, and libs. 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. Inc is a directory for internal header files. This directory is likely to go away soon. Include is a directory which stores all required APR header files for external use. The distinction between internal and external header files will be made soon. Finally, libs is a generated directory. When APR finishes building, it will store it's library files in the libs directory. Creating an APR Type The current design of APR requires that 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. 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: struct ap_file_t { apr_pool_t *cntxt; int filedes; FILE *filehand; ... } In include/apr_file_io.h: typedef struct ap_file_t ap_file_t; 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. The only 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. You may notice the apr_pool_t field. Most APR types have this field. This type is used to allocate memory within APR. Any APR type that has this field should place this field first. If it is important to retrieve the pool from an APR variable, it is possible to use the macro APR_GET_POOL to accomplish this. This macro will only work on types that actually have a pool in them as the first field. On any other type, this macro will cause a seg fault as soon as the pool is used. New Function When creating a new function, please try to adhere to these rules. 1) Result arguments should be the first arguments. 2) If a function needs a context, it should be the last argument. 3) These rules are flexible, especially if it makes the code easier to understand because it mimics a standard function. Documentation 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. The format for the comment block is: /** * 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 */ The last line is not strictly needed. The parser in ScanDoc is not perfect yet, and it can not parse prototypes that are in any form other than return_type program_name(type1 param1, type2 param2, ...) This means that any function prototype that resembles: APR_DECLARE(ap_status_t) ap_foo(int f1, char *f2) will need the deffunc. For an actual example, look at any file in the include directory (ap_tables.h hasn't been done yet). APR Error reporting 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. :-) 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. 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. (Canonical error values are also defined in this section. [Canonical error values are discussed later]). 2) APR_OS_START_STATUS This is platform dependent, and is the offset at which APR status values start. 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. 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: if (open(fname, oflags, 0777) < 0) return errno; 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: if (CreateFile(fname, oflags, sharemod, NULL, createflags, attributes, 0) == INVALID_HANDLE_VALUE return (GetLAstError() + APR_OS_START_SYSERR); 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. 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 are self explanatory. 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. To programmatically correct an error in a running application, the error codes need to be consistent across platforms. This should make sense. To get consistent error codes, APR provides a function ap_canonical_error(). This function will take as input any ap_status_t value, and return a small subset of canonical APR error codes. These codes will be equivalent to Unix errno's. Why is it a small subset? Because we don't want to try to convert everything in the first pass. As more programs require that more error codes are converted, they will be added to this function. Why did APR take this approach? There are two ways to deal with error codes portably. 1) return the same error code across all platforms. 2) return platform specific error codes and convert them when necessary. 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: 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 By keeping the errors platform specific, we can output error strings in two steps. make syscall that fails return error code check for success call error output function step 1 output error string step 2 Less often, programs change their execution based on what error was returned. This is no more expensive using option 2 and it is using option 1, but we put the onus of converting the error code on the programmer themselves. For example, using option 1: make syscall that fails convert to common error code return common error code decide execution based on common error code Using option 2: make syscall that fails return error code convert to common error code (using ap_canonical_error) decide execution based on common error code 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(). On all platforms ap_strerror takes the form: char *ap_strerror(ap_status_t err) { if (err < APR_OS_START_ERRNO2) return (platform dependent error string generator) if (err < APR_OS_START_ERROR) return (platform dependent error string generator for supplemental error values) if (err < APR_OS_SYSERR) return (APR generated error or status string) if (err == 0) return "No error was found" else return "APR doesn't understand this error value" } Notice, this does not handle canonicalized error values well. Those will return "APR doesn't understand this error value" on some platforms and an actual error string on others. To deal with this, just get the string before canonicalizing your error code. The other 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.