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.

This document will discuss the structure of APR, and how best to contribute code to the effort.

APR On Windows and Netware

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.

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.

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

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.

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.

Creating an APR Type

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 exceptions to this rule:

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.

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.

New Function

When creating a new function, please try to adhere to these rules.

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
     */

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 parameters or return values for a function, please be sure to update the documentation.

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.

This includes functions that return TRUE/FALSE values. How that is handled is discussed below

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 its 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.  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.
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 condition.

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 apr_proc_wait, 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 apr_compare_users, which can only return APR_SUCCESS, APR_EMISMATCH, or an error code.

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 should be 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. 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().

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:

The second 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.

    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 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:

    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().