Detailed Information on What APPEND Does (28473)

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The APPEND command is meant to trick MS-DOS-based applications, telling them that files are not where they actually are. This is useful if you have an application that expects a file to be in the current directory and it is actually in some other subdirectory or drive. Programs like this usually use FCBs internally to manipulate files (which do not relate to subdirectories), or they have overlay or data files that are associated with the executable file and have no mechanism for letting you specify where these files are located (environment variables, command-line options, initialization files, installation programs that modify the executable, etc.). The most commonly used example of this is WordStar. Its WS.COM file looks for the file WS.OVL in the current directory. If you have WS.COM in your \BIN directory, WS.OVL stored in your \WS directory, and \BIN in your PATH, WordStar will not be able to find its overlay unless you are in the subdirectory \WS when you invoke WS.COM. Other programs can benefit from APPEND in similar ways.

APPEND tricks these programs by intercepting the following MS-DOS services: If you specify the /X option when you invoke APPEND, it will also intercept the following services: When a program issues one of the calls that APPEND is trapping by trying to open or find a file and the file does not exist, APPEND will look in its path (specified by you when you invoke APPEND, optionally stored in the environment table), and if it finds this file in any of the APPENDed locations, it will return a successful open/find, tricking the application into thinking that the file is in the current directory.

This works in our example, where WS.COM is looking for WS.OVL. However, APPEND does distort the system. APPEND is very useful if you have it pointing to a directory that contains only overlay and supplementary (read- only) files and template and skeleton documents, which you intend to use as a form for new files.

For example, you have the file MAKEFILE in many directories on your system, and you try to create this file in your current directory with your favorite editor (let's assume that you have APPEND pointing to one or more of the directories in which the file MAKEFILE already exists). When your editor invokes the MS-DOS system service HandleOpen for the file MAKEFILE in the current directory, it will not find it. However, then APPEND starts to work and distorts the view of the system and finds this file in another location, one of the directories in the APPEND path.

Your editor assumes this file exists in your current directory (let's also assume that you don't realize that this file shouldn't have existed in your current directory). When you edit the file and then save this file, it will be saved in the current directory, not in the directory that APPEND really got it from. Now, depending on what you're editing, this may or may not seem beneficial. In this case, it might have been useful to have a skeleton MAKEFILE to base your new one on. But let's say that instead of a MAKEFILE, you were editing your list of things to do, TODO.DOC. If you edit this file from many different places on your system (being in different subdirectories when you edit it each time), you will have many copies of this file, each one being different, which is most likely what you did NOT intend to do.

Application programmers who would like to know what is going on "underneath" APPEND have a few options. If the application is robust enough that it does not need APPEND's talents, you could recommend that APPEND not be used with the application. This is a good idea in most cases, since APPEND does distort the application's view of files, and you can run into problems.

Unfortunately, there is no MS-DOS system service designed to return the FULL file specification that APPEND returns, so if you use any of the MS- DOS services that are intercepted by APPEND, you have to gamble that these are in the current directory.

Many system-level programmers would like access to this. One method of gaining more insight into what the system is doing is to determine if APPEND is installed. This can be accomplished by using MS- DOS interrupt 2FH. The following is a code fragment written in C that demonstrates its usage:

\* kbon

    /* get install status of APPEND */ 

    int GetAppendStatus()
    {

        union REGS regs;  /* input/output registers */ 

        regs.h.al = 0x00;  /* get install status */ 
        regs.h.ah = 0xB7;  /* talking to the APPEND */ 
        int86(0x2F, &regs, &regs);  /* invoke interrupt 2FH */ 
        return (regs.h.al);  /* return status code */ 

    } /* GetAppendStatus */ 

\* kboff
				

This MS-DOS service has the following usage: The valid return codes for this service are as follows: Once you have determined if APPEND is installed, you can then obtain the APPEND path by looking for the MS-DOS environment variable APPEND. Unfortunately, if APPEND is invoked without the /E option, this data will not be stored in the MS-DOS environment. In this case, the only recourse for an application that requires this information would be to terminate, telling you that if APPEND is to be used, the /E option is required (this is not unreasonable, since this is the only method for an application to access the APPEND path). Then, once you have the value of this environment variable, you can look into the possible ways that APPEND is altering your view of the system.

In summary, APPEND is a very useful tool if you have programs that can't find their supplementary files. It is an annoyance to other programs, the degree of which depends on how much is pointed to by the APPEND path. Programs that are very critical can avoid possible problems that are raised by APPEND by checking for its existence and warning you of possible problems that may occur due to APPEND's system distortion.