Calling external programs from c

If the two headers are header only i. That way, the symbols in those headers are only visible locally in the respective. If that is not the case, i. I realized that this was what we had done after I finished writing this. The last solution is really my preferred one through I am fine with the first. If I have learned one thing over the years, then its this: Give a developer the opportunity to create a bug by forgetting something about the system and he will do it.

Your C headers include C standard library headers, right? You just broke those too. But this will bite you eventually. I disagree. My C headers usually do not include C standard library headers or similar. Follow Post Reply. Brian Kerrick Nickel. There is probably a better solution, but this is the first thing that came to mind. Jeff Schwab. Chris Mantoulidis. Then you can pipe the output to your program or something.

Depending how you want to design your program, you have a few options. Popen is definitely one. It might make sense to let the gnuplot program come up separately, display your graph and stay there while the processing logic continues. Then you can spawn a process or a thread asynchronously to do that. Lookup how to use system APIs.

Matej Pivoluska. Figure illustrates the architecture of the multithreaded external procedure agent. User sessions 1 and 2 issue requests for callouts to functions in some DLLs. These requests get serviced through heterogeneous services to the multithreaded extproc agent. These requests get handled by the agent's dispatcher threads, which then pass them on to the task threads.

The task thread that is actually handling a request is responsible for loading the respective DLL and calling the function therein.

In this context, a DLL is any dynamically loadable operating-system file that stores external procedures. The value for this environment variable is a colon- separated : list of DLL names qualified with the complete path. For example:. The preferred method to set this environment variable is through the ENVS parameter in the file listener.

You find or write a new external C procedure, then add it to the DLL. When the procedure is in the DLL, you publish it using the call specification mechanism described in the following section. Oracle Database can only use external procedures that are published through a call specification, which maps names, parameter types, and return types for your Java class method or C external procedure to their SQL counterparts.

This new syntax, first introduced for Java class methods, has been extended to C procedures. Specifies a local alias library.

You cannot use a database link to specify a remote library. Therefore, if you enclose the name in double quotes, then it becomes case sensitive. By default, the name is stored in upper case.

Specifies the external C procedure to be called. If you enclose the procedure name in double quotes, then it becomes case sensitive. Specifies the third-generation language in which the external procedure was written.

If you omit this subclause, then the language name defaults to C. Specifies the calling standard under which the external procedure was compiled. The supported calling standard is C. If you omit this subclause, then the calling standard defaults to C. Specifies that a context pointer will be passed to the external procedure.

The context data structure is opaque to the external procedure but is available to service procedures called by the external procedure. Specifies the positions and datatypes of parameters passed to the external procedure. It can also specify parameter properties, such as current length and maximum length, and the preferred parameter passing method by value or by reference. Specifies which parameter holds the name of the agent process that should run this procedure. This is intended for situations where external procedure agents are run using multiple agent processes, to ensure robustness if the agent process of one external procedure fails.

You can pass the name of the agent process corresponding to the name of a database link , and if tnsnames. Both instances must be on the same host. When the agent name is specified this way, it overrides any agent name declared in the alias library.

If no agent name is specified, the default is the extproc agent on the same instance as the calling program. Libunits can be considered analogous to DLLs written, for example, in C--although they map one-to-one with Java classes, whereas DLLs can contain more than one procedure. If the Java method takes no parameters, then you must code an empty parameter list for it.

This is because the methods of many Java classes are called only from other Java classes, or take parameters for which there is no appropriate SQL type.

For both Java class methods and external C procedures, call specifications can be specified in any of the following locations:. Under Oracle Database version 8. Here are some examples showing some of the other locations. This maintains compatibility with the external procedures of Oracle Database version 8. See "Specifying Datatypes" for datatype mappings. Passing parameters to an external C procedure is complicated by several circumstances:.

In the following sections, you learn how to specify a parameter list that deals with these circumstances. The maximum number of parameters that you can pass to a C external procedure is However, if you pass float or double parameters by value, then the maximum is less than How much less depends on the number of such parameters and your operating system.

Authors of such code must be especially careful to guard against such errors. It has no way to verify independently that these types are the actual types expected by the external routine.

If the data types passed are not of the types expected by the external code, the results are undefined, and can easily include memory corruption or even crashing of the IDL program.

Failure to do so will result in undefined behavior. In particular, Fortran programmers are often unaware that Fortran code passes everything by reference, and that C code defaults to passing everything by value. A copy of the value of the argument is passed to the called routine. Any changes made to such a value by the called routine are local to that routine, and do not change the original value of the variable in the calling routine.

The machine address of the argument is passed to the called routine. Any changes made to such a value by the called routine are immediately visible to the caller, because both routines are actually modifying the same memory addresses.

Consult your compiler documentation for details. A calling convention defines the rules used for passing arguments between routines, and specifies such details as how arguments of different types are passed i. A stable and efficient calling convention is critical to the stability of an operating system, and can affect most aspects of the system:.

Microsoft Windows is unique among the platforms supported by IDL in that it has two distinct calling conventions in common use, whereas other systems define a single convention. On single-convention systems, the calling convention is unimportant to application programmers; it is of concern only to hardware designers, authors of compilers, and operating systems.

On a multiple convention system, application programmers need to ensure that their code is compiled to use the proper convention and that calls to that code use the same convention. The Microsoft Calling Conventions are described below. This default can be changed via compiler switches, declspec declarations, or pragmas. CDECL is able to call functions with variable numbers of arguments varargs functions because the caller knows the actual number of arguments passed at runtime, whereas STDCALL cannot call such functions.

This is because the STDARGS routine cannot know efficiently at compile time how many arguments it will be passed at runtime in these situations. IDL therefore has no way to know how your external code was compiled.

If IDL calls your code using the correct convention, it will work correctly. If it calls using the wrong convention, the results are undefined, including memory corruption and possible crashing of the IDL program. Either combination is bad, and can corrupt or kill the program. When the wrong calling convention is used, the process stack can become confused.