LabVIEW
problem of create file
Hi liying,
I'm not sure I understand what you mean by "refurbish", so please let me know if misunderstood the question.
Do you want to keep appending to a file in the beginning of the period and then after a specified time (you say two days) start overwriting the file from the beginning? This is basically a circular buffer using a file as the memory. All you have to do is control where in the fle you want to write to. For that purpose, use the "pos mode" and "pos offset" inputs of the Write Fi;e function. As you can see below, the help for this function describes how to use these inputs:
Have fun!
I'm not sure I understand what you mean by "refurbish", so please let me know if misunderstood the question.
Do you want to keep appending to a file in the beginning of the period and then after a specified time (you say two days) start overwriting the file from the beginning? This is basically a circular buffer using a file as the memory. All you have to do is control where in the fle you want to write to. For that purpose, use the "pos mode" and "pos offset" inputs of the Write Fi;e function. As you can see below, the help for this function describes how to use these inputs:
Have fun!
Message Edited by Philip C. on 06-02-2005 09:53 PM
Hi Liying,
I think that you are halfway there!
It sounds like you are having a CONSTANT value connected to "pos mode" and "pos offset". What you need to do, is create some code that calculates "pos mode" and "pos offset" depending on how long you have been acquiring data. In the first two days, "pos mode" and "pos offset" should be "1 (end)" and "0". Then after two days, when you want to start overwriting the file from the start, "pos mode" and "pos offset" has to be changed to point to the beginning of the file. You can for example choose "0 (start)" and "0" and then increase "pos offset" so you don't keep overwriting the very beginning of the file. You will have to calculate "pos offset" by knowing how many bytes you write to the file at a time.
The Circular Buffer is simply a technique used when accessing a buffer. 'Circular' simply refers to the fact that when the end is reached, it continues from start again. It is basically the same functionality you are referring to - only difference is that you wish to use File I/O and not RAM.
Let me know if you have any questions, thanks.
I think that you are halfway there!
It sounds like you are having a CONSTANT value connected to "pos mode" and "pos offset". What you need to do, is create some code that calculates "pos mode" and "pos offset" depending on how long you have been acquiring data. In the first two days, "pos mode" and "pos offset" should be "1 (end)" and "0". Then after two days, when you want to start overwriting the file from the start, "pos mode" and "pos offset" has to be changed to point to the beginning of the file. You can for example choose "0 (start)" and "0" and then increase "pos offset" so you don't keep overwriting the very beginning of the file. You will have to calculate "pos offset" by knowing how many bytes you write to the file at a time.
The Circular Buffer is simply a technique used when accessing a buffer. 'Circular' simply refers to the fact that when the end is reached, it continues from start again. It is basically the same functionality you are referring to - only difference is that you wish to use File I/O and not RAM.
Let me know if you have any questions, thanks.
Hi liying,
To change the "pos mode" programatically, use a Case Structure or a Select function to pass on the correct choice of "pos mode". In the example below, we are writing to the file relative to the start or end, dependant on whether the Boolean value coming from left is TRUE or FALSE. What you have to do is create the code that determines when to pass a Boolean TRUE and when to pass FALSE.
To change the "pos mode" programatically, use a Case Structure or a Select function to pass on the correct choice of "pos mode". In the example below, we are writing to the file relative to the start or end, dependant on whether the Boolean value coming from left is TRUE or FALSE. What you have to do is create the code that determines when to pass a Boolean TRUE and when to pass FALSE.
Message Edited by Philip C. on 06-07-2005 02:28 AM
