Accessing struct using C library always NULL

Hi,

I am using the Matlab C interface to extract data out of a .mat file containing a 1x50 struct. I am able to do the following successfully:

• Pull structure using matGetNextVariableInfo. Also fills the char* with the name, which matches my struct variable
• Get the correct size of the struct by using mxGetNumberOfElements and mxGetNumberOfFields
• Get the correct first column name by using mxGetFieldNameByNumber
• Get the first field using mxGetField
• Retrieve the correct class name (in my case, a double) from mxGetClassName

After calling mxGetField, I retrieve a non-null pointer to a mxArray, I then try to call mxGetPr on this - but it always ends up with a NULL result. I also tried mxGetData to retrieve a void*, also NULL. I am able to verify that it is not empty by using mxIsEmpty. If I change the row index to one with an empty field, mxIsEmpty returns as expected. It seems like I have everything set up correctly, but nothing I do works for retrieving the actual field data.

Any help is appreciated. Thanks!

Answer 1

[James Tursa]

Typically, it is best just to post your code so we can see exactly what you are doing, instead of posting a description of your code for what you think it is doing.

That being said, the matGetNextVariableInfo( ) function just gets header information for the variable without getting the actual data of the variable. It is intended for you to quickly see what is there without loading potentially a very large variable. So seeing NULL behind a subsequent mxGetPr( ) or mxGetData( ) call would be expected. If you want to get the variable along with its data, you need to use matGetNextVariable( ) instead.

[Beig Serate]

Understood about the code - will definitely do that in the future. But like you said, using matGetNextVariableInfo() was my issue. Thank you for the quick reply and help!