# FW: Queries **URL:** \ **Category:** Hdf-forum archives\ **Created:** [June 4, 2008, 12:18pm UTC](https://forum.hdfgroup.org/t/fw-queries/1014 "2008-06-04T12:18:26Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![ramakrishnan.iyer](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/r/3e96dc/32.png) [@ramakrishnan.iyer](https://forum.hdfgroup.org/u/ramakrishnan.iyer)\ **Post date:** [June 4, 2008, 12:18pm UTC](https://forum.hdfgroup.org/t/fw-queries/1014/1 "2008-06-04T12:18:26Z")
FYI [hdf5\_test.cpp](https://hdf-discourse-1.s3.amazonaws.com/original/1X/92d64d07b9125608e59c24d3914ac27df7cdb15e.cpp) (3.65 KB) [matrix.h5](https://hdf-discourse-1.s3.amazonaws.com/original/1X/c15ef3615407deb94f2524457fd3c32b33e013ff.h5) (4.07 KB) > **···** > > -----Original Message----- > From: Ramakrishnan Iyer > Sent: Wednesday, June 04, 2008 5:25 PM > To: 'hdf-forum-help@hdfgroup.org' > Subject: Queries > > Hi, > > I have the following issues/queries ( am using HDF1.6.5 ). > > 1. I store a dataset which has a datatype of the following nature > > struct data { > > char \*name[1]; > > unsigned int id ; > > hdset\_reg\_ref\_t ref; > > } > > I need to access this dataset every now and then to check whether an > object with the given id is already written . I store an attribute for > the dataset which indicates the number of elements written into the > dataset . For every check I start from the first element to the last > written element and read the contents into a local variable ( of type > struct data defined above ) . The string name has a variable length . > > If there are 5 elements in the dataset I read all the elements in a for > loop one by one into the same local variable . > > Do I need to use the H5Dvlen\_reclaim function after I have read each > element into the local variable and checked for its existence ? What > will happen if I do not use H5Dvlen\_reclaim ? If I use it I get a crash > when this called . > > 2. As a part of one of the objects , I need to store a 4X4 matrix . The > data structure for the element is defined as follows > > struct obj > > { > > double mat[4][4]; > > int id; > > }; > > The type for this is created as follows . > > unsigned int dtsize=sizeof(obj); > > hsize\_t arr\_dims[]={4,4}; //!2 dimensional array > > hid\_t matrix= H5Tarray\_create (H5T\_STD\_U32LE, 2, arr\_dims, NULL); > > objtype=H5Tcreate(H5T\_COMPOUND,dtsize); > > H5Tinsert(objtype,"Trans-Matrix",HOFFSET(obj,mat),matrix); > > H5Tinsert(objtype,"IdHOFFSET(obj,id),H5T\_NATIVE\_UINT); > > I declare a variable of type struct obj and populate the matix and id . > I do the selection of the dataspace and then call H5Dwrite on the > dataset . When I check the output from h5dump this is what is written in > the matrix > > 0,1072693248,0,0,0,0,0,0,0,0,0,1072693248,0,0,0,0 ( I had an identity > matrix as input ) . Why do yiou think this might have occurred . The id > portion was written properly . > > Find attached the sample program and the output file . > > Regards > > Ramakrishnan ---
**Author:** ![ramakrishnan.iyer](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/r/3e96dc/32.png) [@ramakrishnan.iyer](https://forum.hdfgroup.org/u/ramakrishnan.iyer)\ **Post date:** [June 6, 2008, 6:05am UTC](https://forum.hdfgroup.org/t/fw-queries/1014/2 "2008-06-06T06:05:50Z")
I found the mistakes . I had to use H5T\_NATIVE\_DOUBLE instead of H5T\_STD\_U32LE for writing the matrix . For the first problem , I was doing memcpy with wrong type . My question regarding when to use H5Dvlen\_reclaim still stays though . > **···** > > -----Original Message----- > From: Ramakrishnan Iyer > Sent: Wednesday, June 04, 2008 5:25 PM > To: 'hdf-forum-help@hdfgroup.org' > Subject: Queries > > Hi, > > I have the following issues/queries ( am using HDF1.6.5 ). > > 1. I store a dataset which has a datatype of the following nature > > struct data { > > char \*name[1]; > > unsigned int id ; > > hdset\_reg\_ref\_t ref; > > } > > I need to access this dataset every now and then to check whether an > object with the given id is already written . I store an attribute for > the dataset which indicates the number of elements written into the > dataset . For every check I start from the first element to the last > written element and read the contents into a local variable ( of type > struct data defined above ) . The string name has a variable length . > > If there are 5 elements in the dataset I read all the elements in a for > loop one by one into the same local variable . > > Do I need to use the H5Dvlen\_reclaim function after I have read each > element into the local variable and checked for its existence ? What > will happen if I do not use H5Dvlen\_reclaim ? If I use it I get a crash > when this called . > > 2. As a part of one of the objects , I need to store a 4X4 matrix . The > data structure for the element is defined as follows > > struct obj > > { > > double mat[4][4]; > > int id; > > }; > > The type for this is created as follows . > > unsigned int dtsize=sizeof(obj); > > hsize\_t arr\_dims[]={4,4}; //!2 dimensional array > > hid\_t matrix= H5Tarray\_create (H5T\_STD\_U32LE, 2, arr\_dims, NULL); > > objtype=H5Tcreate(H5T\_COMPOUND,dtsize); > > H5Tinsert(objtype,"Trans-Matrix",HOFFSET(obj,mat),matrix); > > H5Tinsert(objtype,"IdHOFFSET(obj,id),H5T\_NATIVE\_UINT); > > I declare a variable of type struct obj and populate the matix and id . > I do the selection of the dataspace and then call H5Dwrite on the > dataset . When I check the output from h5dump this is what is written in > the matrix > > 0,1072693248,0,0,0,0,0,0,0,0,0,1072693248,0,0,0,0 ( I had an identity > matrix as input ) . Why do yiou think this might have occurred . The id > portion was written properly . > > Find attached the sample program and the output file . > > Regards > > Ramakrishnan ---
**Author:** ![Quincey\_Koziol](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/q/dbc845/32.png) [@Quincey\_Koziol](https://forum.hdfgroup.org/u/Quincey_Koziol)\ **Post date:** [June 6, 2008, 2:37pm UTC](https://forum.hdfgroup.org/t/fw-queries/1014/3 "2008-06-06T14:37:18Z")
Hi Ramakrishnan, > I found the mistakes . I had to use H5T\_NATIVE\_DOUBLE instead of H5T\_STD\_U32LE for writing the matrix . > > For the first problem , I was doing memcpy with wrong type . My question regarding when to use H5Dvlen\_reclaim still stays though .   Yes, you should release the memory that the HDF5 library allocates to store the variable length data read from disk. Calling HDvlen\_reclaim is a convenient way to do that, but other ways may work also. You may want to use the H5Pset\_vlen\_mem\_manager, if you'd like more control over the memory allocation & release process.   Quincey > **···** > > On Jun 6, 2008, at 1:05 AM, Ramakrishnan Iyer wrote: > > > -----Original Message----- > > From: Ramakrishnan Iyer > > Sent: Wednesday, June 04, 2008 5:25 PM > > To: 'hdf-forum-help@hdfgroup.org' > > Subject: Queries > > > > Hi, > > > > I have the following issues/queries ( am using HDF1.6.5 ). > > > > 1. I store a dataset which has a datatype of the following nature > > > > struct data { > > > > char \*name[1]; > > > > unsigned int id ; > > > > hdset\_reg\_ref\_t ref; > > > > } > > > > I need to access this dataset every now and then to check whether an object with the given id is already written . I store an attribute for the dataset which indicates the number of elements written into the dataset . For every check I start from the first element to the last written element and read the contents into a local variable ( of type struct data defined above ) . The string name has a variable length . > > > > If there are 5 elements in the dataset I read all the elements in a for loop one by one into the same local variable . > > > > Do I need to use the H5Dvlen\_reclaim function after I have read each element into the local variable and checked for its existence ? What will happen if I do not use H5Dvlen\_reclaim ? If I use it I get a crash when this called . > > > > 2. As a part of one of the objects , I need to store a 4X4 matrix . The data structure for the element is defined as follows > > > > struct obj > > > > { > > > > double mat[4][4]; > > > > int id; > > > > }; > > > > The type for this is created as follows . > > > > unsigned int dtsize=sizeof(obj); > > > > hsize\_t arr\_dims={4,4}; //!2 dimensional array > > > > hid\_t matrix= H5Tarray\_create (H5T\_STD\_U32LE, 2, arr\_dims, NULL); > > > > objtype=H5Tcreate(H5T\_COMPOUND,dtsize); > > H5Tinsert(objtype,"Trans-Matrix",HOFFSET(obj,mat),matrix); > > H5Tinsert(objtype,"IdHOFFSET(obj,id),H5T\_NATIVE\_UINT); > > > > I declare a variable of type struct obj and populate the matix and id . I do the selection of the dataspace and then call H5Dwrite on the dataset . When I check the output from h5dump this is what is written in the matrix > > > > 0,1072693248,0,0,0,0,0,0,0,0,0,1072693248,0,0,0,0 ( I had an identity matrix as input ) . Why do yiou think this might have occurred . The id portion was written properly . > > > > Find attached the sample program and the output file . > > > > Regards > > > > Ramakrishnan > > ---------------------------------------------------------------------- > This mailing list is for HDF software users discussion. > To subscribe to this list, send a message to hdf-forum-subscribe@hdfgroup.org. > To unsubscribe, send a message to hdf-forum-unsubscribe@hdfgroup.org.