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(General Remarks) (File Operations) (Navigating a CGNS File) (Error Handling) (Structural Nodes) (Descriptors) (Physical Data) (Location and Position) (Auxiliary Data) (Grid Specification) (Solution Data) (Grid Connectivity) (Boundary Conditions) (Equation Specification) (Families) (Time-Dependent Data) (Links)

File Operations

Opening and Closing a CGNS File

Functions Modes
ier = cg_open(char *filename, int mode, int *fn); r w m
ier = cg_version(int fn, float *version); r w m
ier = cg_precision(int fn, int *precision); r w m
ier = cg_close(int fn); r w m
ier = cg_is_cgns(const char *filename, int *file_type); r w m
ier = cg_save_as(int fn, const char *filename, int file_type, int follow_links); r w m
ier = cg_set_file_type(int file_type); r w m
ier = cg_get_file_type(int fn, int *file_type); r w m
call cg_open_f(filename, mode, fn, ier) r w m
call cg_version_f(fn, version, ier) r w m
call cg_precision_f(fn, precision, ier) r w m
call cg_close_f(fn, ier) r w m
call cg_is_cgns_f(filename, file_type, ier) r w m
call cg_save_as_f(fn, filename, file_type, follow_links, ier); r w m
call cg_set_file_type_f(file_type, ier); r w m
call cg_get_file_type_f(fn, file_type, ier); r w m


    filename   Name of the CGNS file, including path name if necessary. There is no limit on the length of this character variable.
mode Mode used for opening the file. The modes currently supported are CG_MODE_READ, CG_MODE_WRITE, and CG_MODE_MODIFY.
fn CGNS file index number.
version CGNS version number.
precision Precision used to write the CGNS file. The return value will be one of 32 (32-bit), 64 (64-bit), or 0 if not known.
file_type Type of CGNS file. This will typically be either CG_FILE_ADF or CG_FILE_HDF5 depending on the underlying file format. However, note that when built in 32-bit, there is also an option to create a Version 2.5 CGNS file by setting the file type to CG_FILE_ADF2.
follow_links This flag determines whether links are left intact when saving a CGNS file. If non-zero, then the links will be removed and the data associated with the linked files copied to the new file.
ier Error status.

The function cg_open must always be the first one called. It opens a CGNS file for reading and/or writing and returns an index number fn. The index number serves to identify the CGNS file in subsequent function calls. Several CGNS files can be opened simultaneously. The current limit on the number of files opened at once depends on the platform. On an SGI workstation, this limit is set at 100 (parameter FOPEN_MAX in stdio.h).

The file can be opened in one of the following modes:

    CG_MODE_READ   Read only mode.
CG_MODE_WRITE Write only mode.
CG_MODE_MODIFY Reading and/or writing is allowed.

When the file is opened, if no CGNSLibraryVersion_t node is found, a default value of 1.05 is assumed for the CGNS version number. Note that this corresponds to an old version of the CGNS standard, that doesn't include many data structures supported by the current standard.

The function cg_close must always be the last one called. It closes the CGNS file designated by the index number fn and frees the memory where the CGNS data was kept. When a file is opened for writing, cg_close writes all the CGNS data in memory onto disk prior to closing the file. Consequently, if is omitted, the CGNS file is not written properly.

In order to reduce memory usage and improve execution speed, large arrays such as grid coordinates or flow solutions are not actually stored in memory. Instead, only basic information about the node is kept, while reads and writes of the data is directly to and from the application's memory. An attempt is also made to do the same with unstructured mesh element data.

When a CGNS file is newly created using CG_MODE_WRITE, the default type of database manager used is determined at compile time. If the CGNS library was built with HDF5 version 1.8 or later support, the file type will be CG_FILE_HDF5, otherwise CG_FILE_ADF is used. This may be changed either by setting an environment variable, CGNS_FILETYPE, to one of adf, hdf5, or adf2, or by calling the routine cg_set_file_type prior to the cg_open call. Calling cg_set_file_type with the argument CG_FILE_NONE will reset the library to use the default file type. Note: If the environment variable CGNS_FILETYPE is set, it takes precedence.

For existing files, the function cg_is_cgns may be used to determine if a file is a CGNS file or not, and the type of file (CG_FILE_ADF or CG_FILE_HDF5). If the file is a CGNS file, cg_is_cgns returns CG_OK, otherwise CG_ERROR is returned and file_type is set to CG_FILE_NONE.

The CGNS file identified by fn may be saved to a different filename and type using cg_save_as. In order to save as an HDF5 file, the library must have been built with HDF5 support. ADF support is always built. The function cg_set_file_type sets the default file type for newly created CGNS files. The function cg_get_file_type returns the file type for the CGNS file identified by fn. If the CGNS library is built as 32-bit, the additional file type, CG_FILE_ADF2, is available. This allows creation of a 2.5 compatible CGNS file.

Configuring CGNS Internals

Functions Modes
ier = cg_configure(int option, void *value); r w m
ier = cg_error_handler(void (*)(int, char *)); r w m
ier = cg_set_compress(int compress); r w m
ier = cg_get_compress(int *compress); r w m
ier = cg_set_path(const char *path); r w m
ier = cg_add_path(const char *path); r w m
call cg_configure_f(option, value, ier); r w m
call cg_exit_on_errors_f(flag) r w m
call cg_set_compress_f(compress, ier) r w m
call cg_get_compress_f(compress, ier) r w m
call cg_set_path_f(path, ier) r w m
call cg_add_path_f(path, ier) r w m


value The value to set, type cast as void *. In Fortran the type is TYPE(C_PTR).
compress CGNS compress (rewrite) setting.
path Pathname to search for linked to files when opening a file with external links.
flag Fortran flag to set automatic exit in the case of error.
ier Error status.

The function cg_configure allows particular CGNS library internal options to be configured. The currently supported options and expected values are:

    CG_CONFIG_ERROR   This allows an error call-back function to be defined by the user. The value should be a pointer to a function to receive the error. The function is defined as void err_callback(int is_error, char *errmsg), and will be called for errors and warnings. The first argument, is_error, will be 0 for warning messages, 1 for error messages, and −1 if the program is going to terminate (i.e., a call to cg_error_exit()). The second argument is the error or warning message. If this is defined, warning and error messages will go to the function rather than the terminal. A value of NULL will remove the call-back function.
CG_CONFIG_COMPRESS This is the rewrite-upon-close setting. Note: Prior versions of the library would automatically rewrite the CGNS file when it was closed after being opened in modify mode if there was unused space. This is no longer done due to possible conflicts when using parallel I/O. The previous behavior may be recovered by setting value to a positive integer. In this case, the file will be rewritten if the number of node deletions or modifications are equal to or exceed this number. Setting value to a negative number will force the rewrite when the file is closed. The default value is 0 (no rewrite).
CG_CONFIG_SET_PATH Sets the search path for locating linked-to files. The argument value should be a character string containing one or more directories, formatted the same as for the PATH environment variable. This will replace any current settings. Setting value to NULL will remove all paths. In Fortran, the path must terminate with a NULL character, CHAR(0) or C_NULL_CHAR.
CG_CONFIG_ADD_PATH Adds a directory, or list of directories, to the linked-to file search path. This is the same as CG_CONFIG_SET_PATH, but adds to the path instead of replacing it. In Fortran, the path must terminate with a NULL character, CHAR(0) or C_NULL_CHAR.
CG_CONFIG_FILE_TYPE Sets the default file type for newly created CGNS files. The argument, value should be set to one of CG_FILE_NONE, CG_FILE_ADF, CG_FILE_HDF5, or CG_FILE_ADF2. See the discussion above for cg_set_file_type.
CG_CONFIG_RIND_INDEX This option affects index bounds on structured arrays with rind planes. The SIDS specifies that core array locations begin at index 1. Lower rind planes, if present, would have an index less than 1 (see structured grid indexing conventions). Versions of the Mid-Level Library < 3.4 did not produce this behavior. Index 1 always represented the start of an array: in an array with no rind planes, the core location would have index 1; in an array with 1 rind plane, the core location would have index 2. In version 3.4 of the Mid-Level Library, the API's behavior was fixed to match that specified in the SIDS: core array locations always begin at index 1. This option allows for configuring the library to pre-3.4 indexing behavior (set value to CG_CONFIG_RIND_ZERO) or the new default behavior (set value to CG_CONFIG_RIND_CORE). Note that using CG_CONFIG_RIND_ZERO is considered obsolete, but is provided for backward compatibility. Most users should not select this option and use the default. Values used for this option do not need to be explicitly cast as void*. This option does not change the cgns file in any way; it only modifies the API to the library.
CG_CONFIG_HDF5_COMPRESS Sets the compression level for data written from HDF5. The default is no compression. Setting value to -1 will use the default compression level of 6. The acceptable values are 0 to 9, corresponding to gzip compression levels. This option currently does nothing as compression is not yet implemented.
CG_CONFIG_HDF5_MPI_COMM Sets the MPI communicator for parallel I/O. The default is MPI_COMM_WORLD. The new communicator is given by typecasting it to a void *. This is generally used internally - see cgp_mpi_comm instead.
CG_CONFIG_HDF5_DISKLESS Performs I/O directly to memory and can be used to create temporary CGNS files that never exist on permanent storage. The memory is written to disk depending on the state of CG_CONFIG_HDF5_DISKLESS_WRITE. CGNS will use the core file driver in HDF5 via H5Pset_fapl_core, and the parameters associated with CG_CONFIG_HDF5_DISKLESS are associated with that API. When value is 1, any previously defined driver mode is ignored. Setting value to 0 disables the diskless mode, enabling the previous mode. This configuration is not a valid option when accessing the memory by more than one process.
CG_CONFIG_HDF5_DISKLESS_INCR Value specifies the increment by which allocated memory is to be increased each time more memory is required, in bytes. The default is 10MiB. Ideally, value should be set large enough to minimize repeated increases. The type of value is size_t in C and C_SIZE_T in Fortran. Due to a bug with gfortran, it is advisable to use C_LOC or C_FUNLOC in-line of the call instead of using a variable.
CG_CONFIG_HDF5_DISKLESS_WRITE Value indicates whether to write (value=1) the memory contents to disk when the file is closed. Otherwise, value=0 does not persist the memory to disk.
CG_CONFIG_HDF5_ALIGNMENT Configures HDF5's H5Pset_alignment and sets the alignment, value[1], properties of a file access property list so that any file object greater than or equal in size to a threshold, value[0], bytes will be aligned on an address which is a multiple of alignment.
CG_CONFIG_HDF5_MD_BLOCK_SIZE Configures HDF5's H5Pset_meta_block_size and sets the minimum size, value (in bytes), of metadata block allocations.
CG_CONFIG_HDF5_BUFFER Configures HDF5's H5Pset_buffer and sets the maximum size, value (in bytes), for the type conversion buffer and background buffer.
CG_CONFIG_HDF5_SIEVE_BUF_SIZE Configures HDF5's H5Pset_sieve_buf_size and sets the maximum size, value (in bytes), of the data sieve buffer.
CG_CONFIG_RESET Value indicates the configuration values to reset to their default values. Currently, only CG_CONFIG_RESET_HDF5 is a valid value and will reset all the CG_CONFIG_HDF5_* parameters, excluding CG_CONFIG_HDF5_MPI_COMM and CG_CONFIG_HDF5_DISKLESS, to their default values.

The routines cg_error_handler, cg_set_compress, cg_set_path, cg_add_path, and cg_set_file_type are convenience functions built on top of cg_configure.

There is no Fortran counterpart for function cg_error_handler. The Fortran function cg_exit_on_error_f routine can be used in place of cg_error_handler. If flag is non-zero, then when an error is encountered, the library will print the error message and exit with a code of 1. Setting flag to zero (the default) prevents this, and the error is returned to the user code.

Note: The HDF5 implementation does not support search paths for linked files. The links need to be either absolute or relative pathnames. As a result, it is recommended that the search path options not be used as they may be removed in future versions.

Interfacing with CGIO

Functions Modes
ier = cg_get_cgio(int fn, int *cgio_num); r w m
ier = cg_root_id(int fn, double *rootid); r w m
call cg_get_cgio_f(fn, cgio_num, ier) r w m
call cg_root_id_f(fn, rootid, ier) r w m


    fn   CGNS file index number.
cgio_num CGIO indentifier for the CGNS file.
rootid Root node identifier for the CGNS file.
ier Error status.

These allow for the use of the low-level CGIO functions in conjunction with the Mid Level Library. The function cg_get_cgio returns the CGIO database identifier for the specified CGNS file, which is used in the CGIO routines. The root node identifier for the CGNS file is returned by cg_root_id.