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NetCDF User's Guide for C

8 Attributes

Attributes may be associated with each netCDF variable to specify such properties as units, special values, maximum and minimum valid values, scaling factors, and offsets. Attributes for a netCDF dataset are defined when the dataset is first created, while the netCDF dataset is in define mode. Additional attributes may be added later by reentering define mode. A netCDF attribute has a netCDF variable to which it is assigned, a name, a type, a length, and a sequence of one or more values. An attribute is designated by its variable ID and name. When an attribute name is not known, it may be designated by its variable ID and number in order to determine its name, using the function nc_inq_attname.

The attributes associated with a variable are typically defined immediately after the variable is created, while still in define mode. The data type, length, and value of an attribute may be changed even when in data mode, as long as the changed attribute requires no more space than the attribute as originally defined.

It is also possible to have attributes that are not associated with any variable. These are called global attributes and are identified by using NC_GLOBAL as a variable pseudo-ID. Global attributes are usually related to the netCDF dataset as a whole and may be used for purposes such as providing a title or processing history for a netCDF dataset.

Operations supported on attributes are:

8.1 Attribute Conventions

Names commencing with underscore ('_') are reserved for use by the netCDF library. Most generic applications that process netCDF datasets assume standard attribute conventions and it is strongly recommended that these be followed unless there are good reasons for not doing so. Below we list the names and meanings of recommended standard attributes that have proven useful. Note that some of these (e.g. units, valid_range, scale_factor) assume numeric data and should not be used with character data.
unitsA character string that specifies the units used for the variable's data. Unidata has developed a freely-available library of routines to convert between character string and binary forms of unit specifications and to perform various useful operations on the binary forms. This library is used in some netCDF applications. Using the recommended units syntax permits data represented in conformable units to be automatically converted to common units for arithmetic operations. See Appendix A "Units," page 113, for more information.
long_nameA long descriptive name. This could be used for labeling plots, for example. If a variable has no long_name attribute assigned, the variable name should be used as a default.
valid_minA scalar specifying the minimum valid value for this variable.
valid_maxA scalar specifying the maximum valid value for this variable.
valid_rangeA vector of two numbers specifying the minimum and maximum valid values for this variable, equivalent to specifying values for both valid_min and valid_max attributes. Any of these attributes define the valid range. The attribute valid_range must not be defined if either valid_min or valid_max is defined.

Generic applications should treat values outside the valid range as missing. The type of each valid_range, valid_min and valid_max attribute should match the type of its variable (except that for byte data, these can be of a signed integral type to specify the intended range).

If neither valid_min, valid_max nor valid_range is defined then generic applications should define a valid range as follows. If the data type is byte and _FillValue is not explicitly defined, then the valid range should include all possible values. Otherwise, the valid range should exclude the _FillValue (whether defined explicitly or by default) as follows. If the _FillValue is positive then it defines a valid maximum, otherwise it defines a valid minimum. For integer types, there should be a difference of 1 between the _FillValue and this valid minimum or maximum. For floating point types, the difference should be twice the minimum possible (1 in the least significant bit) to allow for rounding error.

scale_factorIf present for a variable, the data are to be multiplied by this factor after the data are read by the application that accesses the data.
add_offsetIf present for a variable, this number is to be added to the data after it is read by the application that accesses the data. If both scale_factor and add_offset attributes are present, the data are first scaled before the offset is added. The attributes scale_factor and add_offset can be used together to provide simple data compression to store low-resolution floating-point data as small integers in a netCDF dataset. When scaled data are written, the application should first subtract the offset and then divide by the scale factor.

When scale_factor and add_offset are used for packing, the associated variable (containing the packed data) is typically of type byte or short, whereas the unpacked values are intended to be of type float or double. The attributes scale_factor and add_offset should both be of the type intended for the unpacked data, e.g. float or double.

_FillValueThe _FillValue attribute specifies the fill value used to pre-fill disk space allocated to the variable. Such pre-fill occurs unless nofill mode is set using nc_set_fill. See Section 5.12 "Set Fill Mode for Writes: nc_set_fill," page 39, for details. The fill value is returned when reading values that were never written. If _FillValue is defined then it should be scalar and of the same type as the variable. It is not necessary to define your own _FillValue attribute for a variable if the default fill value for the type of the variable is adequate. However, use of the default fill value for data type byte is not recommended. Note that if you change the value of this attribute, the changed value applies only to subsequent writes; previously written data are not changed.

Generic applications often need to write a value to represent undefined or missing values. The fill value provides an appropriate value for this purpose because it is normally outside the valid range and therefore treated as missing when read by generic applications. It is legal (but not recommended) for the fill value to be within the valid range.

See Section 7.16 "Fill Values," page 78, for more information.

missing_valueThis attribute is not treated in any special way by the library or conforming generic applications, but is often useful documentation and may be used by specific applications. The missing_value attribute can be a scalar or vector containing values indicating missing data. These values should all be outside the valid range so that generic applications will treat them as missing.
signednessDeprecated attribute, originally designed to indicate whether byte values should be treated as signed or unsigned. The attributes valid_min and valid_max may be used for this purpose. For example, if you intend that a byte variable store only nonnegative values, you can use valid_min = 0 and valid_max = 255. This attribute is ignored by the netCDF library.
C_formatA character array providing the format that should be used by C applications to print values for this variable. For example, if you know a variable is only accurate to three significant digits, it would be appropriate to define the C_format attribute as "%.3g". The ncdump utility program uses this attribute for variables for which it is defined. The format applies to the scaled (internal) type and value, regardless of the presence of the scaling attributes scale_factor and add_offset.
FORTRAN_formatA character array providing the format that should be used by FORTRAN applications to print values for this variable. For example, if you know a variable is only accurate to three significant digits, it would be appropriate to define the FORTRAN_format attribute as "(G10.3)".
titleA global attribute that is a character array providing a succinct description of what is in the dataset.
historyA global attribute for an audit trail. This is a character array with a line for each invocation of a program that has modified the dataset. Well-behaved generic netCDF applications should append a line containing: date, time of day, user name, program name and command arguments.
ConventionsIf present, 'Conventions' is a global attribute that is a character array for the name of the conventions followed by the dataset, in the form of a string that is interpreted as a directory name relative to a directory that is a repository of documents describing sets of discipline-specific conventions. This permits a hierarchical structure for conventions and provides a place where descriptions and examples of the conventions may be maintained by the defining institutions and groups. The conventions directory name is currently interpreted relative to the directory pub/netcdf/Conventions/ on the host machine ftp.unidata.ucar.edu. Alternatively, a full URL specification may be used to name a WWW site where documents that describe the conventions are maintained.
 For example, if a group named NUWG agrees upon a set of conventions for dimension names, variable names, required attributes, and netCDF representations for certain discipline-specific data structures, they may store a document describing the agreed-upon conventions in a dataset in the NUWG/ subdirectory of the Conventions directory. Datasets that followed these conventions would contain a global Conventions attribute with value "NUWG".
 Later, if the group agrees upon some additional conventions for a specific subset of NUWG data, for example time series data, the description of the additional conventions might be stored in the NUWG/Time_series/ subdirectory, and datasets that adhered to these additional conventions would use the global Conventions attribute with value "NUWG/Time_series", implying that this dataset adheres to the NUWG conventions and also to the additional NUWG time-series conventions.

8.2 Create an Attribute: nc_put_att_type

The function nc_put_att_type adds or changes a variable attribute or global attribute of an open netCDF dataset. If this attribute is new, or if the space required to store the attribute is greater than before, the netCDF dataset must be in define mode.

Usage

Although it's possible to create attributes of all types, text and double attributes are adequate for most purposes.

int nc_put_att_text   (int ncid, int varid, const char *name, 
                                      size_t len, const char *tp);
int nc_put_att_uchar  (int ncid, int varid, const char *name,
                          nc_type xtype, size_t len, const unsigned char *up);
int nc_put_att_schar  (int ncid, int varid, const char *name,
                          nc_type xtype, size_t len, const signed char *cp);
int nc_put_att_short  (int ncid, int varid, const char *name,
                       nc_type xtype, size_t len, const short *sp);
int nc_put_att_int    (int ncid, int varid, const char *name,
                       nc_type xtype, size_t len, const int *ip);
int nc_put_att_long   (int ncid, int varid, const char *name,
                       nc_type xtype, size_t len, const long *lp);
int nc_put_att_float  (int ncid, int varid, const char *name,
                       nc_type xtype, size_t len, const float *fp);
int nc_put_att_double (int ncid, int varid, const char *name,
                       nc_type xtype, size_t len, const double *dp);

ncidNetCDF ID, from a previous call to nc_open or nc_create.

varidVariable ID of the variable to which the attribute will be assigned or NC_GLOBAL for a global attribute.

nameAttribute name. Must begin with an alphabetic character, followed by zero or more alphanumeric characters including the underscore ('_'). Case is significant. Attribute name conventions are assumed by some netCDF generic applications, e.g., units as the name for a string attribute that gives the units for a netCDF variable. See Section 8.1 "Attribute Conventions,"  page 81, for examples of attribute conventions. 

xtypeOne of the set of predefined netCDF external data types. The type of this parameter, nc_type, is defined in the netCDF header file. The valid netCDF external data types are NC_BYTE, NC_CHAR, NC_SHORT, NC_INT, NC_FLOAT, and NC_DOUBLE. Although it's possible to create attributes of all types, NC_CHAR and NC_DOUBLE attributes are adequate for most purposes. 

lenNumber of values provided for the attribute.

tp, up, cp, sp, ip, lp, fp, or dpPointer to one or more values. If the type of values differs from the netCDF attribute type specified as xtype, type conversion will occur. See Section 3.3 "Type Conversion,"  page 20, for details.
Errors

nc_put_att_type returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:

Example

Here is an example using nc_put_att_double to add a variable attribute named valid_range for a netCDF variable named rh and a global attribute named title to an existing netCDF dataset named foo.nc: 

#include <netcdf.h>
   ... 
int  status;                            /* error status */
int  ncid;                              /* netCDF ID */
int  rh_id;                             /* variable ID */
static double rh_range[] = {0.0, 100.0};/* attribute vals */
static char title[] = "example netCDF dataset";
   ... 
status = nc_open("foo.nc", NC_WRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_redef(ncid);                /* enter define mode */
if (status != NC_NOERR) handle_error(status);
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_put_att_double (ncid, rh_id, "valid_range", 
                            NC_DOUBLE, 2, rh_range);
if (status != NC_NOERR) handle_error(status);
status = nc_put_att_text (ncid, NC_GLOBAL, "title",
                          NC_CHAR, strlen(title), title)
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_enddef(ncid);               /* leave define mode */
if (status != NC_NOERR) handle_error(status);

8.3 Get Information about an Attribute: nc_inq_att Family

This family of functions returns information about a netCDF attribute. All but one of these functions require the variable ID and attribute name; the exception isnc_inq_attname. Information about an attribute includes its type, length, name, and number. See the nc_get_att family for getting attribute values.

The function nc_inq_attname gets the name of an attribute, given its variable ID and number. This function is useful in generic applications that need to get the names of all the attributes associated with a variable, since attributes are accessed by name rather than number in all other attribute functions. The number of an attribute is more volatile than the name, since it can change when other attributes of the same variable are deleted. This is why an attribute number is not called an attribute ID.

The function nc_inq_att returns the attribute's type and length. The other functions each return just one item of information about an attribute.

Usage 


int nc_inq_att    (int ncid, int varid, const char *name,
                   nc_type *xtypep, size_t *lenp);
int nc_inq_atttype(int ncid, int varid, const char *name,
                   nc_type *xtypep);
int nc_inq_attlen  (int ncid, int varid, const char *name, size_t *lenp);
int nc_inq_attname(int ncid, int varid, int attnum, char *name);
int nc_inq_attid   (int ncid, int varid, const char *name, int *attnump); 

ncidNetCDF ID, from a previous call to nc_open or nc_create.

varidVariable ID of the attribute's variable, or NC_GLOBAL for a global attribute.

nameAttribute name. For nc_inq_attname, this is a pointer to the location for the returned attribute name.

xtypepPointer to location for returned attribute type, one of the set of predefined netCDF external data types. The type of this parameter, nc_type, is defined in the netCDF header file. The valid netCDF external data types are NC_BYTE, NC_CHAR, NC_SHORT, NC_INT, NC_FLOAT, and NC_DOUBLE. If this parameter is given as '0' (a null pointer), no type will be returned so no variable to hold the type needs to be declared. 

lenpPointer to location for returned number of values currently stored in the attribute. For attributes of type NC_CHAR, you should not assume that this includes a trailing zero byte; it doesn't if the attribute was stored without a trailing zero byte, for example from a FORTRAN program. Before using the value as a C string, make sure it is null-terminated. If this parameter is given as '0' (a null pointer), no length will be returned so no variable to hold this information needs to be declared.

attnumFor nc_inq_attname, attribute number. The attributes for each variable are numbered from 0 (the first attribute) to natts-1, where natts is the number of attributes for the variable, as returned from a call to nc_inq_varnatts. 

attnumpFor nc_inq_attid, pointer to location for returned attribute number that specifies which attribute this is for this variable (or which global attribute). If you already know the attribute name, knowing its number is not very useful, because accessing information about an attribute requires its name.
Errors

Each function returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:

Example

Here is an example using nc_inq_att to find out the type and length of a variable attribute named valid_range for a netCDF variable named rh and a global attribute named title in an existing netCDF dataset named foo.nc: 

#include <netcdf.h>
   ... 
int  status;               /* error status */
int  ncid;                 /* netCDF ID */
int  rh_id;                /* variable ID */
nc_type vr_type, t_type;   /* attribute types */
int  vr_len, t_len;        /* attribute lengths */

   ... 
status = nc_open("foo.nc", NC_NOWRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_inq_att (ncid, rh_id, "valid_range", &vr_type, &vr_len);
if (status != NC_NOERR) handle_error(status);
status = nc_inq_att (ncid, NC_GLOBAL, "title", &t_type, &t_len);
if (status != NC_NOERR) handle_error(status);

8.4 Get Attribute's Values:nc_get_att_type

Members of the nc_get_att_type family of functions get the value(s) of a netCDF attribute, given its variable ID and name.

Usage 

int nc_get_att_text   (int ncid, int varid, const char *name, 
                       char *tp);
int nc_get_att_uchar  (int ncid, int varid, const char *name,
                       unsigned char *up);
int nc_get_att_schar  (int ncid, int varid, const char *name,
                       signed char *cp);
int nc_get_att_short  (int ncid, int varid, const char *name,
                       short *sp);
int nc_get_att_int    (int ncid, int varid, const char *name,
                       int *ip);
int nc_get_att_long   (int ncid, int varid, const char *name,
                       long *lp);
int nc_get_att_float  (int ncid, int varid, const char *name,
                       float *fp);
int nc_get_att_double (int ncid, int varid, const char *name, 
                       double *dp);

ncidNetCDF ID, from a previous call to nc_open or nc_create.

varidVariable ID of the attribute's variable, or NC_GLOBAL for a global attribute.

nameAttribute name.

tp, up, cp, sp, ip, lp, fp, or dpPointer to location for returned attribute value(s). All elements of the vector of attribute values are returned, so you must allocate enough space to hold them. For attributes of type NC_CHAR, you should not assume that the returned values include a trailing zero byte; they won't if the attribute was stored without a trailing zero byte, for example from a FORTRAN program. Before using the value as a C string, make sure it is null-terminated. If you don't know how much space to reserve, call nc_inq_attlen first to find out the length of the attribute.
Errors

nc_get_att_type returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:

Example

Here is an example using nc_get_att_double to determine the values of a variable attribute named valid_range for a netCDF variable named rh and a global attribute named title in an existing netCDF dataset named foo.nc. In this example, it is assumed that we don't know how many values will be returned, but that we do know the types of the attributes. Hence, to allocate enough space to store them, we must first inquire about the length of the attributes.

#include <netcdf.h>
   ... 
int  status;               /* error status */
int  ncid;                 /* netCDF ID */
int  rh_id;                /* variable ID */
int  vr_len, t_len;        /* attribute lengths */
double *vr_val;            /* ptr to attribute values */
char *title;               /* ptr to attribute values */
extern char *malloc();     /* memory allocator */

   ... 
status = nc_open("foo.nc", NC_NOWRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
   ... 
/* find out how much space is needed for attribute values */
status = nc_inq_attlen (ncid, rh_id, "valid_range", &vr_len);
if (status != NC_NOERR) handle_error(status);
status = nc_inq_attlen (ncid, NC_GLOBAL, "title", &t_len);
if (status != NC_NOERR) handle_error(status);

/* allocate required space before retrieving values */
vr_val = (double *) malloc(vr_len * sizeof(double));
title = (char *) malloc(t_len + 1);  /* + 1 for trailing null */

/* get attribute values */
status = nc_get_att_double(ncid, rh_id, "valid_range", vr_val);
if (status != NC_NOERR) handle_error(status);
status = nc_get_att_text(ncid, NC_GLOBAL, "title", title);
if (status != NC_NOERR) handle_error(status);
title[t_len] = '\0';       /* null terminate */
   ...

8.5 Copy Attribute from One NetCDF to Another: nc_copy_att

The function nc_copy_att copies an attribute from one open netCDF dataset to another. It can also be used to copy an attribute from one variable to another within the same netCDF.

Usage 


int nc_copy_att (int ncid_in, int varid_in, const char *name,
                 int ncid_out, int varid_out);


ncid_inThe netCDF ID of an input netCDF dataset from which the attribute will be copied, from a previous call to nc_open or nc_create.

varid_inID of the variable in the input netCDF dataset from which the attribute will be copied, or NC_GLOBAL for a global attribute.

nameName of the attribute in the input netCDF dataset to be copied.

ncid_outThe netCDF ID of the output netCDF dataset to which the attribute will be copied, from a previous call to nc_open or nc_create. It is permissible for the input and output netCDF IDs to be the same. The output netCDF dataset should be in define mode if the attribute to be copied does not already exist for the target variable, or if it would cause an existing target attribute to grow. 

varid_outID of the variable in the output netCDF dataset to which the attribute will be copied, or NC_GLOBAL to copy to a global attribute.
Errors

nc_copy_att returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:

Example

Here is an example using nc_copy_att to copy the variable attribute units from the variable rh in an existing netCDF dataset named foo.nc to the variable avgrh in another existing netCDF dataset named bar.nc, assuming that the variable avgrh already exists, but does not yet have a units attribute: 

#include <netcdf.h>
   ... 
int  status;               /* error status */
int  ncid1, ncid2;         /* netCDF IDs */
int  rh_id, avgrh_id;      /* variable IDs */
   ... 
status = nc_open("foo.nc", NC_NOWRITE, ncid1);
if (status != NC_NOERR) handle_error(status);
status = nc_open("bar.nc", NC_WRITE, ncid2);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_inq_varid (ncid1, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
status = nc_inq_varid (ncid2, "avgrh", &avgrh_id);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_redef(ncid2);  /* enter define mode */
if (status != NC_NOERR) handle_error(status);
/* copy variable attribute from "rh" to "avgrh" */
status = nc_copy_att(ncid1, rh_id, "units", ncid2, avgrh_id);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_enddef(ncid2); /* leave define mode */
if (status != NC_NOERR) handle_error(status);

8.6 Rename an Attribute: nc_rename_att

The function nc_rename_att changes the name of an attribute. If the new name is longer than the original name, the netCDF dataset must be in define mode. You cannot rename an attribute to have the same name as another attribute of the same variable.

Usage 

int nc_rename_att (int ncid, int varid, const char* name, 
                   const char* newname);

ncidNetCDF ID, from a previous call to nc_open or nc_create

varidID of the attribute's variable, or NC_GLOBAL for a global attribute

nameThe current attribute name.

newnameThe new name to be assigned to the specified attribute. If the new name is longer than the current name, the netCDF dataset must be in define mode.
Errors

nc_rename_att returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:

Example

Here is an example using nc_rename_att to rename the variable attribute units to Units for a variable rh in an existing netCDF dataset named foo.nc: 

#include <netcdf.h>
   ... 
int  status;      /* error status */
int  ncid;        /* netCDF ID */
int  rh_id;       /* variable id */
   ... 
status = nc_open("foo.nc", NC_NOWRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
   ... 
/* rename attribute */
status = nc_rename_att(ncid, rh_id, "units", "Units");
if (status != NC_NOERR) handle_error(status);

8.7 Delete an Attribute: nc_del_att

The function nc_del_att deletes a netCDF attribute from an open netCDF dataset. The netCDF dataset must be in define mode.

Usage 

int nc_del_att (int ncid, int varid, const char* name);

ncidNetCDF ID, from a previous call to nc_open or nc_create.

varidID of the attribute's variable, or NC_GLOBAL for a global attribute.

nameThe name of the attribute to be deleted.
Errors

nc_del_att returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:

Example

Here is an example using nc_del_att to delete the variable attribute Units for a variable rh in an existing netCDF dataset named foo.nc: 

#include <netcdf.h>
   ... 
int  status;      /* error status */
int  ncid;        /* netCDF ID */
int  rh_id;       /* variable ID */
   ... 
status = nc_open("foo.nc", NC_WRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
   ... 
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
   ... 
/* delete attribute */
status = nc_redef(ncid);        /* enter define mode */
if (status != NC_NOERR) handle_error(status);
status = nc_del_att(ncid, rh_id, "Units");
if (status != NC_NOERR) handle_error(status);
status = nc_enddef(ncid);       /* leave define mode */
if (status != NC_NOERR) handle_error(status);
8.1 - Attribute Conventions
8.2 - Create an Attribute: nc_put_att_type
Usage -
Example -
8.3 - Get Information about an Attribute: nc_inq_att Family
Usage -
Example -
8.4 - Get Attribute's Values:nc_get_att_type
Usage -
Example -
8.5 - Copy Attribute from One NetCDF to Another: nc_copy_att
Usage -
Example -
8.6 - Rename an Attribute: nc_rename_att
Usage -
Example -
8.7 - Delete an Attribute: nc_del_att
Usage -
Example -
NetCDF User's Guide for C - 5 JUN 1997
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