Routines (alphabetical) > Routines: C > CDF Routines > CDF_OPEN

# CDF_OPEN

The CDF_OPEN function opens an existing Common Data Format file.

## Examples

id = CDF_OPEN('open_close.cdf') ; Open a file.

; ... other CDF_ commands ...

CDF_CLOSE, id ; Close the cdf file.

## Syntax

Result = CDF_OPEN(Filename [, /READONLY] )

## Return Value

If successful, the CDF ID for the file is returned.

## Arguments

### Filename

A scalar string containing the name of the file to be created. Note that if the desired filename has a `.cdf` ending, you can omit the extension and specify just the first part of the filename. For example, specifying “mydata” would open the file `mydata.cdf`.

## Keywords

### READONLY

Set this keyword to open the specified CDF file in readonly mode. Opening a file in readonly mode will result in significant performance improvement if there is a large amount of metadata and a large portion of the metadata is read.

## Version History

Pre 4.0 | Introduced
7.0.3 | READONLY keyword added