octave: Opening and Closing Files
14.2.1 Opening and Closing Files
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When reading data from a file it must be opened for reading first, and
likewise when writing to a file. The ‘fopen’ function returns a pointer
to an open file that is ready to be read or written. Once all data has
been read from or written to the opened file it should be closed. The
‘fclose’ function does this. The following code illustrates the basic
pattern for writing to a file, but a very similar pattern is used when
reading a file.
filename = "myfile.txt";
fid = fopen (filename, "w");
# Do the actual I/O here...
fclose (fid);
-- : FID = fopen (NAME)
-- : FID = fopen (NAME, MODE)
-- : FID = fopen (NAME, MODE, ARCH)
-- : [FID, MSG] = fopen (...)
-- : FID_LIST = fopen ("all")
-- : [FILE, MODE, ARCH] = fopen (FID)
Open a file for low-level I/O or query open files and file
descriptors.
The first form of the ‘fopen’ function opens the named file with
the specified mode (read-write, read-only, etc.) and architecture
interpretation (IEEE big endian, IEEE little endian, etc.), and
returns an integer value that may be used to refer to the file
later. If an error occurs, FID is set to −1 and MSG contains the
corresponding system error message. The MODE is a one or two
character string that specifies whether the file is to be opened
for reading, writing, or both.
The second form of the ‘fopen’ function returns a vector of file
ids corresponding to all the currently open files, excluding the
‘stdin’, ‘stdout’, and ‘stderr’ streams.
The third form of the ‘fopen’ function returns information about
the open file given its file id.
For example,
myfile = fopen ("splat.dat", "r", "ieee-le");
opens the file ‘splat.dat’ for reading. If necessary, binary
numeric values will be read assuming they are stored in IEEE format
with the least significant bit first, and then converted to the
native representation.
Opening a file that is already open simply opens it again and
returns a separate file id. It is not an error to open a file
several times, though writing to the same file through several
different file ids may produce unexpected results.
The possible values of MODE are
‘r’ (default)
Open a file for reading.
‘w’
Open a file for writing. The previous contents are discarded.
‘a’
Open or create a file for writing at the end of the file.
‘r+’
Open an existing file for reading and writing.
‘w+’
Open a file for reading or writing. The previous contents are
discarded.
‘a+’
Open or create a file for reading or writing at the end of the
file.
Append a "t" to the mode string to open the file in text mode or a
"b" to open in binary mode. On Windows systems, text mode reading
and writing automatically converts linefeeds to the appropriate
line end character for the system (carriage-return linefeed on
Windows). The default when no mode is specified is binary.
Additionally, you may append a "z" to the mode string to open a
gzipped file for reading or writing. For this to be successful,
you must also open the file in binary mode.
The parameter ARCH is a string specifying the default data format
for the file. Valid values for ARCH are:
"native" or "n" (default)
The format of the current machine.
"ieee-be" or "b"
IEEE big endian format.
"ieee-le" or "l"
IEEE little endian format.
However, conversions are currently only supported for ‘native’,
‘ieee-be’, and ‘ieee-le’ formats.
When opening a new file that does not yet exist, permissions will
be set to ‘0666 - UMASK’.
Compatibility Note: Octave opens files using buffered I/O. Small
writes are accumulated until an internal buffer is filled, and then
everything is written in a single operation. This is very
efficient and improves performance. MATLAB, however, opens files
using flushed I/O where every write operation is immediately
performed. If the write operation must be performed immediately
after data has been written then the write should be followed by a
call to ‘fflush’ to flush the internal buffer.
DONTPRINTYET See also: 
fclose XREFfclose, fgets XREFfgets, *noteDONTPRINTYET See also: fclose XREFfclose, fgets XREFfgets,
fgetl XREFfgetl, fscanf XREFfscanf, fread XREFfread,
DONTPRINTYET fputs XREFfputs, fdisp XREFfdisp, *notefprintf:
DONTPRINTYET fputs XREFfputs, fdisp XREFfdisp, fprintf
XREFfprintf, fwrite XREFfwrite, fskipl XREFfskipl,
DONTPRINTYET fseek XREFfseek, frewind XREFfrewind, *noteftell:
DONTPRINTYET DONTPRINTYET fseek XREFfseek, frewind XREFfrewind, ftell
XREFftell, feof XREFfeof, ferror XREFferror, *noteDONTPRINTYET DONTPRINTYET DONTPRINTYET fseek XREFfseek, frewind XREFfrewind, ftell
XREFftell, feof XREFfeof, ferror XREFferror,
fclear XREFfclear, fflush XREFfflush, *notefreport:
DONTPRINTYET DONTPRINTYET DONTPRINTYET fseek XREFfseek, frewind XREFfrewind, ftell
XREFftell, feof XREFfeof, ferror XREFferror,
fclear XREFfclear, fflush XREFfflush, freport
XREFfreport, umask XREFumask.
-- : fclose (FID)
-- : fclose ("all")
-- : STATUS = fclose ("all")
Close the file specified by the file descriptor FID.
If successful, ‘fclose’ returns 0, otherwise, it returns -1. The
second form of the ‘fclose’ call closes all open files except
‘stdin’, ‘stdout’, ‘stderr’, and any FIDs associated with gnuplot.
DONTPRINTYET See also: fopen XREFfopen, fflush XREFfflush, *noteDONTPRINTYET See also: fopen XREFfopen, fflush XREFfflush,
freport XREFfreport.
-- : is_valid_file_id (FID)
Return true if FID refers to an open file.
See also: freport XREFfreport, fopen XREFfopen.
Info Catalog
octave: C-Style I/O Functions
octave: Simple Output