14.2.16 Binary I/O

Octave can read and write binary data using the functions fread and fwrite, which are patterned after the standard C functions with the same names. They are able to automatically swap the byte order of integer data and convert among the supported floating point formats as the data are read.

: val = fread (fid)

: val = fread (fid, size)

: val = fread (fid, size, precision)

: val = fread (fid, size, precision, skip)

: val = fread (fid, size, precision, skip, arch)

: [val, count] = fread (…)

Read binary data from the file specified by the file descriptor fid.

The optional argument size specifies the amount of data to read and may be one of

Inf

Read as much as possible, returning a column vector.

nr

Read up to nr elements, returning a column vector.

[nr, Inf]

Read as much as possible, returning a matrix with nr rows. If the number of elements read is not an exact multiple of nr, the last column is padded with zeros.

[nr, nc]

Read up to nr * nc elements, returning a matrix with nr rows. If the number of elements read is not an exact multiple of nr, the last column is padded with zeros.

If size is omitted, a value of Inf is assumed.

The optional argument precision is a string specifying the type of data to read and may be one of

"uint8" (default)

8-bit unsigned integer.

"int8"

"integer*1"

8-bit signed integer.

"uint16"

"ushort"

"unsigned short"

16-bit unsigned integer.

"int16"

"integer*2"

"short"

16-bit signed integer.

"uint"

"uint32"

"unsigned int"

"ulong"

"unsigned long"

32-bit unsigned integer.

"int"

"int32"

"integer*4"

"long"

32-bit signed integer.

"uint64"

64-bit unsigned integer.

"int64"

"integer*8"

64-bit signed integer.

"single"

"float"

"float32"

"real*4"

32-bit floating point number.

"double"

"float64"

"real*8"

64-bit floating point number.

"char"

"char*1"

8-bit single character.

"uchar"

"unsigned char"

8-bit unsigned character.

"schar"

"signed char"

8-bit signed character.

The default precision is "uint8".

The precision argument may also specify an optional repeat count. For example, ‘32*single’ causes fread to read a block of 32 single precision floating point numbers. Reading in blocks is useful in combination with the skip argument.

The precision argument may also specify a type conversion. For example, ‘int16=>int32’ causes fread to read 16-bit integer values and return an array of 32-bit integer values. By default, fread returns a double precision array. The special form ‘*TYPE’ is shorthand for ‘TYPE=>TYPE’.

The conversion and repeat counts may be combined. For example, the specification ‘32*single=>single’ causes fread to read blocks of single precision floating point values and return an array of single precision values instead of the default array of double precision values.

The optional argument skip specifies the number of bytes to skip after each element (or block of elements) is read. If it is not specified, a value of 0 is assumed. If the final block read is not complete, the final skip is omitted. For example,

fread (f, 10, "3*single=>single", 8)

will omit the final 8-byte skip because the last read will not be a complete block of 3 values.

The optional argument arch is a string specifying the data format for the file. Valid values are

"native" or "n"

The format of the current machine.

"ieee-be" or "b"

IEEE big endian.

"ieee-le" or "l"

IEEE little endian.

If no arch is given the value used in the call to fopen which created the file descriptor is used. Otherwise, the value specified with fread overrides that of fopen and determines the data format.

The output argument val contains the data read from the file.

The optional return value count contains the number of elements read.

See also: fwrite, fgets, fgetl, fscanf, fopen.

: fwrite (fid, data)

: fwrite (fid, data, precision)

: fwrite (fid, data, precision, skip)

: fwrite (fid, data, precision, skip, arch)

: count = fwrite (…)

Write data in binary form to the file specified by the file descriptor fid, returning the number of values count successfully written to the file.

The argument data is a matrix of values that are to be written to the file. The values are extracted in column-major order.

The remaining arguments precision, skip, and arch are optional, and are interpreted as described for fread.

The behavior of fwrite is undefined if the values in data are too large to fit in the specified precision.

See also: fread, fputs, fprintf, fopen.