the Problem of Distribution memory of MATLAB struct？

clear all 
clc
testStruct.a=int8(10);
a=int8(10)
whos testStruct
whos a

the result is：

a =

     10

    Name            Size            Bytes  Class     Attributes

    testStruct      1x1               125  struct              

    Name      Size            Bytes  Class    Attributes

    a         1x1                 1  int8

Variable a takes up 8 bytes, however, testStruct.a takes up 125 bytes。why？ my email:[e-mail]，think you very much。

[James Tursa]

The whos result is misleading for this comparison. For the following discussion the numbers cited assume a 64-bit system and that the overhead for a MATLAB variable is 124 bytes (this is approx).

First, every MATLAB variable has an overhead structure called an mxArray that contains information such as class, size, data pointers, sharing pointers, etc. So for variable "a" above, the data for "a" is only showing 1 byte, but the mxArray for this variable is taking up an additional 124 bytes of memory. So the total amount of memory actually used by variable "a" is 125 bytes. (actually probably more due to memory alignment effects)

For the struct testStruct, it too has this mxArray overhead. So there is 124 bytes right there. Then the data area of this struct is actually mxArray pointers, another 8 bytes. Then there is memory for the field name "a", so another 2 bytes minimum for that. Then the data itself is actually a MATLAB variable, so the overhead and data for that come to another 125 bytes. So the total for the struct testStruct is actually 124 + 8 + 2 + 125 = 259 bytes.

Bottom line is that you can't really compare the whos result of structs to numeric variables since whos does not tell you the whole memory story, it only reports on the data areas.

Answer 1

[Walter Roberson]

The structure as a whole takes up 125 bytes, not just the .a field of the structure.

The structure includes the overhead to list all of the field names and the types and sizes of each of the fields.

[deng]

MATLAB how to achieve the streaming file read and write similar to the C language, you very much think, email:[e-mail]

Is there similar to the structure of the C language by matlab? the structure itself is not occupied by the definition of memory byte. Only elements of the structure occupy byte. I want to read the function of the actual flow file, for example (using C language):

if true
 TagRFHeader struct
{
// Offset in bytes
Short rh_tag; / / if 0xfnff for  0x00ff header, old file 00
Short rh_data; / / constant 1024 (obsolete: obsolete obsolete) 02
Short rh_nsamp; / / samples per scan 04
Short rh_bits per data word; / / bits (8 or 16) 06
Short rh_zero; / / Offset (0x80 or 0x8000 depends on rh_bits) 08
Float rhf_sps; / / scans per second 10
Float rhf_spm; / / scans per meter 14
Float rhf_mpm; / / meters per mark 18
Float rhf_position; / / position (NS) 22
Float rhf_range; / / range (NS) 26
Short rh_npass num of passes for 2-D; / / files 30
Float rhb_cdt; / / Creation date & time 32
Float rhb_mdt; / / Last modification date & time 36
Short rh_rgain offset to range gain; / / function 40
Short rh_nrgain size of range gain; / / function 42
Short rh_text; / / offset to text 44
Short rh_ntext; / / size of text 46
Short rh_proc; offset to processing / / history 48
Short rh_nproc; size of processing / / history 50
Short rh_nchan; / / number of channels 52
Float rhf_epsr; / / average dielectric constant 54
Float rhf_top; / / position in meters 58
Float rhf_depth; / / range in meters 62
Double rh_coordX; / / X coordinates 66
Float rhf_servo_level; / / gain servo level 74
Char reserved[3]; / / reserved 78
Char rh_accomp; / / Ant Conf component 81
Short rh_sconfig; / / setup config number 82
Short rh_spp; / / scans per pass 84
Short rh_linenum; / / Line Number 86
Double rh_coordY; / / Y coordinates 88
Char rh_lineorder:4 / / 96;
Char rh_slicetype:4 / / 96;
Char rh_dtype / / 97;
Char rh_antname[14]; / / Antenna name 98
Char rh_pass0TX:4; / / Activ Transmit mask 112
Char rh_pass1TX:4; / / Activ Transmit mask 112
Char rh_version:3; / / 1 - No 2 - GPS 113 GPS;
Char rh_system:5; for SIR3000 113 / 3
Char rh_name[12]; / / Initial File Name 114
Short rh_chksum; / / checksum for header 126
} sirFileHeader;

end

/ * Read the file by c language* /

if true
Main void (argc int, argv char**) {
*fsir *fsegy, FILE;
I int = 0;
If (argc < 3) {
Printf ("use: sir2segy <input Sir Please file> <output segy file>\n");
Exit (0);
}
FSIR = fopen (argv[1], "RB");
If (FSIR = = NULL) {
Printf ("Sir file doesn't exist the, check\n please");
Exit (0);
}
}
Fseek (FSIR, 0L, SEEK_SET); / / to move the pointer to the file header
FREAD (&sirFileHeader, 128,1, FSIR); / / read 128 bytes from the file
//......
// The following omitted
end

Question: MATLAB how to achieve the streaming file read and write similar to the C language, you very much think, email:[e-mail]

[Walter Roberson]

See memmapfile()

[deng]

Thank you very much, see you give the answer is very inspired, is there a function as C language structure when the binary stream file to be wirte.

if true
 m = memmapfile('mybinary.bin',...
'Format',{'uint16',[50 1],'pressure';...
'uint16',[50,1],'temperature';...
'double',[5,10],'volume'},'Repeat',1)

end

Following the correct way of writing:

if true
    fileID = fopen('outputfile1.bin','w');
fwrite(fileID,m.Data.pressure,'uint16');
fwrite(fileID,m.Data.temperature,'uint16');
fwrite(fileID,m.Ddata.volume.temperature,'uint16');
fclose(fileID);

end

But in the case of very many variables this kind of writing is very troublesome, in the C language can use the structure to achieve a number of variables to write, for example

if true
  % code
end

if true
   TagRFHeader struct
{
// Offset in bytes
Short rh_tag; / / if 0xfnff for  0x00ff header, old file 00
Short rh_data; / / constant 1024 (obsolete: obsolete obsolete) 02
Short rh_nsamp; / / samples per scan 04
Short rh_bits per data word; / / bits (8 or 16) 06
Short rh_zero; / / Offset (0x80 or 0x8000 depends on rh_bits) 08
Float rhf_sps; / / scans per second 10
Float rhf_spm; / / scans per meter 14
Float rhf_mpm; / / meters per mark 18
Float rhf_position; / / position (NS) 22
Float rhf_range; / / range (NS) 26
}sirFileHeader;

end

Write file statement by C language

if true
  fwrite(sirFileHeader,30,1,fileout)
end

it is very simple to write many variables to the file by C language. Is there a function that is similar C language on writing stream file in matlab.

[Walter Roberson]

memmapfile() can be used for writing files as well as for reading files.

There is no provided MATLAB function that takes a structure memory or structure array and writes it out. You can write such a function. Remember that you need to write it recursively, since C structures and MATLAB structures can have substructures.

Also remember that in MATLAB, strings are character vectors whose length is variable, but when you write out C structures, you need to write out a fixed length and you need to have a NUL (binary 0) after the last used character. Be careful because if you try to embed NUL characters in your MATLAB string, char(0), operations involving formatting might consider that to be the end of the string.

[deng]

I see.Think you very much.

[Walter Roberson]

deng comments to my original Answer:

hello, problem of memmapfile function. think you very muchu. http://www.mathworks.com/matlabcentral/answers/293260-problem-of-memmapfile-function