I have a large binary data file with some ASCII formatted metadata header at the beginning. To read this header, I use 'string-oriented' read functions like fgetl(~), fscanf(~, '%s', ~) or fread(~, ~, '*char'). In Matlab versions prior to R2020a (I have R2014b and R2019b) this worked just fine, however in the R2020a something changed.
Now the very first, but only the first, attempt to read any string from the file will use extensive amount of memory and freezes the whole thing. I have a guess that Matlab is trying to read the whole file into memory. And in my case the file itself is larger than available RAM which probably cause the freezing.
Here what I do:
fd = fopen('file.name', 'r');
arr1 = fread(fd, 1);
arr2 = fread(fd, 1);
fclose(fd);
fd = fopen('file.name', 'r');
arr1 = fread(fd, 1);
arr2 = fread(fd, 1, '*char');
arr3 = fread(fd, 1, '*char');
fclose(fd);
1) It does not matter what part of the file I read.
2) All numeric type returning read functions are always fast.
3) The first string returning read function is always slow and does not matter what function I use (as long as it returns string).
4) All successive string reads are as fast as numeric ones.
5) Once the read function returns the string the memory is released.
6) File position pointer is always at expected position (does not move to end of the file).
7) It does not matter if the file is opened in text or binary mode.
8) The issue is presented both on Windows and Linux.
Any idea?
