File Holes¶

A file hole is a portion of a regular file that contains NUL characters and is not stored in any data block on disk. Holes are a long‐standing feature of Unix files. For instance, the following Unix command creates a file in which the first bytes are a hole:

$ echo -n "X" | dd of=/tmp/hole bs=1024 seek=6
$

Now /tmp/hole has 6,145 characters (6,144 NUL characters plus an X character), yet the file occupies just one data block on disk.

File holes were introduced to avoid wasting disk space. They are used extensively by database applications and, more generally, by all applications that perform hashing on files.

See http://www.oreilly.com/catalog/linuxkernel2/chapter/ch17.pdf for more information.

Reading¶

The size of binary files is taken from the size of the file on the file system. If the file has holes these will read as blocks of NUL (zero) data, as there is no elegant way to detect Unix file holes. In general, you probably want to use the -unfill filter to find and remove large swathes of zero bytes.

Writing¶

In producing a binary file, srec_cat(1) honours the address information and places the data into the binary file at the addresses specified in the hex file. This usually results on holes in the file. Sometimes alarmingly large file sizes are reported as a result.

If you are on a brain‐dead operating system without file holes then there are going to be real data blocks containing real zero bytes, and consuming real amounts of disk space. Upgrade - I suggest Linux.

To make a file of the size you expect, use

to find the lowest address, then use

where n is the lowest address present in the foo.s19 file, as reported by srec_info(1). The negative offset serves to move the data down to have an origin of zero.