Question

I need a function which will create a file with fixed size in linux. Function like truncate or fopen,fseek,fclose, is not a solution because, they will fill opened file with zeros, but it is not necessary and I have no time for this. So is there some function, which will only open a file with fixed length and not fill buffer?

Thanks in advance.

Answer 1

The system call truncate(2) doesn’t fill the file with zeros. It simply advances the file’s reported size and leaves holes in it.

When you read from it, you do get zeros, but that’s just a convenience of the OS.

The truncate() and ftruncate() functions cause the regular file named
by path or referenced by fd to be truncated to a size of precisely
length bytes.

If the file previously was shorter, it is extended, and the extended
part reads as null bytes (‘\0’).

About holes (from TLPI):

The existence of holes means that a file’s nominal size may be larger
than the amount of disk storage it utilizes (in some cases,
considerably larger).

Filesystems and holes:

Rather than allocate blocks of null bytes for the holes in a file, the
file system can just mark (with the value 0) appropriate pointers in
the i-node and in the indirect pointer blocks to indicate that they
don’t refer to actual disk blocks.

As Per Johansson notes, this is dependent of the filesystem.

Most native UNIX file systems support the concept of file holes, but
many nonnative file systems (e.g., Microsoft’s VFAT) do not. On a
file system that doesn’t support holes, explicit null bytes are
written to the file.