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Mozna tohle:

fcntl(2):

F_FREESP

Free storage space associated with a section of the
ordinary file fildes. The section is specified by a
variable of data type struct flock pointed to by arg.
The data type struct flock is defined in the
header (see fcntl.h(3HEAD)) and is described below. Note
that all file systems might not support all possible
variations of F_FREESP arguments. In particular, many
file systems allow space to be freed only at the end of
a file.

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