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sizeof(7): miscellaneous edits

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Suggested by:	pstef
Reviewed by:	imp, pstef (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D37683
This commit is contained in:
Konstantin Belousov 2022-12-13 16:07:32 +02:00
parent 57bee0817f
commit 9e0d976d95
1 changed files with 31 additions and 9 deletions
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40
share/man/man7/sizeof.7
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@ -34,17 +34,38 @@ operator
.br .br
.Nm Vt expression .Nm Vt expression
.Sh DESCRIPTION .Sh DESCRIPTION
The size of primitive data types in C may differ The
.Nm
operator yields the size of its operand.
The
.Nm
operator cannot be applied to incomplete types and expressions
with incomplete types (e.g.
.Vt void ,
or forward-defined
.Vt struct foo ),
and function types.
.Pp
The size of primitive (non-derived) data types in C may differ
across hardware platforms and implementations. across hardware platforms and implementations.
They are defined by corresponding Application Binary Interface (ABI)
specifications, see
.Xr arch 7
for details about ABI used by
.Fx .
It may be necessary or useful for a program to be able It may be necessary or useful for a program to be able
to determine the storage size of a data type or object. to determine the storage size of a data type or object
to account for the platform specifics.
.Pp .Pp
The unary The unary
.Nm .Nm
operator yields the storage size of an expression or operator yields the storage size of an expression or
data type in data type in
.Em char sized units . .Em char sized units
As a result, 'sizeof(char)' is always guaranteed to be 1. (C language bytes).
As a result,
.Ql sizeof(char)
is always guaranteed to be 1.
(The number of bits per (The number of bits per
.Vt char .Vt char
is given by the is given by the
@ -72,8 +93,7 @@ on an ILP32 vs. an LP64 system:
.Pp .Pp
When applied to a simple variable or data type, When applied to a simple variable or data type,
.Nm .Nm
returns the storage size of the data type of the returns the storage size of the data type of the object:
object:
.Bl -column -offset indent \ .Bl -column -offset indent \
".Li sizeof(struct flex)" ".Sy Result (ILP32)" ".Sy Result (LP64)" ".Li sizeof(struct flex)" ".Sy Result (ILP32)" ".Sy Result (LP64)"
.It Sy Object or type \ .It Sy Object or type \
@ -128,7 +148,7 @@ platforms, as they are based on character units.
.Pp .Pp
When applied to a struct or union, When applied to a struct or union,
.Nm .Nm
returns the total number of units in the object, returns the total number of bytes in the object,
including any internal or trailing padding used to including any internal or trailing padding used to
align the object in memory. align the object in memory.
This result may thus be larger than if the storage This result may thus be larger than if the storage
@ -251,8 +271,10 @@ if ((buf = malloc(BUFSIZ)) == NULL) {
.Ed .Ed
.Pp .Pp
In that case, the operator will return the storage In that case, the operator will return the storage
size of the pointer ('sizeof(char *)'), not the size of the pointer (
allocated memory! .Ql sizeof(char *)
), not the
allocated memory.
.Pp .Pp
.Nm .Nm
determines the determines the