Re: strcpy(3) examples use of sizeof

[Jan Schaumann <jschauma%netmeister.org@localhost>]

David Holland <dholland-docs%netbsd.org@localhost> wrote:
> On Sat, Dec 10, 2022 at 05:24:24PM -0500, Jan Schaumann wrote:
>  > Martin Husemann <martin%duskware.de@localhost> wrote:
>  > How do you feel about adding a separate manual page
>  > for the sizeof operator?  (See attached proposal,
>  > though my mandoc is not very good and likely needs
>  > clean up.)
> 
> some kibitzing on the page itself, since I'm not by any means
> convinced this would be a bad page to have in the system:

Thanks for your feedback.

This is the version that was added to FreeBSD:
https://cgit.freebsd.org/src/tree/share/man/man7/sizeof.7

If we're interested in adding it, then we can base it
off that version, which has seen a bunch of edits
(including at least some that you mentioned) since the
discussion here.

That version also has at least one FreeBSD specific
reference to their arch(7) page, which we'd want to
remove.

The attached version would then be a proposal based
off the one imported into FreeBSD.

-Jan

.\"
.\" Copyright (C) 2022 Jan Schaumann <jschauma%netmeister.org@localhost>.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd December 26, 2022
.Dt sizeof 7
.Os
.Sh NAME
.Nm sizeof
operator
.Nd yield the storage size of the given operand
.Sh SYNTAX
.Nm Vt ( type )
.br
.Nm Vt expression
.Sh DESCRIPTION
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.
defined by corresponding Application Binary Interface (ABI)
data models.
It may be necessary or useful for a program to be able
to determine the storage size of a data type or object
to account for the platform specifics.
.Pp
The unary
.Nm
operator yields the storage size of an expression or
data type in
.Em char sized units
(C language bytes).
As a result,
.Ql sizeof(char)
is defined to be 1.
(The number of bits per
.Vt char
is given by the
.Dv CHAR_BIT
definition in the
.In limits.h
header; many systems also provide the "number of bits
per byte" definition as
.Dv NBBY
in the
.In sys/param.h
header.)
.Sh EXAMPLES
Different platforms may use different data models.
For example, systems on which integers, longs, and
pointers are using 32 bits (e.g., i386) are referred
to as using the "ILP32" data model, systems using
64 bit longs and pointers (e.g., amd64 / x86_64)
as the "LP64" data model.
.Pp
The following examples illustrate the possible results
of calling
.Nm
on an ILP32 vs. an LP64 system:
.Pp
When applied to a simple variable or data type,
.Nm
returns the storage size of the data type of the object:
.Bl -column -offset indent \
    ".Li sizeof(struct flex)" ".Sy Result (ILP32)" ".Sy Result (LP64)"
.It Sy Object or type \
 Ta Sy Result (ILP32) \
 Ta Sy Result (LP64)
.It Li sizeof(char) \
 Ta 1 \
 Ta 1
.It Li sizeof(int) \
 Ta 4 \
 Ta 4
.It Li sizeof(long) \
 Ta 4 \
 Ta 8
.It Li sizeof(float) \
 Ta 4 \
 Ta 4
.It Li sizeof(double) \
 Ta 8 \
 Ta 8
.It Li sizeof(char *) \
 Ta 4 \
 Ta 8
.El
.Pp
For initialized data or uninitialized arrays of a
fixed size known at compile time,
.Nm
will return the actual storage size:
.Bd -literal -offset indent
#define DATA "1234567890"
char buf1[] = "abc";
char buf2[1024];
char buf3[1024] = { 'a', 'b', 'c' };
.Ed
.Bl -column -offset indent \
    ".Li sizeof(struct flex)" ".Sy Result"
.It Sy Object or type \
 Ta Sy Result
.It Li sizeof(DATA) \
 Ta 11
.It Li sizeof(buf1) \
 Ta 4
.It Li sizeof(buf2) \
 Ta 1024
.It Li sizeof(buf3) \
 Ta 1024
.El
.Pp
The examples above are the same for ILP32 and LP64
platforms, as they are based on character units.
.Pp
When applied to a struct or union,
.Nm
returns the total number of bytes in the object,
including any internal or trailing padding used to
align the object in memory.
This result may thus be larger than if the storage
size of each individual member had been added:
.Bd -literal -offset indent
struct s1 {
        char c;
};

struct s2 {
        char *s;
        int i;
};

struct s3 {
        char *s;
        int i;
        int j;
};

struct s4 {
	int i;
	uint64_t i64;
};

struct s5 {
        struct s1 a;
        struct s2 b;
        struct s3 c;
        struct s4 d;
};
.Ed
.Bl -column -offset indent \
    ".Li sizeof(struct flex)" ".Sy Result (ILP32)  " ".Sy Result (LP64)"
.It Sy Object or type \
 Ta Sy Result (ILP32) \
 Ta Sy Result (LP64)
.It Li sizeof(struct s1) \
 Ta 1 \
 Ta 1
.It Li sizeof(struct s2) \
 Ta 8 \
 Ta 16
.It Li sizeof(struct s3) \
 Ta 12 \
 Ta 16
.It Li sizeof(struct s4) \
 Ta 12 \
 Ta 16
.It Li sizeof(struct s5) \
 Ta 36  \
 Ta 56
.El
.Pp
When applied to a struct containing a flexible array
member,
.Nm
returns the size of the struct
.Em without
the array, although again possibly including any
padding the compiler deemed appropriate:
.Bd -literal -offset indent
struct flex {
        char c;
        long b;
        char array[];
}
.Ed
.Bl -column -offset indent \
    ".Li sizeof(struct flex)" ".Sy Result (ILP32)  " ".Sy Result (LP64)"
.It Sy Object or type \
 Ta Sy Result (ILP32) \
 Ta Sy Result (LP64)
.It Li sizeof(struct flex) \
 Ta 8 \
 Ta 16
.El
.Pp
One of the more common uses of the
.Nm
operator is to determine the correct amount of memory
to allocate:
.Bd -literal -offset indent
int *nums = calloc(512, sizeof(int));
.Ed
.Pp
The
.Nm
operator can be used to calculate the number of
elements in an array by dividing the size of the array
by the size of one of its elements:
.Bd -literal -offset indent
int nums[] = { 1, 2, 3, 4, 5 };
const int howmany = sizeof(nums) / sizeof(nums[0]);
.Ed
.Pp
Many systems provide this shortcut as the macro
.Dv ntimes()
via the
.In sys/param.h
header file.
.Sh RESULT
The result of the
.Nm
operator is an unsigned integer type, defined in the
.Dv stddef.h
header as a
.Vt size_t .
.Sh NOTES
It is a common mistake to apply
.Nm
to a dynamically allocated array:
.Bd -literal -offset indent
char *buf;
if ((buf = malloc(BUFSIZ)) == NULL) {
        perror("malloc");
}
/* Warning: wrong! */
(void)snprintf(buf, sizeof(buf), fmt, data);
.Ed
.Pp
In that case, the operator will return the storage
size of the pointer (
.Ql sizeof(char *)
), not the
allocated memory.
.Pp
.Nm
determines the
.Ev size
of the result of the expression given, but
.Em does not
evaluate the expression:
.Bd -literal -offset indent
int a = 42;
printf("%ld - %d\\n", sizeof(a = 10), a); /* Result: "4 - 42" */
.Ed
.Pp
Since it is evaluated by the compiler and not the
preprocessor, the
.Nm
operator cannot be used in a preprocessor expression.
.Sh SEE ALSO
.Xr arch 7 ,
.Xr operator 7
.Sh STANDARDS
The
.Nm
operator conforms to
.St -ansiC .
.Pp
Handling of flexible array members in structures
conforms to
.St -isoC-99 .
.Sh AUTHORS
This manual page was written by
.An Jan Schaumann Aq Mt jschauma%netmeister.org@localhost .