sizeof(7)		Miscellaneous Information Manual	       sizeof(7)

NAME
     sizeof operator  yield the storage size of the given operand

SYNTAX
     sizeof (type)
     sizeof expression

DESCRIPTION
     The unary sizeof operator yields the storage size of an expression or data
     type in bytes.

DETAILS
     The size of data types in C (such as e.g., integers or pointers) may differ
     across hardware platforms and implementations.  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.

     As it may be necessary or useful for a program to be able to determine the
     storage size of a data type or object, sizeof yields that size in char
     sized units.  As a result, 'sizeof(char)' is always guaranteed to be 1.
     (The number of bits per char is given by the CHAR_BIT definition in the
     <limits.h> header; many systems also provide the "number of bits per byte"
     definition as NBBY in the <sys/param.h> header.)

EXAMPLES
     The following examples illustrate the possible results of calling sizeof on
     an ILP32 vs. an LP64 system:

     When applied to a simple variable or data type, sizeof returns the storage
     size of the data type of the object:

	   Object or type	      Result (ILP32)	    Result (LP64)
	   sizeof(char)		      1			    1
	   sizeof(int)		      4			    4
	   sizeof(long)		      4			    8
	   sizeof(float)	      4			    4
	   sizeof(double)	      8			    8
	   sizeof(char *)	      4			    8

     For initialized data or uninitialized arrays of a fixed size known at
     compile time, sizeof will return the correct storage size:

	   #define DATA "1234567890"
	   char buf1[] = "abc";
	   char buf2[1024];
	   char buf3[1024] = { 'a', 'b', 'c' };

	   Object or type	      Result
	   sizeof(DATA)		      11
	   sizeof(buf1)		      4
	   sizeof(buf2)		      1024
	   sizeof(buf3)		      1024

     The examples above are the same for ILP32 and LP64 platforms, as they are
     based on character units.

     When applied to a struct or union, sizeof 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:

	   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;
	   }

	   Object or type	      Result (ILP32)	      Result (LP64)
	   sizeof(struct s1)	      1			      1
	   sizeof(struct s2)	      8			      16
	   sizeof(struct s3)	      12		      16
	   sizeof(struct s4)	      12		      16
	   sizeof(struct s5)	      36		      56

     When applied to a struct containing a flexible array member, sizeof returns
     the size of the struct without the array, although again possibly including
     any padding the compiler deemed appropriate:

	   struct flex {
		   char c;
		   long b;
		   char array[];
	   }

	   Object or type	      Result (ILP32)	      Result (LP64)
	   sizeof(struct flex)	      8			      16

     One of the more common uses of the sizeof operator is to determine the
     correct amount of memory to allocate:

	   int *nums = calloc(512, sizeof(int));

     The sizeof 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:

	   int nums[] = { 1, 2, 3, 4, 5 };
	   const int howmany = sizeof(nums) / sizeof(nums[0]);

     Many systems provide this shortcut as the macro ntimes() via the
     sys/param.h header file.

RESULT
     The result of the sizeof operator is an unsigned integer type, defined in
     the stddef.h header as a size_t.

NOTES
     It is a common mistake to apply sizeof to a dynamically allocated array:

	   char *buf;
	   if ((buf = malloc(BUFSIZ)) == NULL) {
		   perror("malloc");
	   }
	   /* Warning: wrong! */
	   (void)strncat(buf, input, sizeof(buf) - 1);

     In that case, the operator will return the storage size of the pointer
     ('sizeof(char *)'), not the allocated memory.

     sizeof determines the size of the result of the expression given, but does
     not evaluate the expression:

	   int a = 42;
	   printf("%ld - %d\n", sizeof(a = 10), a); /* Result: "4 - 42" */

     Since it is evaluated by the compiler and not the preprocessor, the sizeof
     operator cannot be used in a preprocessor expression.

     The sizeof operator cannot be used on a bit-field object, a function type,
     or an incomplete type.

SEE ALSO
     arch(7), operator(7)

STANDARDS
     The sizeof operator conforms to ANSI X3.159-1989 (ANSIC89).

     Handling of flexible array members in structures conforms to ISO/IEC
     9899:1999 (ISOC99).

AUTHORS
     This manual page was written by Jan Schaumann <jschauma@netmeister.org>.

NetBSD 9.3			December 12, 2022		      NetBSD 9.3