FreeBSD Manual Pages
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 sizeof operator yields the size of its operand. The sizeof opera- tor cannot be applied to incomplete types and expressions with incom- plete types (e.g. void, or forward-defined struct foo ), and function types. The size of primitive (non-derived) data types in C may differ across hardware platforms and implementations. They are defined by corre- sponding Application Binary Interface (ABI) specifications, see arch(7) for details about ABI used by FreeBSD. 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. The unary sizeof operator yields the storage size of an expression or data type in char sized units (C language bytes). 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 Different platforms may use different data models. For example, sys- tems 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. 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 re- turns 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 ar- ray: 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. SEE ALSO arch(7), operator(7) STANDARDS The sizeof operator conforms to ANSI X3.159-1989 ("ANSI C89"). Handling of flexible array members in structures conforms to ISO/IEC 9899:1999 ("ISO C99"). AUTHORS This manual page was written by Jan Schaumann <jschauma@netmeister.org>. FreeBSD 15.0 December 12, 2022 sizeof(7)
NAME | SYNTAX | DESCRIPTION | EXAMPLES | RESULT | NOTES | SEE ALSO | STANDARDS | AUTHORS
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