Manual Pages  — sizeof

sizeof operator – yield the storage size of the given operand

SYNTAX DESCRIPTION EXAMPLES RESULT NOTES SEE ALSO STANDARDS AUTHORS

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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.

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:
".Li sizeof(struct flex)" ".Sy Result (ILP32)" ".Sy Result (LP64)"

Object or type Ta Sy Result (ILP32) Ta Sy Result (LP64)

sizeof(char) Ta 1 Ta 1

sizeof(int) Ta 4 Ta 4

sizeof(long) Ta 4 Ta 8

sizeof(float) Ta 4 Ta 4

sizeof(double) Ta 8 Ta 8

sizeof(char *) Ta 4 Ta 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' };

".Li sizeof(struct flex)" ".Sy Result"

Object or type Ta Sy Result

sizeof(DATA) Ta 11

sizeof(buf1) Ta 4

sizeof(buf2) Ta 1024

sizeof(buf3) Ta 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;
};

".Li sizeof(struct flex)" ".Sy Result (ILP32) " ".Sy Result (LP64)"

Object or type Ta Sy Result (ILP32) Ta Sy Result (LP64)

sizeof(struct s1) Ta 1 Ta 1

sizeof(struct s2) Ta 8 Ta 16

sizeof(struct s3) Ta 12 Ta 16

sizeof(struct s4) Ta 12 Ta 16

sizeof(struct s5) Ta 36 Ta 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[];
}

".Li sizeof(struct flex)" ".Sy Result (ILP32) " ".Sy Result (LP64)"

Object or type Ta Sy Result (ILP32) Ta Sy Result (LP64)

sizeof(struct flex) Ta 8 Ta 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.

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

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 , 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 conforms to -ansiC.

Handling of flexible array members in structures conforms to ISO/IEC 9899:1999 ("ISO C99").

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