What is size_t in C?

From Wikipedia:

According to the 1999 ISO C standard (C99), size_t is an unsigned integer type of at least 16 bit (see sections 7.17 and 7.18.3). size_tis an unsigned data type defined by several C/C++ standards, e.g. the C99 ISO/IEC 9899 standard, that is defined in stddef.h.1 It can be further imported by inclusion of stdlib.h as this file internally sub includes stddef.h. This type is used to represent the size of an object. Library functions that take or return sizes expect them to be of type or have the return type of size_t. Further, the most frequently used compiler-based operator sizeof should evaluate to a constant value that is compatible with size_t.

As an implication, size_t is a type guaranteed to hold any array index.

---

size_t is an unsigned type. So, it cannot represent any negative values(<0). You use it when you are counting something, and are sure that it cannot be negative. For example, strlen() returns a size_t because the length of a string has to be at least 0.

In your example, if your loop index is going to be always greater than 0, it might make sense to use size_t, or any other unsigned data type.

When you use a size_t object, you have to make sure that in all the contexts it is used, including arithmetic, you want non-negative values. For example, let's say you have:

size_t s1 = strlen(str1);
size_t s2 = strlen(str2);

and you want to find the difference of the lengths of str2 and str1. You cannot do:

int diff = s2 - s1; /* bad */

This is because the value assigned to diff is always going to be a positive number, even when s2 < s1, because the calculation is done with unsigned types. In this case, depending upon what your use case is, you might be better off using int (or long long) for s1 and s2.

There are some functions in C/POSIX that could/should use size_t, but don't because of historical reasons. For example, the second parameter to fgets should ideally be size_t, but is int.

---

size_t is a type that can hold any array index.

Depending on the implementation, it can be any of:

unsigned char

unsigned short

unsigned int

unsigned long

unsigned long long

Here's how size_t is defined in stddef.h of my machine:

typedef unsigned long size_t;

---

If you are the empirical type,

echo | gcc -E -xc -include 'stddef.h' - | grep size_t

Output for Ubuntu 14.04 64-bit GCC 4.8:

typedef long unsigned int size_t;

Note that stddef.h is provided by GCC and not glibc under src/gcc/ginclude/stddef.h in GCC 4.2.

Interesting C99 appearances

malloc takes size_t as an argument, so it determines the maximum size that may be allocated. And since it is also returned by sizeof, I think it limits the maximum size of any array. See also: What is the maximum size of an array in C?

---

The manpage for types.h says:

size_t shall be an unsigned integer type

---

To go into why size_t needed to exist and how we got here:

In pragmatic terms, size_t and ptrdiff_t are guaranteed to be 64 bits wide on a 64-bit implementation, 32 bits wide on a 32-bit implementation, and so on. They could not force any existing type to mean that, on every compiler, without breaking legacy code.

A size_t or ptrdiff_t is not necessarily the same as an intptr_t or uintptr_t. They were different on certain architectures that were still in use when size_t and ptrdiff_t were added to the Standard in the late 1980s, and becoming obsolete when C99 added many new types but not gone yet (such as 16-bit Windows). The x86 in 16-bit protected mode had a segmented memory where the largest possible array or structure could be only 65,536 bytes in size, but a far pointer needed to be 32 bits wide, wider than the registers. On those, intptr_t would have been 32 bits wide but size_t and ptrdiff_t could be 16 bits wide and fit in a register. And who knew what kind of operating system might be written in the future? In theory, the i386 architecture offers a 32-bit segmentation model with 48-bit pointers that no operating system has ever actually used.

The type of a memory offset could not be long because far too much legacy code assumes that long is exactly 32 bits wide. This assumption was even built into the UNIX and Windows APIs. Unfortunately, a lot of other legacy code also assumed that a long is wide enough to hold a pointer, a file offset, the number of seconds that have elapsed since 1970, and so on. POSIX now provides a standardized way to force the latter assumption to be true instead of the former, but neither is a portable assumption to make.

It couldn’t be int because only a tiny handful of compilers in the ’90s made int 64 bits wide. Then they really got weird by keeping long 32 bits wide. The next revision of the Standard declared it illegal for int to be wider than long, but int is still 32 bits wide on most 64-bit systems.

It couldn’t be long long int, which anyway was added later, since that was created to be at least 64 bits wide even on 32-bit systems.

So, a new type was needed. Even if it weren’t, all those other types meant something other than an offset within an array or object. And if there was one lesson from the fiasco of 32-to-64-bit migration, it was to be specific about what properties a type needed to have, and not use one that meant different things in different programs.

---

Since nobody has yet mentioned it, the primary linguistic significance of size_t is that the sizeof operator returns a value of that type. Likewise, the primary significance of ptrdiff_t is that subtracting one pointer from another will yield a value of that type. Library functions that accept it do so because it will allow such functions to work with objects whose size exceeds UINT_MAX on systems where such objects could exist, without forcing callers to waste code passing a value larger than "unsigned int" on systems where the larger type would suffice for all possible objects.

---

size_t and int are not interchangeable. For instance on 64-bit Linux size_t is 64-bit in size (i.e. sizeof(void*)) but int is 32-bit.

Also note that size_t is unsigned. If you need signed version then there is ssize_t on some platforms and it would be more relevant to your example.

As a general rule I would suggest using int for most general cases and only use size_t/ssize_t when there is a specific need for it (with mmap() for example).

---

size_t is an unsigned integer data type which can assign only 0 and greater than 0 integer values. It measure bytes of any object's size and is returned by sizeof operator.

const is the syntax representation of size_t, but without const you can run the program.

const size_t number;

size_t regularly used for array indexing and loop counting. If the compiler is 32-bit it would work on unsigned int. If the compiler is 64-bit it would work on unsigned long long int also. There for maximum size of size_t depending on the compiler type.

size_t already defined in the <stdio.h> header file, but it can also be defined by the <stddef.h>, <stdlib.h>, <string.h>, <time.h>, and <wchar.h> headers.

Example (with const)

#include <stdio.h>

int main()
{
    const size_t value = 200;
    size_t i;
    int arr[value];

    for (i = 0 ; i < value ; ++i)
    {
        arr[i] = i;
    }

    size_t size = sizeof(arr);
    printf("size = %zu\n", size);
}

Output: size = 800

Example (without const)

#include <stdio.h>

int main()
{
    size_t value = 200;
    size_t i;
    int arr[value];

    for (i = 0; i < value; ++i)
    {
        arr[i] = i;
    }

    size_t size = sizeof(arr);
    printf("size = %zu\n", size);
}

Output: size = 800

---

size_t is unsigned integer data type. On systems using the GNU C Library, this will be unsigned int or unsigned long int. size_t is commonly used for array indexing and loop counting.

---

In general, if you are starting at 0 and going upward, always use an unsigned type to avoid an overflow taking you into a negative value situation. This is critically important, because if your array bounds happens to be less than the max of your loop, but your loop max happens to be greater than the max of your type, you will wrap around negative and you may experience a segmentation fault (SIGSEGV). So, in general, never use int for a loop starting at 0 and going upwards. Use an unsigned.

---

size_t is a typedef which is used to represent the size of any object in bytes. (Typedefs are used to create an additional name/alias for another data type, but does not create a new type.)

Find it defined in stddef.h as follows:

typedef unsigned long long size_t;

size_t is also defined in the <stdio.h>.

size_t is used as the return type by the sizeof operator.

Use size_t, in conjunction with sizeof, to define the data type of the array size argument as follows:

#include <stdio.h>

void disp_ary(int *ary, size_t ary_size)
{
    for (int i = 0; i < ary_size; i++)
    {
        printf("%d ", ary[i]);
    }
}
 
int main(void)
{
    int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
    int ary_size = sizeof(arr)/sizeof(int);
    disp_ary(arr, ary_size);
    return 0;
}

size_t is guaranteed to be big enough to contain the size of the biggest object the host system can handle.

Note that an array's size limitation is really a factor the system's stack size limitations where this code is compiled and executed. You should be able to adjust the stack size at link time (see ld commands's --stack-size parameter).

To give you an idea of approximate stack sizes:

4K on an embedded device

1M on Win10

7.4M on Linux

Many C library functions like malloc, memcpy and strlen declare their arguments and return type as size_t.

size_t affords the programmer with the ability to deal with different types, by adding/subtracting the number of elements required instead of using the offset in bytes.

Let's get a deeper appreciate for what size_t can do for us by examining its usage in pointer arithmetic operations of a C string and an integer array:

Here's an example using a C string:

const char* reverse(char *orig)
{
  size_t len = strlen(orig);
  char *rev = orig + len - 1;
  while (rev >= orig)
  {
    printf("%c", *rev);
    rev = rev - 1;  // <= See below
  }
  return rev;
}

int main() {
  char *string = "123";
  printf("%c", reverse(string));
}
// Output: 321

0x7ff626939004 "123"  // <= orig
0x7ff626939006 "3"    // <= rev - 1 of 3
0x7ff626939005 "23"   // <= rev - 2 of 3
0x7ff626939004 "123"  // <= rev - 3 of 3
0x7ff6aade9003 ""     // <= rev is indeterminant. This can be exploited as an out of bounds bug to read memory contents that this program has no business reading.

That's not very helpful in understanding the benefits of using size_t since a character is one byte, regardless of your architecture.

When we're dealing with numerical types, size_t becomes very beneficial.

size_t type is like an integer with benefits that can hold a physical memory address; That address changes its size according to the type of platform in which it is executed.

Here's how we can leverage sizeof and size_t when passing an array of ints:

void print_reverse(int *orig, size_t ary_size)
{
  int *rev = orig + ary_size - 1;
  while (rev >= orig)
  {
    printf("%i", *rev);
    rev = rev - 1;
  }
}

int main()
{
  int nums[] = {1, 2, 3};
  print_reverse(nums, sizeof(nums)/sizeof(*nums));

  return 0;
}

0x617d3ffb44 1  // <= orig
0x617d3ffb4c 3  // <= rev - 1 of 3
0x617d3ffb48 2  // <= rev - 2 of 3
0x617d3ffb44 1  // <= rev - 3 of 3

Above, we see than an int takes 4 bytes (and since there are 8 bits per byte, an int occupies 32 bits).

If we were to create an array of longs we'd discover that a long takes 64 bits on a linux64 operating system, but only 32 bits on a Win64 system. Hence, using t_size, will save a lot of coding and potential bugs, especially when running C code that performs Address Arithmetic on different architectures.

So the moral of this story is "Use size_t and let your C-compiler do the error-prone work of pointer arithmetic."

---

size_t or any unsigned type might be seen used as loop variable as loop variables are typically greater than or equal to 0.

When we use a size_t object, we have to make sure that in all the contexts it is used, including arithmetic, we want only non-negative values. For instance, following program would definitely give the unexpected result:

// C program to demonstrate that size_t or
// any unsigned int type should be used 
// carefully when used in a loop

#include<stdio.h>
int main()
{
const size_t N = 10;
int a[N];

// This is fine
for (size_t n = 0; n < N; ++n)
a[n] = n;

// But reverse cycles are tricky for unsigned 
// types as can lead to infinite loop
for (size_t n = N-1; n >= 0; --n)
printf("%d ", a[n]);
}

Output
Infinite loop and then segmentation fault

---

This is a platform-specific typedef. For example, on a particular machine, it might be unsigned int or unsigned long. You should use this definition for more portability of your code.

---

From my understanding, size_t is an unsigned integer whose bit size is large enough to hold a pointer of the native architecture.

So:

sizeof(size_t) >= sizeof(void*)

---