What is size_t data type in C language?

size_t is an unsigned integer data type which is defined in various header files like:

<stddef.h>, <stdio.h>, <stdlib.h>, <string.h>, <time.h>, <wchar.h>

It’s a type which is used to represent sizes of objects in bytes, hence it can be returned by the sizeof operator. So it is guaranteed to be big enough to contain the size of the biggest object that system can handle. Basically the maximum permissible size is dependent on the compiler. If compiler is 32 bit then it is nothing other than typedef(i.e., alias) for unsigned int but if compiler is 64 bit then it would be a typedef for unsigned long long. In short size_t is never negative.
Therefore many library function of C language declare their argument and return type as size_t like malloc, memcpy and strlen. For instance,

// Declaration of various library function
// Here argument of n refers maximum blocks that can be
// allocated which is guaranteed to be non-negative
void *malloc(size_t n);
// While copying 'n' bytes from 's2' to 's1'
// n must be non-negative integer
void *memcpy(void *s1, void const *s2, size_t n);
// strlen() use size_t because lengt of string has
// to be atleast 0
size_t strlen(char const *s);

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

Note: 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
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]);

Infinite loop and then segmentation fauilt

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