uint16_t is unsigned 16-bit integer.
unsigned short int is unsigned short integer, but the size is implementation dependent. The standard only says it's at least 16-bit (i.e, minimum value of UINT_MAX is 65535). In practice, it usually is 16-bit, but you can't take that as guaranteed.
Note:
uint16_t.inttypes.h and stdint.h are both introduced in C99. If you are using C89, define your own type.uint16_t may not be provided in certain implementation(See reference below), but unsigned short int is always available.Reference: C11(ISO/IEC 9899:201x) §7.20 Integer types
For each type described herein that the implementation provides) shall declare that typedef name and define the associated macros. Conversely, for each type described herein that the implementation does not provide, shall not declare that typedef name nor shall it define the associated macros. An implementation shall provide those types described as ‘‘required’’, but need not provide any of the others (described as ‘optional’’).
uint16_t is guaranteed to be a unsigned integer that is 16 bits large
unsigned short int is guaranteed to be a unsigned short integer, where short integer is defined by the compiler (and potentially compiler flags) you are currently using. For most compilers for x86 hardware a short integer is 16 bits large.
Also note that per the ANSI C standard only the minimum size of 16 bits is defined, the maximum size is up to the developer of the compiler
Minimum Type Limits
Any compiler conforming to the Standard must also respect the following limits with respect to the range of values any particular type may accept. Note that these are lower limits: an implementation is free to exceed any or all of these. Note also that the minimum range for a char is dependent on whether or not a char is considered to be signed or unsigned.
Type Minimum Range
signed char -127 to +127 unsigned char 0 to 255 short int -32767 to +32767 unsigned short int 0 to 65535
Source: Stackoverflow.com