6.1.0860
C,
c-comma
CORE
 
( char -- )

Reserve space for one character in the data space and store char in the space. If the data-space pointer is character aligned when C, begins execution, it will remain character aligned when C, finishes execution. An ambiguous condition exists if the data-space pointer is not character-aligned prior to execution of C,.

See:
3.3.3 Data space, 3.3.3.1 Address alignment.

Testing:
HERE 1 C,
HERE 2 C,
CONSTANT 2NDC
CONSTANT 1STC

T{    1STC 2NDC U< -> <TRUE> }T \ HERE MUST GROW WITH ALLOT
T{      1STC CHAR+ ->  2NDC  }T \ ... BY ONE CHAR
T{  1STC 1 CHARS + ->  2NDC  }T
T{ 1STC C@ 2NDC C@ ->   1 2  }T
T{       3 1STC C! ->        }T
T{ 1STC C@ 2NDC C@ ->   3 2  }T
T{       4 2NDC C! ->        }T
T{ 1STC C@ 2NDC C@ ->   3 4  }T