subroutine checksum(buffer,length,sum32) C Calculate a 32-bit 1's complement checksum of the input buffer, adding C it to the value of sum32. This algorithm assumes that the buffer C length is a multiple of 4 bytes. C a double precision value (which has at least 48 bits of precision) C is used to accumulate the checksum because standard Fortran does not C support an unsigned integer datatype. C buffer - integer buffer to be summed C length - number of bytes in the buffer (must be multiple of 4) C sum32 - double precision checksum value (The calculated checksum C is added to the input value of sum32 to produce the C output value of sum32) integer buffer(*),length,i,hibits double precision sum32,word32 parameter (word32=4.294967296D+09) C (word32 is equal to 2**32) C LENGTH must be less than 2**15, otherwise precision may be lost C in the sum if (length .gt. 32768)then print *, 'Error: size of block to sum is too large' return end if do i=1,length/4 if (buffer(i) .ge. 0)then sum32=sum32+buffer(i) else C sign bit is set, so add the equivalent unsigned value sum32=sum32+(word32+buffer(i)) end if end do C fold any overflow bits beyond 32 back into the word 10 hibits=sum32/word32 if (hibits .gt. 0)then sum32=sum32-(hibits*word32)+hibits go to 10 end if end