5.3.1 Unsigned integers ¶

If the -funsigned option is given, GNU Fortran supports unsigned integers according to J3/24-116. The data type is called UNSIGNED. For an unsigned type with n bits, it implements integer arithmetic modulo 2**n, comparable to the unsigned data type in C.

The data type has KIND numbers comparable to other Fortran data types, which can be selected via the SELECTED_UNSIGNED_KIND function.

Mixed arithmetic, comparisons and assignment between UNSIGNED and other types are only possible via explicit conversion. Conversion from UNSIGNED to other types is done via type conversion functions like INT or REAL. Conversion from other types to UNSIGNED is done via UINT. Unsigned variables cannot be used as index variables in DO loops or as array indices.

Unsigned numbers have a trailing u as suffix, optionally followed by a KIND number separated by an underscore.

Input and output can be done using the ‘I’, ‘B’, ‘O’ and ‘Z’ descriptors, plus unformatted I/O.

Here is a small, somewhat contrived example of their use:

program main
  use iso_fortran_env, only : uint64
  unsigned(kind=uint64) :: v
  v = huge(v) - 32u_uint64
  print *,v
end program main

which outputs the number 18446744073709551583.

Arithmetic operations work on unsigned integers, also for exponentiation. As an extension to J3/24-116.txt, unary minus and exponentiation of unsigned integers are permitted unless -pedantic is in force.

In intrinsic procedures, unsigned arguments are typically permitted for arguments for the data to be processed, analogous to the use of REAL arguments. Unsigned values are prohibited as index variables in DO loops and as array indices.

Unsigned numbers can be read and written using list-directed, formatted and unformatted I/O. For formatted I/O, the ‘B’, ‘I’, ‘O’ and ‘Z’ descriptors are valid. Negative values and values that would overflow are rejected with -pedantic.

SELECT CASE is supported for unsigned integers.

The following intrinsics take unsigned arguments:

• BGE, see BGE — Bitwise greater than or equal to
• BGT, see BGT — Bitwise greater than
• BIT_SIZE, see BIT_SIZE — Bit size inquiry function
• BLE, see BLE — Bitwise less than or equal to
• BLT, see BLT — Bitwise less than
• CMPLX, see CMPLX — Complex conversion function
• CSHIFT, see CSHIFT — Circular shift elements of an array
• DIGITS, see DIGITS — Significant binary digits function
• DOT_PRODUCT, see DOT_PRODUCT — Dot product function
• DSHIFTL, see DSHIFTL — Combined left shift
• DSHIFTR, see DSHIFTR — Combined right shift
• EOSHIFT, see EOSHIFT — End-off shift elements of an array
• FINDLOC, see FINDLOC — Search an array for a value
• HUGE, see HUGE — Largest number of a kind
• IALL, see IALL — Bitwise AND of array elements
• IAND, see IAND — Bitwise logical and
• IANY, see IANY — Bitwise OR of array elements
• IBCLR, see IBCLR — Clear bit
• IBITS, see IBITS — Bit extraction
• IBSET, see IBSET — Set bit
• IEOR, see IEOR — Bitwise logical exclusive or
• INT, see INT — Convert to integer type
• IOR, see IOR — Bitwise logical or
• IPARITY, see IPARITY — Bitwise XOR of array elements
• ISHFT, see ISHFT — Shift bits
• ISHFTC, see ISHFTC — Shift bits circularly
• MATMUL, see MATMUL — matrix multiplication
• MAX, see MAX — Maximum value of an argument list
• MAXLOC, see MAXLOC — Location of the maximum value within an array
• MAXVAL, see MAXVAL — Maximum value of an array
• MERGE, see MERGE — Merge variables
• MERGE_BITS, see MERGE_BITS — Merge of bits under mask
• MIN, see MIN — Minimum value of an argument list
• MINLOC, see MINLOC — Location of the minimum value within an array
• MINVAL, see MINVAL — Minimum value of an array
• MOD, see MOD — Remainder function
• MODULO, see MODULO — Modulo function
• MVBITS, see MVBITS — Move bits from one integer to another
• NOT, see NOT — Logical negation
• OUT_OF_RANGE, see OUT_OF_RANGE — Range check for numerical conversion
• PRODUCT, see PRODUCT — Product of array elements
• RANDOM_NUMBER, see RANDOM_NUMBER — Pseudo-random number
• RANGE, see RANGE — Decimal exponent range
• REAL, see REAL — Convert to real type
• SHIFTA, see SHIFTA — Right shift with fill
• SHIFTL, see SHIFTL — Left shift
• SHIFTR, see SHIFTR — Right shift
• SUM, see SUM — Sum of array elements
• TRANSPOSE, see TRANSPOSE — Transpose an array of rank two
• TRANSFER, see TRANSFER — Transfer bit patterns

The following intrinsics are enabled with -funsigned:

• UINT, see UINT – Convert to UNSIGNED type
• UMASKL, see UMASKL — Unsigned left justified mask
• UMASKR, see UMASKR — Unsigned right justified mask
• SELECTED_UNSIGNED_KIND, see SELECTED_UNSIGNED_KIND — Choose unsigned kind

The following constants have been added to the intrinsic ISO_C_BINDING module: c_unsigned, c_unsigned_short, c_unsigned_char, c_unsigned_long, c_unsigned_long_long, c_uintmax_t, c_uint8_t, c_uint16_t, c_uint32_t, c_uint64_t, c_uint128_t, c_uint_fast8_t, c_uint_fast16_t, c_uint_fast32_t, c_uint_fast64_t, c_uint_fast128_t, c_uint_least8_t, c_uint_least16_t, c_uint_least32_t, c_uint_least64_t and c_uint_least128_t.

The following constants have been added to the intrinsic ISO_FORTRAN_ENV module: uint8, uint16, uint32 and uint64.