4. Characters

Here is a technical overview of handling characters and strings in Fortran 2018, structured according to internal logic, memory allocation behavior, and interface boundaries.

---

1. Core Allocation and Memory Layout

In Fortran 2018, strings are declared using the character keyword. The storage size is determined by two parameters: len (length) and kind (character set encoding).

Static Character Arrays

Static declarations allocate a fixed-size contiguous memory block. Unused trailing bytes are automatically padded with blanks (0x20).

character(len=64) :: buffer
buffer = "sys_init" ! Bytes 9 to 64 are padded with spaces

Deferred-Length (Dynamic Allocation)

To eliminate memory overhead and manual tracking of string lengths, use deferred-length characters via the allocatable attribute. The runtime environment handles reallocation automatically upon assignment.

character(len=:), allocatable :: dynamic_payload

dynamic_payload = "kernel_panic_log_data" ! Allocated at length 21
dynamic_payload = "short_err"              ! Automatically reallocated to length 9

---

2. String Manipulation Logic

Truncation and Padding Prevention

Standard assignments to fixed-length strings trigger truncation or blank padding. To manage data flow accurately, utilize intrinsic functions to control whitespace logic.

character(len=10) :: standard_str
character(len=20) :: explicit_str

standard_str = "data"
! adjustl shifts characters left, removing leading whitespaces
! trim strips all trailing blanks
explicit_str = adjustl(standard_str) // "_valid" 

Substring Slicing

Fortran uses a 1-based indexing system for slicing contiguous memory blocks.

character(len=8) :: target
target = "12345678"
print *, target(3:6) ! Returns "3456"

---

3. ABI Interoperability with C (ISO_C_BINDING)

Fortran 2018 ensures strict binary compatibility with C types through the intrinsic module iso_c_binding.

Null-Termination Specification

Fortran strings do not implicitly terminate with a null character (\0). To pass a string safely to a C interface without causing a buffer overflow, you must append c_null_char.

use, intrinsic :: iso_c_binding, only : c_char, c_null_char
implicit none

character(kind=c_char, len=:), allocatable :: c_compatible_string

! Explicit null-termination for C ABI enforcement
c_compatible_string = "fetch_framebuffer" // c_null_char

---

4. Performance Specifications

Operation	Mechanism	Overhead Considerations
fixed_len = "str"	Static copy + Blank padding	Minimal CPU cycles, fixed memory footprint.
alloc_len = "str"	Heap reallocation check	Triggers malloc/free wrappers if bounds mismatch.
trim(string)	Runtime descriptor scan	Scans backward to identify the last non-blank byte.