[flang-commits] [flang] [llvm] [flang] Implement 'F_C_STRING' library function (Fortran 2023) (PR #174474)
Caroline Newcombe via flang-commits
flang-commits at lists.llvm.org
Tue Jan 6 09:37:36 PST 2026
https://github.com/cenewcombe updated https://github.com/llvm/llvm-project/pull/174474
>From e071e86d4414e754d9acb8000a27ffd92d39b9f2 Mon Sep 17 00:00:00 2001
From: Caroline Newcombe <caroline.newcombe at hpe.com>
Date: Wed, 17 Dec 2025 15:04:20 -0600
Subject: [PATCH 1/2] [flang] Implement 'F_C_STRING' library function (Fortran
2023)
---
flang-rt/lib/runtime/CMakeLists.txt | 1 +
flang/module/iso_c_binding.f90 | 16 ++++++-
flang/test/Integration/f_c_string.f90 | 60 +++++++++++++++++++++++++++
3 files changed, 76 insertions(+), 1 deletion(-)
create mode 100644 flang/test/Integration/f_c_string.f90
diff --git a/flang-rt/lib/runtime/CMakeLists.txt b/flang-rt/lib/runtime/CMakeLists.txt
index 7fa8c2cb95417..d18f24e6c786a 100644
--- a/flang-rt/lib/runtime/CMakeLists.txt
+++ b/flang-rt/lib/runtime/CMakeLists.txt
@@ -75,6 +75,7 @@ set(supported_sources
# List of source not used for GPU offloading.
set(host_sources
${FLANG_SOURCE_DIR}/module/iso_fortran_env_impl.f90
+ ${FLANG_SOURCE_DIR}/module/iso_c_binding.f90
command.cpp
complex-powi.cpp
complex-reduction.c
diff --git a/flang/module/iso_c_binding.f90 b/flang/module/iso_c_binding.f90
index 8e3f78cea51b7..34e7b6280f54d 100644
--- a/flang/module/iso_c_binding.f90
+++ b/flang/module/iso_c_binding.f90
@@ -29,7 +29,7 @@ module iso_c_binding
private
public :: c_associated, c_funloc, c_funptr, c_f_pointer, c_loc, &
- c_null_funptr, c_null_ptr, c_ptr, c_sizeof, &
+ c_null_funptr, c_null_ptr, c_ptr, c_sizeof, f_c_string, &
operator(==), operator(/=)
! Table 18.2 (in clause 18.3.1)
@@ -145,4 +145,18 @@ subroutine c_f_procpointer(cptr, fptr)
! TODO: implement
end subroutine c_f_procpointer
+ ! F_C_STRING - Convert Fortran string to C null-terminated string
+ ! Fortran 2023 standard intrinsic
+ pure function f_c_string(string, asis) result(res)
+ character(kind=c_char, len=*), intent(in) :: string
+ logical, optional, intent(in) :: asis
+ character(kind=c_char, len=:), allocatable :: res
+
+ if (present(asis) .and. asis) then
+ res = string // c_null_char
+ else
+ res = trim(string) // c_null_char
+ end if
+ end function f_c_string
+
end module iso_c_binding
diff --git a/flang/test/Integration/f_c_string.f90 b/flang/test/Integration/f_c_string.f90
new file mode 100644
index 0000000000000..79ac10556a740
--- /dev/null
+++ b/flang/test/Integration/f_c_string.f90
@@ -0,0 +1,60 @@
+! RUN: %flang %s -o %t && %t | FileCheck %s
+! Test F_C_STRING library function
+
+program test_f_c_string
+ use iso_c_binding
+ implicit none
+
+ character(len=20) :: str
+ character(len=:), allocatable :: result
+ logical :: flag
+
+ ! Test 1: Basic trimming
+ str = 'hello '
+ result = f_c_string(str(1:10))
+ ! CHECK: Test 1: 6
+ print '(A,I0)', 'Test 1: ', len(result)
+ if (result /= 'hello' // c_null_char) error stop 'Test 1 failed'
+
+ ! Test 2: ASIS=.TRUE. (keep blanks)
+ result = f_c_string(str(1:10), .true.)
+ ! CHECK: Test 2: 11
+ print '(A,I0)', 'Test 2: ', len(result)
+ if (result /= 'hello ' // c_null_char) error stop 'Test 2 failed'
+
+ ! Test 3: ASIS=.FALSE. (explicit trim)
+ result = f_c_string(str(1:10), .false.)
+ ! CHECK: Test 3: 6
+ print '(A,I0)', 'Test 3: ', len(result)
+ if (result /= 'hello' // c_null_char) error stop 'Test 3 failed'
+
+ ! Test 4: Variable ASIS
+ flag = .true.
+ str = 'abc '
+ result = f_c_string(str(1:6), flag)
+ ! CHECK: Test 4: 7
+ print '(A,I0)', 'Test 4: ', len(result)
+ if (len(result) /= 7) error stop 'Test 4 failed'
+
+ flag = .false.
+ result = f_c_string(str(1:6), flag)
+ ! CHECK: Test 5: 4
+ print '(A,I0)', 'Test 5: ', len(result)
+ if (len(result) /= 4) error stop 'Test 5 failed'
+
+ ! Test 6: Empty (all blanks)
+ result = f_c_string(' ')
+ ! CHECK: Test 6: 1
+ print '(A,I0)', 'Test 6: ', len(result)
+ if (result /= c_null_char) error stop 'Test 6 failed'
+
+ ! Test 7: Internal blanks preserved
+ result = f_c_string('a b c ')
+ ! CHECK: Test 7: 6
+ print '(A,I0)', 'Test 7: ', len(result)
+ if (result /= 'a b c' // c_null_char) error stop 'Test 7 failed'
+
+ ! CHECK: PASS
+ print *, 'PASS'
+
+end program test_f_c_string
>From 9d061994b4648e8fa5c3da64d384c0dadbc4ebb6 Mon Sep 17 00:00:00 2001
From: Caroline Newcombe <caroline.newcombe at hpe.com>
Date: Tue, 6 Jan 2026 11:37:21 -0600
Subject: [PATCH 2/2] Swap the integration test for semantics checking, due to
testing limitations
---
flang/test/Integration/f_c_string.f90 | 60 ---------------------------
flang/test/Semantics/f_c_string.f90 | 39 +++++++++++++++++
2 files changed, 39 insertions(+), 60 deletions(-)
delete mode 100644 flang/test/Integration/f_c_string.f90
create mode 100644 flang/test/Semantics/f_c_string.f90
diff --git a/flang/test/Integration/f_c_string.f90 b/flang/test/Integration/f_c_string.f90
deleted file mode 100644
index 79ac10556a740..0000000000000
--- a/flang/test/Integration/f_c_string.f90
+++ /dev/null
@@ -1,60 +0,0 @@
-! RUN: %flang %s -o %t && %t | FileCheck %s
-! Test F_C_STRING library function
-
-program test_f_c_string
- use iso_c_binding
- implicit none
-
- character(len=20) :: str
- character(len=:), allocatable :: result
- logical :: flag
-
- ! Test 1: Basic trimming
- str = 'hello '
- result = f_c_string(str(1:10))
- ! CHECK: Test 1: 6
- print '(A,I0)', 'Test 1: ', len(result)
- if (result /= 'hello' // c_null_char) error stop 'Test 1 failed'
-
- ! Test 2: ASIS=.TRUE. (keep blanks)
- result = f_c_string(str(1:10), .true.)
- ! CHECK: Test 2: 11
- print '(A,I0)', 'Test 2: ', len(result)
- if (result /= 'hello ' // c_null_char) error stop 'Test 2 failed'
-
- ! Test 3: ASIS=.FALSE. (explicit trim)
- result = f_c_string(str(1:10), .false.)
- ! CHECK: Test 3: 6
- print '(A,I0)', 'Test 3: ', len(result)
- if (result /= 'hello' // c_null_char) error stop 'Test 3 failed'
-
- ! Test 4: Variable ASIS
- flag = .true.
- str = 'abc '
- result = f_c_string(str(1:6), flag)
- ! CHECK: Test 4: 7
- print '(A,I0)', 'Test 4: ', len(result)
- if (len(result) /= 7) error stop 'Test 4 failed'
-
- flag = .false.
- result = f_c_string(str(1:6), flag)
- ! CHECK: Test 5: 4
- print '(A,I0)', 'Test 5: ', len(result)
- if (len(result) /= 4) error stop 'Test 5 failed'
-
- ! Test 6: Empty (all blanks)
- result = f_c_string(' ')
- ! CHECK: Test 6: 1
- print '(A,I0)', 'Test 6: ', len(result)
- if (result /= c_null_char) error stop 'Test 6 failed'
-
- ! Test 7: Internal blanks preserved
- result = f_c_string('a b c ')
- ! CHECK: Test 7: 6
- print '(A,I0)', 'Test 7: ', len(result)
- if (result /= 'a b c' // c_null_char) error stop 'Test 7 failed'
-
- ! CHECK: PASS
- print *, 'PASS'
-
-end program test_f_c_string
diff --git a/flang/test/Semantics/f_c_string.f90 b/flang/test/Semantics/f_c_string.f90
new file mode 100644
index 0000000000000..172cbc49c7c8c
--- /dev/null
+++ b/flang/test/Semantics/f_c_string.f90
@@ -0,0 +1,39 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+! Test semantic checking of F_C_STRING from ISO_C_BINDING
+
+program test
+ use iso_c_binding
+ implicit none
+
+ character(len=20) :: str
+ character(len=:), allocatable :: result
+ logical :: flag
+ integer :: n
+
+ ! Valid usages
+ result = f_c_string('hello')
+ result = f_c_string(str)
+ result = f_c_string(str, .true.)
+ result = f_c_string(str, .false.)
+ result = f_c_string(str, flag)
+ result = f_c_string(string=str)
+ result = f_c_string(string=str, asis=.true.)
+ result = f_c_string(asis=.false., string=str)
+
+ ! Invalid: missing required argument
+ !ERROR: missing mandatory 'string=' argument
+ result = f_c_string()
+
+ ! Invalid: too many arguments
+ !ERROR: No intrinsic or generic 'f_c_string' matches the actual arguments
+ result = f_c_string(str, .true., .false.)
+
+ ! Invalid: non-character first argument
+ !ERROR: No intrinsic or generic 'f_c_string' matches the actual arguments
+ result = f_c_string(n)
+
+ ! Invalid: non-logical second argument
+ !ERROR: No intrinsic or generic 'f_c_string' matches the actual arguments
+ result = f_c_string(str, n)
+
+end program
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