diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4991c31d..530d7c39 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -4,7 +4,7 @@ on:
   - pull_request
 jobs:
   test:
-    name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }}
+    name: Julia ${{ matrix.version }} - ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
@@ -14,19 +14,12 @@ jobs:
           - '1'
         os:
           - ubuntu-latest
-#          - macOS-latest
-        arch:
-          - x86
-          - x64
-        exclude:
-          - os: macOS-latest
-            arch: x86
+          - macOS-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: julia-actions/setup-julia@v2
+      - uses: actions/checkout@v6
+      - uses: julia-actions/setup-julia@v3
         with:
           version: ${{ matrix.version }}
-          arch: ${{ matrix.arch }}
           show-versioninfo: true
       - uses: actions/cache@v3
         env:
@@ -38,10 +31,10 @@ jobs:
             ${{ runner.os }}-test-${{ env.cache-name }}-
             ${{ runner.os }}-test-
             ${{ runner.os }}-
-      - uses: julia-actions/julia-buildpkg@latest
-      - uses: julia-actions/julia-runtest@latest
+      - uses: julia-actions/julia-buildpkg@v1
+      - uses: julia-actions/julia-runtest@v1
       - uses: julia-actions/julia-processcoverage@v1
-      - uses: codecov/codecov-action@v4
+      - uses: codecov/codecov-action@v6
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
           file: lcov.info
diff --git a/src/libfasttransforms.jl b/src/libfasttransforms.jl
index 3ce492d9..dd694b15 100644
--- a/src/libfasttransforms.jl
+++ b/src/libfasttransforms.jl
@@ -49,6 +49,34 @@ function renew!(x::AbstractArray{BigFloat})
     return x
 end
 
+# Obtain a raw C pointer to the mpfr_t struct underlying a BigFloat.
+# In Julia < 1.12, BigFloat was a mutable struct whose fields matched mpfr_t directly,
+# so pointer_from_objref gives us the mpfr_t*.
+# In Julia >= 1.12, BigFloat is an immutable struct wrapping Memory{Limb}, and the
+# mpfr_t struct is stored at the start of that Memory.
+@static if VERSION >= v"1.12"
+    _mpfr_ptr(b::BigFloat) = Ptr{mpfr_t}(pointer(getfield(b, :d)))
+else
+    _mpfr_ptr(b::BigFloat) = Ptr{mpfr_t}(pointer_from_objref(b))
+end
+
+function _mpfr_ptrs(x::StridedVector{BigFloat})
+    return [_mpfr_ptr(xi) for xi in x]
+end
+
+function _mpfr_ptrs(x::StridedMatrix{BigFloat})
+    m, n = size(x)
+    s = stride(x, 2)
+    len = n > 0 ? s * (n - 1) + m : 0
+    ptrs = Vector{Ptr{mpfr_t}}(undef, len)
+    for j in 0:n-1
+        for i in 0:m-1
+            @inbounds ptrs[i + j*s + 1] = _mpfr_ptr(x[i+1, j+1])
+        end
+    end
+    return ptrs
+end
+
 function horner!(f::Vector{Float64}, c::StridedVector{Float64}, x::Vector{Float64})
     @assert length(x) == length(f)
     ccall((:ft_horner, libfasttransforms), Cvoid, (Cint, Ptr{Float64}, Cint, Cint, Ptr{Float64}, Ptr{Float64}), length(c), c, stride(c, 1), length(x), x, f)
@@ -983,19 +1011,31 @@ for (fJ, fC) in ((:lmul!, :ft_mpfr_trmv_ptr),
         function $fJ(p::FTPlan{BigFloat, 1}, x::StridedVector{BigFloat})
             checksize(p, x)
             checkstride(p, x)
-            ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{BigFloat}, Int32), 'N', p.n, p, p.n, renew!(x), Base.MPFR.ROUNDING_MODE[])
+            renew!(x)
+            GC.@preserve x begin
+                ptrs = _mpfr_ptrs(x)
+                ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{Ptr{mpfr_t}}, Int32), 'N', p.n, p, p.n, ptrs, Base.MPFR.ROUNDING_MODE[])
+            end
             return x
         end
         function $fJ(p::AdjointFTPlan{BigFloat, FTPlan{BigFloat, 1, K}}, x::StridedVector{BigFloat}) where K
             checksize(p, x)
             checkstride(p, x)
-            ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{BigFloat}, Int32), 'T', p.parent.n, p, p.parent.n, renew!(x), Base.MPFR.ROUNDING_MODE[])
+            renew!(x)
+            GC.@preserve x begin
+                ptrs = _mpfr_ptrs(x)
+                ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{Ptr{mpfr_t}}, Int32), 'T', p.parent.n, p, p.parent.n, ptrs, Base.MPFR.ROUNDING_MODE[])
+            end
             return x
         end
         function $fJ(p::TransposeFTPlan{BigFloat, FTPlan{BigFloat, 1, K}}, x::StridedVector{BigFloat}) where K
             checksize(p, x)
             checkstride(p, x)
-            ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{BigFloat}, Int32), 'T', p.parent.n, p, p.parent.n, renew!(x), Base.MPFR.ROUNDING_MODE[])
+            renew!(x)
+            GC.@preserve x begin
+                ptrs = _mpfr_ptrs(x)
+                ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{Ptr{mpfr_t}}, Int32), 'T', p.parent.n, p, p.parent.n, ptrs, Base.MPFR.ROUNDING_MODE[])
+            end
             return x
         end
     end
@@ -1083,19 +1123,31 @@ for (fJ, fC) in ((:lmul!, :ft_mpfr_trmm_ptr),
         function $fJ(p::FTPlan{BigFloat, 1}, x::StridedMatrix{BigFloat})
             checksize(p, x)
             checkstride(p, x)
-            ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{BigFloat}, Cint, Cint, Int32), 'N', p.n, p, p.n, renew!(x), stride(x, 2), size(x, 2), Base.MPFR.ROUNDING_MODE[])
+            renew!(x)
+            GC.@preserve x begin
+                ptrs = _mpfr_ptrs(x)
+                ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{Ptr{mpfr_t}}, Cint, Cint, Int32), 'N', p.n, p, p.n, ptrs, stride(x, 2), size(x, 2), Base.MPFR.ROUNDING_MODE[])
+            end
             return x
         end
         function $fJ(p::AdjointFTPlan{BigFloat, FTPlan{BigFloat, 1, K}}, x::StridedMatrix{BigFloat}) where K
             checksize(p, x)
             checkstride(p, x)
-            ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{BigFloat}, Cint, Cint, Int32), 'T', p.parent.n, p, p.parent.n, renew!(x), stride(x, 2), size(x, 2), Base.MPFR.ROUNDING_MODE[])
+            renew!(x)
+            GC.@preserve x begin
+                ptrs = _mpfr_ptrs(x)
+                ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{Ptr{mpfr_t}}, Cint, Cint, Int32), 'T', p.parent.n, p, p.parent.n, ptrs, stride(x, 2), size(x, 2), Base.MPFR.ROUNDING_MODE[])
+            end
             return x
         end
         function $fJ(p::TransposeFTPlan{BigFloat, FTPlan{BigFloat, 1, K}}, x::StridedMatrix{BigFloat}) where K
             checksize(p, x)
             checkstride(p, x)
-            ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{BigFloat}, Cint, Cint, Int32), 'T', p.parent.n, p, p.parent.n, renew!(x), stride(x, 2), size(x, 2), Base.MPFR.ROUNDING_MODE[])
+            renew!(x)
+            GC.@preserve x begin
+                ptrs = _mpfr_ptrs(x)
+                ccall(($(string(fC)), libfasttransforms), Cvoid, (Cint, Cint, Ptr{mpfr_t}, Cint, Ptr{Ptr{mpfr_t}}, Cint, Cint, Int32), 'T', p.parent.n, p, p.parent.n, ptrs, stride(x, 2), size(x, 2), Base.MPFR.ROUNDING_MODE[])
+            end
             return x
         end
     end