diff --git a/pkgs/by-name/fa/fastcap/fastcap-mulglobal-add-ualloc-declaration.patch b/pkgs/by-name/fa/fastcap/fastcap-mulglobal-add-ualloc-declaration.patch
new file mode 100644
index 000000000000..4ca9628bb905
--- /dev/null
+++ b/pkgs/by-name/fa/fastcap/fastcap-mulglobal-add-ualloc-declaration.patch
@@ -0,0 +1,12 @@
+Fix implicit int truncating points in 64 bit systems.
+--- a/src/mulGlobal.h
++++ b/src/mulGlobal.h
+@@ -77,6 +77,8 @@
+ /* #define MALCORE malloc */
+ #define MALCORE ualloc
+ 
++extern char *ualloc(unsigned int);
++
+ /* counts of memory usage by multipole matrix type */
+ extern long memcount;
+ extern long memQ2M;
diff --git a/pkgs/by-name/fa/fastcap/package.nix b/pkgs/by-name/fa/fastcap/package.nix
index cc6cb1a41d38..b0f5b9490943 100644
--- a/pkgs/by-name/fa/fastcap/package.nix
+++ b/pkgs/by-name/fa/fastcap/package.nix
@@ -19,6 +19,7 @@ stdenv.mkDerivation (finalAttrs: {
   patches = [
     ./fastcap-mulglobal-drop-conflicting-lib.patch
     ./fastcap-mulsetup-add-forward-declarations.patch
+    ./fastcap-mulglobal-add-ualloc-declaration.patch
   ];
 
   nativeBuildInputs = [
@@ -28,7 +29,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   postPatch = ''
     substituteInPlace ./doc/Makefile \
-      --replace '/bin/rm' 'rm'
+      --replace-fail '/bin/rm' 'rm'
 
     for f in "doc/*.tex" ; do
       sed -i -E $f \
@@ -54,8 +55,14 @@ stdenv.mkDerivation (finalAttrs: {
     "all"
   ];
 
-  # GCC 14 makes these errors by default
-  env.NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration -Wno-error=return-mismatch -Wno-error=implicit-int";
+  env.NIX_CFLAGS_COMPILE = toString [
+    # gcc14
+    "-Wno-error=implicit-function-declaration"
+    "-Wno-error=return-mismatch"
+    "-Wno-error=implicit-int"
+    # gcc15
+    "-std=gnu17"
+  ];
 
   outputs = [
     "out"