Attempt to fix the PR

  - If ran using 1 mpi process, the rcv_buffer variable has to be filled by the packed parmesh;
  - the pointer toward the buffer provided to the mpipack_parmesh function is modified by the function so cannot be used anymore after the function call.

src/mergemesh_pmmg.c

@@ -1100,8 +1100,8 @@ int PMMG_gather_parmesh( PMMG_pParMesh parmesh,
 
   size_t     pack_size_tot,next_disp;
   int        *rcv_pack_size,ier,ier_glob,k,*displs,ier_pack;
-  int        nprocs,root,pack_size;
-  char       *rcv_buffer,*buffer,*ptr;
+  int        nprocs,root,pack_size,buf_idx;
+  char       *rcv_buffer,*ptr_to_free,*buffer;
 
   nprocs        = parmesh->nprocs;
   root          = parmesh->info.root;
@@ -1157,13 +1157,30 @@ int PMMG_gather_parmesh( PMMG_pParMesh parmesh,
       }
       displs[k] = next_disp;
     }
+
+    /* On root, we will gather all the meshes in rcv_buffer so we have to
+     * compute the total pack size */
     pack_size_tot        = (size_t)(displs[nprocs-1])+(size_t)(rcv_pack_size[nprocs-1]);
     assert ( pack_size_tot < SIZE_MAX && "SIZE_MAX overflow" );
-    PMMG_MALLOC( parmesh,rcv_buffer,pack_size_tot,char,"rcv_buffer",ier=0);
+
+    /* root will write directly in the suitable position of rcv_buffer */
+    buf_idx = parmesh->myrank;
   }
+  else {
+    /* on ranks other than root we just need to store the local mesh so buffer
+     * will be of size pack_size */
+    pack_size_tot = pack_size;
+    /* we will write the mesh at the starting position */
+    buf_idx = 0;
+  }
+
+  PMMG_MALLOC( parmesh,rcv_buffer,pack_size_tot,char,"rcv_buffer",ier=0);
 
   /* Parmesh compression */
-  PMMG_MALLOC ( parmesh,buffer,pack_size,char,"buffer to send",ier=0 );
+  buffer = &rcv_buffer[buf_idx];
+
+  /* Save input allocated address to avoid arrors at unalloc */
+  ptr_to_free = rcv_buffer;
 
 #ifndef NDEBUG
   /* Remark: in release mode, a non allocated buffer used in gatherv creates a
@@ -1174,29 +1191,40 @@ int PMMG_gather_parmesh( PMMG_pParMesh parmesh,
   }
 #endif
 
-  ptr = buffer;
+  /* /!\ mpipack_parmesh and mpiunpack_parmesh are modifying the buffer pointer
+   * making it not valid for realloc / unalloc */
+
+  /* Save adress of buffer because it will be set the the end of the char array
+  by the \a PMMG_mpipack_parmesh function */
+  char *buffer_to_send = buffer;
   ier_pack = PMMG_mpipack_parmesh ( parmesh ,&buffer );
+
+  /* Do not use \a buffer pointer after this call: it points toward the end of
+   * the packed array which is useless */
+  buffer = NULL;
+
   assert ( ier_pack );
 
   /* Gather the packed parmeshes */
   ier = MG_MIN ( ier, ier_pack );
-
-  // Remplacer MPI_Gatherv par une boucle Send/Recv
+
+  /* Here the gatherv call has been replaced by a send/recv to avoid errors when
+   * displacements overflow the INT_MAX value */
   if (parmesh->myrank == root) {
-    for (int i = 0; i < nprocs; ++i) {
-      if (i != root) {
-        MPI_CHECK(MPI_Recv(rcv_buffer + displs[i], rcv_pack_size[i], MPI_CHAR, i, 0, parmesh->comm, MPI_STATUS_IGNORE), ier = 0);
+    int i;
+    for ( i = 0; i < nprocs; ++i ) {
+      if ( i != root ) {
+        MPI_CHECK(
+          MPI_Recv(rcv_buffer + displs[i], rcv_pack_size[i], MPI_CHAR, i,
+                   MPI_MERGEMESH_TAG, parmesh->comm, MPI_STATUS_IGNORE),
+          ier = 0);
       }
     }
   } else {
-    MPI_CHECK(MPI_Send(ptr, pack_size, MPI_CHAR, root, 0, parmesh->comm), ier = 0);
+    MPI_CHECK(
+      MPI_Send(buffer_to_send, pack_size, MPI_CHAR, root, MPI_MERGEMESH_TAG,parmesh->comm),
+      ier = 0);
   }
-  /*MPI_CHECK( MPI_Gatherv ( ptr,pack_size,MPI_CHAR,
-                           rcv_buffer,rcv_pack_size,
-                           displs,MPI_CHAR,root,parmesh->comm ),ier=0 );*/
-
-
-  PMMG_DEL_MEM(parmesh,ptr,char,"buffer to send");
 
   /** 4: Unpack parmeshes */
 #ifndef NDEBUG
@@ -1207,7 +1235,6 @@ int PMMG_gather_parmesh( PMMG_pParMesh parmesh,
 #endif
 
   if ( parmesh->myrank == root ) {
-    ptr = rcv_buffer;
     for ( k=0; k<nprocs; ++k ) {
       ier_pack = PMMG_mpiunpack_parmesh ( parmesh,(*rcv_grps),k,(*rcv_int_node_comm)+k,
                                           (*rcv_next_node_comm)+k,(*rcv_ext_node_comm)+k,
@@ -1220,7 +1247,9 @@ int PMMG_gather_parmesh( PMMG_pParMesh parmesh,
   /* Free temporary arrays */
   PMMG_DEL_MEM(parmesh,rcv_pack_size,int,"rcv_pack_size");
   PMMG_DEL_MEM(parmesh,displs,int,"displs");
-  PMMG_DEL_MEM(parmesh,ptr ,char,"rcv_buffer");
+  /* the address of rcv_buffer is modified by packing/unpacking so it is needed
+   * to send the initially allocated address stored in to the unalloc macro */
+  PMMG_DEL_MEM(parmesh,ptr_to_free,char,"rcv_buffer");
 
   return ier;
 }

src/mpi_pmmg.h

@@ -47,7 +47,7 @@
 #define MPI_TRANSFER_GRP_TAG            8000
 #define MPI_COMMUNICATORS_REF_TAG       9000
 #define MPI_ANALYS_TAG                 10000
-
+#define MPI_MERGEMESH_TAG              11000
 
 #define MPI_CHECK(func_call,on_failure) do {                            \
     int mpi_ret_val;                                                    \

src/mpipack_pmmg.c

@@ -1132,6 +1132,8 @@ int PMMG_mpipack_grp ( PMMG_pGrp grp,char **buffer ) {
  * buffer pointer at the end of the written area. The parmesh groups must have
  * been merged before entering this function.
  *
+ * \remark the \a buffer pointer is modified (shifted) thus, after this
+ * function, it cannot be used for deallocation anymore
  */
 int PMMG_mpipack_parmesh ( PMMG_pParMesh parmesh ,char **buffer ) {
   PMMG_pGrp grp;

src/mpiunpack_pmmg.c

@@ -1112,6 +1112,9 @@ int PMMG_mpiunpack_grp ( PMMG_pParMesh parmesh,PMMG_pGrp listgrp,int igrp,char *
  * pointer toward a buffer of type "x". Then we can get the variable value by
  * dereferencing the adress of the buffer.
  *
+ * \remark the \a buffer pointer is modified (shifted) thus, after this
+ * function, it cannot be used for deallocation anymore
+ *
  */
 int PMMG_mpiunpack_parmesh ( PMMG_pParMesh parmesh,PMMG_pGrp listgrp,int igrp,
                              PMMG_pInt_comm int_node_comm,