Index: README-blksz
===================================================================
--- README-blksz	(.../trunk)	(revision 0)
+++ README-blksz	(.../branches/blksz_improvement)	(revision 531)
@@ -0,0 +1,27 @@
+    Using environment variables to control buffering
+    ------------------------------------------------
+
+    This version of netCDF sets buffer sizes according to the
+values of certain environment variables as follows:
+
+    "NC_BLKSZ_filename" where "filename" is the prefix-part 
+of the netcdf file being opened, from the last "/" to the 
+first ".". For example, when opening "/home/me/MyFile.nc.001",
+netCDF will look for an environment variable "NC_BLKSZ_MyFile".
+
+    Next, netCDF looks for the generic "NC_BLKSZ". An earlier
+implementation of this feature used "NC_BLOCKSIZE" instead,
+so finally, if a blocksize has not been found, "NC_BLOCKSIZE"
+is used. If none of these are set, netCDF uses the pagesize,
+as per the earlier netCDF versions.
+
+    The buffer size can be an integer, optionally suffixed 
+with "k", K", "m", "M", "kb", "MB" etc (case-insensitive)
+for kilobytes and megabytes. Alternately, if the buffer size
+is set to "FILE", and the file already exists, netCDF will 
+use the size of the existing file as a buffer size.
+
+    A warning: the actual memory space allocated is roughly 
+twice the buffer size requested, this is an implementation 
+detail within netCDF.
+
Index: libsrc/posixio.c
===================================================================
--- libsrc/posixio.c	(.../trunk)	(revision 531)
+++ libsrc/posixio.c	(.../branches/blksz_improvement)	(revision 531)
@@ -15,6 +15,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <string.h>
+#include <ctype.h>
 #ifdef _MSC_VER /* Microsoft Compilers */
 #include <io.h>
 #else
@@ -104,7 +105,84 @@
 	return (size_t) 2 * pagesize();
 }
 
+/* sjl: allow the user to specify buffer sizes via env vars */
 
+size_t decode_size(const char* path, const char* size_string)
+{
+  /* only should be called from get_user_chunk_size */
+  size_t user_chunk_size = 0;
+  size_t len;
+  int i;
+  struct stat sb;
+  char *new_string = NULL;
+
+  /* make case insensitive */
+  len = strlen(size_string) + 1;
+  new_string = (char*) malloc(sizeof(char)*len);
+  for (i=0; i<len; i++) {
+    if (islower((int)size_string[i]))
+      new_string[i] = (char) toupper((int)size_string[i]);
+    else
+      new_string[i] = size_string[i];
+  }
+
+  /* first check: "FILE" means "make the buffer as big as the file" */
+  if (!strcmp(new_string, "FILE")) {
+    if (stat(path, &sb) != -1) 
+      user_chunk_size = M_RNDUP(sb.st_size);
+  /* otherwise, look for a number of bytes/kb/mb */
+  } else {
+      user_chunk_size = atoi(new_string);
+      if (rindex(new_string,'K') != NULL) user_chunk_size *= 1024;
+      else if (rindex(new_string,'M') != NULL) user_chunk_size *= 1024*1024;
+  }
+  free(new_string);
+  return user_chunk_size;
+}
+
+void get_user_chunk_size(const char* path, size_t *sizehint) 
+{
+  char *env_var_name = NULL;
+  char *env_var_value = NULL;
+  const char *fname, *c;
+  size_t len1, len;
+  size_t user_chunk_size = 0;
+
+  /* first, is a buffer size specified for this file?
+   * look for an env var called NC_BLKSZ_filename where filename
+   * is the file name, not including its path, up to the first "."
+   * eg if path is output/file01.nc then set the environment
+   * variable NC_BLKSZ_file01 to the desired buffer size */
+  if (path != NULL) 
+  {
+    fname = (c=rindex(path, '/')) ? c++ : path;
+    len1  = strcspn(fname,".");
+    len   = len1 + strlen("NC_BLKSZ_") + 1;
+    env_var_name = (char*) malloc(sizeof(char)*len);
+    strncat(strcpy(env_var_name,"NC_BLKSZ_"),fname,len1);
+    env_var_name[len-1] = '\0';
+    env_var_value = getenv(env_var_name);
+    free (env_var_name);
+    if (env_var_value != NULL)
+      user_chunk_size = decode_size(path, env_var_value);
+  }
+
+  /* next, is an all-files buffer size given? */
+  if (path != NULL && user_chunk_size <= 0) 
+  {
+    env_var_value = getenv("NC_BLKSZ");
+    /* earlier versions used NC_BLOCKSIZE instead, check for that */
+    if (env_var_value == NULL) env_var_value = getenv("NC_BLOCKSIZE");
+    if (env_var_value != NULL)
+      user_chunk_size = decode_size(path, env_var_value);
+  }
+
+  /* update the sizehint accordingly. it will be used in _open and _create */
+  if (user_chunk_size > *sizehint) *sizehint = user_chunk_size;
+
+  return;
+}
+
 /*
  * Sortof like ftruncate, except won't make the
  * file shorter.
@@ -164,6 +242,7 @@
 	}
 	return ENOERR;
 }
+
 /* End OS */
 /* Begin px */
 
@@ -1460,6 +1539,7 @@
 	}
 	*((int *)&nciop->fd) = fd; /* cast away const */
 
+        get_user_chunk_size(path, sizehintp); /* sjl: update sizehint wrt env vars */
 	if(*sizehintp < NCIO_MINBLOCKSIZE || *sizehintp > NCIO_MAXBLOCKSIZE)
 	{
 		/* Use default */
@@ -1584,6 +1664,7 @@
 	}
 	*((int *)&nciop->fd) = fd; /* cast away const */
 
+        get_user_chunk_size(path, sizehintp); /* sjl: update sizehint wrt env vars */
 	if(*sizehintp < NCIO_MINBLOCKSIZE || *sizehintp > NCIO_MAXBLOCKSIZE)
 	{
 		/* Use default */