provide more generic implementation of Comm::read_lines_from_file() in utils

src/utils.cpp

@@ -225,6 +225,36 @@ void utils::sfread(const char *srcname, int srcline, void *s, size_t size,
 
 /* ------------------------------------------------------------------ */
 
+/* read N lines and broadcast */
+int utils::read_lines_from_file(FILE *fp, int nlines, int maxline,
+                                char *buffer, int me, MPI_Comm comm)
+{
+  char *ptr = buffer;
+  *ptr = '\0';
+
+  if (me == 0) {
+    if (fp) {
+      for (int i = 0; i < nlines; i++) {
+        ptr = fgets(ptr,maxline,fp);
+        if (!ptr) break; // EOF?
+        // advance ptr to end of string and append newline char if needed.
+        ptr += strlen(ptr);
+        if (*(--ptr) != '\n') *(++ptr) = '\n';
+        // ensure buffer is null terminated. null char is start of next line.
+        *(++ptr) = '\0';
+      }
+    }
+  }
+
+  int n = strlen(buffer);
+  MPI_Bcast(&n,1,MPI_INT,0,comm);
+  if (n == 0) return 1;
+  MPI_Bcast(buffer,n+1,MPI_CHAR,0,comm);
+  return 0;
+}
+
+/* ------------------------------------------------------------------ */
+
 std::string utils::check_packages_for_style(const std::string &style,
                                             const std::string &name,
                                             LAMMPS *lmp)

src/utils.h

@@ -17,9 +17,12 @@
 /*! \file utils.h */
 
 #include "lmptype.h"
+
+#include <mpi.h>
+
+#include <cstdio>
 #include <string>
 #include <vector>
-#include <cstdio>
 
 namespace LAMMPS_NS {
 
@@ -89,6 +92,26 @@ namespace LAMMPS_NS {
     void sfread(const char *srcname, int srcline, void *s, size_t size,
                 size_t num, FILE *fp, const char *filename, Error *error);
 
+    /** Read N lines of text from file into buffer and broadcast them
+     *
+     * This function uses repeated calls to fread() to fill a buffer with
+     * newline terminated text.  If a line does not end in a newline (e.g.
+     * at the end of a file), it is added.  The caller has to allocate an
+     * nlines by maxline sized buffer for storing the text data.
+     * Reading is done by MPI rank 0 of the given communicator only, and
+     * thus only MPI rank 0 needs to provide a valid file pointer.
+     *
+     *  \param fp       file pointer used by fread
+     *  \param nlines   number of lines to be read
+     *  \param maxline  maximum length of a single line
+     *  \param buffer   buffer for storing the data.
+     *  \param me       MPI rank of calling process in MPI communicator
+     *  \param comm     MPI communicator for broadcast
+     *  \return         1 if the read was short, 0 if read was succesful */
+
+    int read_lines_from_file(FILE *fp, int nlines, int maxline,
+                             char *buffer, int me, MPI_Comm comm);
+
     /** Report if a requested style is in a package or may have a typo
      *
      *  \param style type of style that is to be checked for

unittest/formats/test_file_operations.cpp

@@ -11,13 +11,13 @@
    See the README file in the top-level LAMMPS directory.
 ------------------------------------------------------------------------- */
 
+#include "../testing/core.h"
 #include "info.h"
 #include "input.h"
 #include "lammps.h"
 #include "utils.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "../testing/core.h"
 
 #include <cstdio>
 #include <mpi.h>
@@ -28,6 +28,7 @@ using namespace LAMMPS_NS;
 using testing::MatchesRegex;
 using testing::StrEq;
 
+using utils::read_lines_from_file;
 using utils::sfgets;
 using utils::sfread;
 using utils::split_words;
@@ -124,6 +125,37 @@ TEST_F(FileOperationsTest, safe_fread)
     fclose(fp);
 }
 
+TEST_F(FileOperationsTest, read_lines_from_file)
+{
+    char *buf = new char[MAX_BUF_SIZE];
+    FILE *fp       = nullptr;
+    MPI_Comm world = MPI_COMM_WORLD;
+    int me, rv;
+    memset(buf, 0, MAX_BUF_SIZE);
+
+    rv = utils::read_lines_from_file(nullptr, 1, MAX_BUF_SIZE, buf, me, world);
+    ASSERT_EQ(rv, 1);
+
+    MPI_Comm_rank(world, &me);
+    if (me == 0) {
+        fp = fopen("safe_file_read_test.txt", "r");
+        ASSERT_NE(fp, nullptr);
+    } else
+        ASSERT_EQ(fp, nullptr);
+
+    rv = utils::read_lines_from_file(fp, 2, MAX_BUF_SIZE / 2, buf, me, world);
+    ASSERT_EQ(rv, 0);
+    ASSERT_THAT(buf, StrEq("one line\ntwo_lines\n"));
+
+    rv = utils::read_lines_from_file(fp, 2, MAX_BUF_SIZE / 2, buf, me, world);
+    ASSERT_EQ(rv, 0);
+    ASSERT_THAT(buf, StrEq("\nno newline\n"));
+
+    rv = utils::read_lines_from_file(fp, 2, MAX_BUF_SIZE / 2, buf, me, world);
+    ASSERT_EQ(rv, 1);
+    delete[] buf;
+}
+
 TEST_F(FileOperationsTest, logmesg)
 {
     char buf[8];