Detect the glibc version in CMake and link librt if it is required.

For the Makefile, unconditionally link librt with no detection.

CMakeLists.txt

@@ -2,6 +2,81 @@ project (ndhc)
 
 cmake_minimum_required (VERSION 2.6)
 
+####################################
+
+# Computes the realtionship between two version strings.  A version
+# string is a number delineated by '.'s such as 1.3.2 and 0.99.9.1.
+# You can feed version strings with different number of dot versions,
+# and the shorter version number will be padded with zeros: 9.2 <
+# 9.2.1 will actually compare 9.2.0 < 9.2.1.
+#
+# Input: a_in - value, not variable
+#        b_in - value, not variable
+#        result_out - variable with value:
+#                         -1 : a_in <  b_in
+#                          0 : a_in == b_in
+#                          1 : a_in >  b_in
+#
+# Written by James Bigler.
+MACRO(COMPARE_VERSION_STRINGS a_in b_in result_out)
+  # Since SEPARATE_ARGUMENTS using ' ' as the separation token,
+  # replace '.' with ' ' to allow easy tokenization of the string.
+  STRING(REPLACE "." " " a ${a_in})
+  STRING(REPLACE "." " " b ${b_in})
+  SEPARATE_ARGUMENTS(a)
+  SEPARATE_ARGUMENTS(b)
+
+  # Check the size of each list to see if they are equal.
+  LIST(LENGTH a a_length)
+  LIST(LENGTH b b_length)
+
+  # Pad the shorter list with zeros.
+
+  # Note that range needs to be one less than the length as the for
+  # loop is inclusive (silly CMake).
+  IF(a_length LESS b_length)
+    # a is shorter
+    SET(shorter a)
+    MATH(EXPR range "${b_length} - 1")
+    MATH(EXPR pad_range "${b_length} - ${a_length} - 1")
+  ELSE(a_length LESS b_length)
+    # b is shorter
+    SET(shorter b)
+    MATH(EXPR range "${a_length} - 1")
+    MATH(EXPR pad_range "${a_length} - ${b_length} - 1")
+  ENDIF(a_length LESS b_length)
+
+  # PAD out if we need to
+  IF(NOT pad_range LESS 0)
+    FOREACH(pad RANGE ${pad_range})
+      # Since shorter is an alias for b, we need to get to it by by dereferencing shorter.
+      LIST(APPEND ${shorter} 0)
+    ENDFOREACH(pad RANGE ${pad_range})
+  ENDIF(NOT pad_range LESS 0)
+
+  SET(result 0)
+  FOREACH(index RANGE ${range})
+    IF(result EQUAL 0)
+      # Only continue to compare things as long as they are equal
+      LIST(GET a ${index} a_version)
+      LIST(GET b ${index} b_version)
+      # LESS
+      IF(a_version LESS b_version)
+        SET(result -1)
+      ENDIF(a_version LESS b_version)
+      # GREATER
+      IF(a_version GREATER b_version)
+        SET(result 1)
+      ENDIF(a_version GREATER b_version)
+    ENDIF(result EQUAL 0)
+  ENDFOREACH(index)
+
+  # Copy out the return result
+  SET(${result_out} ${result})
+ENDMACRO(COMPARE_VERSION_STRINGS)
+
+####################################
+
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu99 -pedantic -Wall -Wextra -Wformat=2 -Wformat-nonliteral -Wformat-security -Wshadow -Wpointer-arith -Wmissing-prototypes -D_GNU_SOURCE -DNK_USE_CAPABILITY")
 set(CMAKE_CXX_FLAGS "${CMAKE_C_FLAGS} -std=gnu99 -pedantic -Wall -Wextra -Wformat=2 -Wformat-nonliteral -Wformat-security -Wshadow -Wpointer-arith -Wmissing-prototypes -D_GNU_SOURCE -DNK_USE_CAPABILITY")
 
@@ -21,6 +96,31 @@ if (NOT (${OSNAME} STREQUAL "Linux"))
   return()
 endif()
 
+set(GLIBC_DETECT_TEST_C
+  "
+  #include <stdio.h>
+  int main(int argc, char *argv[]) {
+  printf(\"%d.%d\", __GLIBC__, __GLIBC_MINOR__);
+  return 0;
+  }
+  ")
+file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/detect/glibc.c "${GLIBC_DETECT_TEST_C}\n")
+try_run(GLIBC_DETECTION GLIBC_COMPILE
+  ${CMAKE_CURRENT_BINARY_DIR}/detect ${CMAKE_CURRENT_BINARY_DIR}/detect/glibc.c
+  RUN_OUTPUT_VARIABLE GLIBC_VERSION)
+if (GLIBC_VERSION AND GLIBC_COMPILE)
+  message("glibc version detected as ${GLIBC_VERSION}")
+else()
+  message("Could not detect glibc version.")
+endif()
+
+COMPARE_VERSION_STRINGS(${GLIBC_VERSION} "2.17" GLIBC_MUST_LINK_RT)
+if (${GLIBC_MUST_LINK_RT} LESS 0)
+  message("glibc <2.17 detected; linking against librt.")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -lrt")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -lrt")
+endif()
+
 execute_process(
   COMMAND uname -m
   COMMAND tr "\n" " "

Makefile

@@ -15,6 +15,9 @@ CC = gcc
 AR = ar
 RANLIB = ranlib
 CFLAGS = -O2 -s -std=gnu99 -pedantic -Wall -D_GNU_SOURCE
+# Not required for glibc >= 2.17, but older glibcs are still common.
+# The CMake build script will perform detection, but this Makefile is simple.
+LINK_LIBS = -lrt
 
 all: makedir ifchd-parse.o ncmlib.a ndhc
 
@@ -36,7 +39,7 @@ ncmlib.a: $(NCM_OBJS)
 	$(RANLIB) $(BUILD_DIR)/$@
 
 ndhc: $(NDHC_OBJS) ifchd-parse.o
-	$(CC) $(CFLAGS) $(NCM_INC) -o $(BUILD_DIR)/$@ $(subst src/,$(OBJ_DIR)/src/,$(NDHC_OBJS)) $(BUILD_DIR)/ncmlib.a $(BUILD_DIR)/objs/src/ifchd-parse.o
+	$(CC) $(CFLAGS) $(NCM_INC) -o $(BUILD_DIR)/$@ $(subst src/,$(OBJ_DIR)/src/,$(NDHC_OBJS)) $(BUILD_DIR)/ncmlib.a $(BUILD_DIR)/objs/src/ifchd-parse.o $(LINK_LIBS)
 
 .PHONY: all clean