让busybox里的free命令可以显示page cache

busybox的free命令不能显示page cache，导致于其功能大打折扣，因为page cache大小很大，在用户申请内存时可以回收，打上如下补丁后，busybox的free命令变得完整：

commit 2219fd301a7f319258ad4be9217cd0d6db9240d4
Author: Barry Song 
Date:   Mon Mar 26 11:47:10 2012 +0800

    busybox: update free command to include page cache
   
    eg.
    sh-4.2$ free
                        total         used         free       shared      buffers       cached
    Mem:               402924        49232       353692            0          668        32160
    -/+ buffers/cache:              16404       386520
    Swap:                  0            0            0
   
    Change-Id: Ic9de9f8cd0051da8fd61ae8238a1d2573587223b
    Signed-off-by: Barry Song 

diff --git a/procps/free.c b/procps/free.c
index efbac5b..6abf1e8 100644
--- a/procps/free.c
+++ b/procps/free.c
@@ -29,11 +29,35 @@ static unsigned long long scale(unsigned long d)
         return ((unsigned long long)d * G.mem_unit) >> G_unit_steps;
 }
 
+static unsigned long get_cache_from_meminfo(void)
+{
+#define LINE_LEN 256
+       FILE *fp;
+       char str[LINE_LEN];
+       unsigned long long cached;
+       if((fp = fopen("/proc/meminfo","rt")) == NULL) {
+               printf("Cantnot open /proc/meminfo");
+               exit(1);
+       }
+
+       while(1) {
+               fgets(str, LINE_LEN, fp);
+               if (!strncmp(str, "Cached:", strlen("Cached:")))
+                       break;
+       }
+
+       cached = atoi(str + strlen("Cached:"));
+
+       fclose(fp);
+
+       return cached;
+}
 
 int free_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int free_main(int argc UNUSED_PARAM, char **argv IF_NOT_DESKTOP(UNUSED_PARAM))
 {
         struct sysinfo info;
+       unsigned long long cached;
 
         INIT_G();
 
@@ -60,43 +84,45 @@ int free_main(int argc UNUSED_PARAM, char **argv IF_NOT_DESKTOP(UNUSED_PARAM))
 #endif
 
         sysinfo(&info);
+       cached = get_cache_from_meminfo();
 
         /* Kernels prior to 2.4.x will return info.mem_unit==0, so cope... */
         G.mem_unit = (info.mem_unit ? info.mem_unit : 1);
 
-       printf("     %13s%13s%13s%13s%13s\n",
+       printf("           %13s%13s%13s%13s%13s%13s\n",
                 "total",
                 "used",
                 "free",
-               "shared", "buffers" /* swap and total don't have these columns */
+               "shared", "buffers", "cached" /* swap and total don't have these columns */
                 /* procps version 3.2.8 also shows "cached" column, but
                  * sysinfo() does not provide this value, need to parse
                  * /proc/meminfo instead and get "Cached: NNN kB" from there.
                  */
         );
 
-#define FIELDS_5 "%13llu%13llu%13llu%13llu%13llu\n"
-#define FIELDS_3 (FIELDS_5 + 2*6)
-#define FIELDS_2 (FIELDS_5 + 3*6)
+#define FIELDS_6 "%13llu%13llu%13llu%13llu%13llu%13llu\n"
+#define FIELDS_3 (FIELDS_6 + 3*6)
+#define FIELDS_2 (FIELDS_6 + 4*6)
 
-       printf("Mem: ");
-       printf(FIELDS_5,
+       printf("Mem:       ");
+       printf(FIELDS_6,
                 scale(info.totalram),
                 scale(info.totalram - info.freeram),
                 scale(info.freeram),
                 scale(info.sharedram),
-               scale(info.bufferram)
+               scale(info.bufferram),
+               cached
         );
         /* Show alternate, more meaningful busy/free numbers by counting
          * buffer cache as free memory (make it "-/+ buffers/cache"
          * if/when we add support for "cached" column): */
-       printf("-/+ buffers:      ");
+       printf("-/+ buffers/cache:      ");
         printf(FIELDS_2,
-               scale(info.totalram - info.freeram - info.bufferram),
-               scale(info.freeram + info.bufferram)
+               scale(info.totalram - info.freeram - info.bufferram) - cached,
+               scale(info.freeram + info.bufferram) + cached
         );
 #if BB_MMU
-       printf("Swap:");
+       printf("Swap:     ");
         printf(FIELDS_3,
                 scale(info.totalswap),
                 scale(info.totalswap - info.freeswap),