# cat /proc/meminfo 
MemTotal:        3880404 kB
MemFree:         3182248 kB
MemAvailable:    3396580 kB
Buffers:           39588 kB
Cached:           355616 kB
SwapCached:            0 kB
Active:           318708 kB
Inactive:         252380 kB
Active(anon):     176120 kB
Inactive(anon):      248 kB
Active(file):     142588 kB
Inactive(file):   252132 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:        175940 kB
Mapped:            54184 kB
Shmem:               488 kB
Slab:              62040 kB
SReclaimable:      48712 kB
SUnreclaim:        13328 kB
KernelStack:        2672 kB
PageTables:         6080 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     1940200 kB
Committed_AS:     615340 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       14160 kB
VmallocChunk:   34359715580 kB
HardwareCorrupted:     0 kB
AnonHugePages:     36864 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       53120 kB
DirectMap2M:     3092480 kB
DirectMap1G:     3145728 kB

MemUsed  = MemTotal - MemFree

free = MemFree + Buffers + Cahched

MemUsedPrec = 100*(MemTotal - MemFree - Buffers - Cahched)/MemTotal

实际可挪用的内存数: free+cache+buffer,
实际可使用的内存数: used-cache-buffer (total-free-cache-buffer)。
内存占用率：(total-available) / total * 100 

struct MEM_INFO
{
    unsigned int total;
    unsigned int free;
    unsigned int buffers;
    unsigned int cached;
    unsigned int swap_cached;
    unsigned int swap_total;
    unsigned int swap_free;
    unsigned int available;
};
typedef struct MEM_INFO Mem_info;

void  get_mem_occupy (Mem_info *o)
{
    FILE* fpMemInfo = fopen("/proc/meminfo", "r");
    if (NULL == fpMemInfo)
    {
        return ;
    }
    int i = 0;
    int value;
    char name[1024];
    char line[1024];
    int nFiledNumber = 2;
    int nMemberNumber = 5;
    while (fgets(line, sizeof(line) - 1, fpMemInfo))
    {
        if (sscanf(line, "%s%u", name, &value) != nFiledNumber)
        {
            continue;
        }
        if (0 == strcmp(name, "MemTotal:"))
        {
            ++i;
            o->total = value;
        }
        else if (0 == strcmp(name, "MemFree:"))
        {
            ++i;
            o->free = value;
        }
        else if (0 == strcmp(name, "MemAvailable:"))
        {
            ++i;
            o->available = value;
        }
        else if (0 == strcmp(name, "Buffers:"))
        {
            ++i;
            o->buffers = value;
        }
        else if (0 == strcmp(name, "Cached:"))
        {
            ++i;
            o->cached = value;
        }
        if (i == nMemberNumber)
        {
            break;
        }
    }
    // system("free");
    // system("cat /proc/meminfo");
    // printf("MemTotal      : %d\n",o->total);
    // printf("MemFree       : %d\n",o->free);
    // printf("MemAvailable  : %d\n",o->available);
    // printf("MemBuffers    : %d\n",o->buffers);
    // printf("MemCached     : %d\n",o->cached);
    // printf("MemSwapCached : %d\n",o->swap_cached);
    // printf("MemSwapTotal  : %d\n",o->swap_total);
    // printf("MemSwapFree   : %d\n",o->swap_free);
    fclose(fpMemInfo);
}

float cal_mem_occupy(Mem_info *o)
{
    return (100.0 * (o->total - o->available) / o->total);
}

Mem_info omem;
while(1)
{
    // printf("-------------------- Mem occupy -------------------\n");
    get_mem_occupy(&omem);
    printf("Mem Usage(%): %8.4f\n", cal_mem_occupy(&omem));
    printf("\n");
}