总内存大小/proc/meminfo

Question

 struct sysinfo sys_info;
 int32_t total_ram = 0;    
 if (sysinfo(&sys_info) != -1)
   total_ram = (sys_info.totalram * sys_info.mem_unit)/1024;

上述代码中的total_ram值为3671864。但是/proc/meminfo显示了一个不同的值。

cat /proc/meminfo | grep MemTotal
MemTotal:       16255004 kB

他们为什么不一样？在Linux中获得物理RAM大小的正确方法是什么？

Answer 1

这是溢出造成的。如果涉及的数字超过40亿(如4GB+内存)，请确保使用64bit+类型：

 struct sysinfo sys_info;
 int32_t total_ram = 0;    
 if (sysinfo(&sys_info) != -1)
   total_ram = ((uint64_t) sys_info.totalram * sys_info.mem_unit)/1024;

下面是一个完整的示例：

#include <stdint.h>
#include <stdio.h>
#include <sys/sysinfo.h>

int main() {
  struct sysinfo sys_info;
  int32_t before, after;
  if (sysinfo(&sys_info) == -1) return 1;

  before = (sys_info.totalram * sys_info.mem_unit)/1024;
  after = ((uint64_t)sys_info.totalram * sys_info.mem_unit)/1024;
  printf("32bit intermediate calculations gives %d\n", before);
  printf("64bit intermediate calculations gives %d\n", after);
  return 0;
}

编译和运行时：

$ gcc foo.c -o foo -m32 -Wall -Werror -ansi -pedantic && ./foo
32bit intermediate calculations gives 2994988
64bit intermediate calculations gives 61715244