man sysinfo

return information on overall system statistics;

#include <sys/sysinfo.h>

int sysinfo(struct sysinfo *info);

struct sysinfo{

long uptime;

......

unsigned long totalram;//Total useable main memorysize

unsigned long freeram;//Available memory size(可用页的个数)

  ......

};

usage:

struct sysinfo aa;

sysinfo(&aa);

printf(...,aa.freeram);