C语言 在linux中使用stdio.h库却找不到popen函数

写了一段代码尝试获取主机主频信息：

double get_cpu_info(){
    double cpuclockrate;
    FILE *fp;
    char str[81];
    if(fp=popen("cat /proc/cpuinfo|grep cpu\\ MHz|sed -e 's/.*:[^0-9]//'","r")==NULL){
        printf("无法读取CPU主频信息\n");
    }else{
        fgets(str,80,fp);
        double spd=atof(str);
        cpuclockrate = spd;
        pclose(fp);
    }
    return cpuclockrate;
}

make编译提醒我没有对popen函数进行声明。

[root@iZ2ze2y6fwj9mvujtsiya4Z test_mem]# make
gcc -c systeminfo.c
systeminfo.c: In function ‘get_cpu_info’:
systeminfo.c:56:10: warning: assignment makes pointer from integer without a cast [enabled by default]
     if(fp=popen("cat /proc/cpuinfo|grep cpu\\ MHz|sed -e 's/.*:[^0-9]//'","r")==NULL){
          ^
gcc -o main main.o systeminfo.o cpuusage.o

就提示信息看，不是没有popen，而是返回值fp的类型不兼容，少一个强制转换