O_RDONLY’ undeclared 异常

O_RDONLY’ undeclared 错误求助
大家好！我在ununtu系统下用gcc编译自已写的小程序出现如题的错误，求指教是否要包含什么头文件，然后gcc的命令要怎么写？

我编译时的命令是 gcc -o 8_1 8_1.c -lm

×××××××××××××源文件如下×××××××××××××××××

C/C++ code

#include <stdio.h>#include <sys/types.h>#include <unistd.h>void filecopy(int fd1,int fd2);int main(int argc, char *argv[]){    int fd1,fd2;    if (3 != argc)        return -1;    if ((fd1 = open(argv+1,O_RDONLY,0)) == -1)        return -2;    if ((fd2 = open(argv+2,O_RDONLY,0)) == -1)        return -2;    filecopy(fd1,fd2);}void filecopy(int fd1,int fd2){    int n;    char *str[100];    while ((n = read(fd1,str,100)) !=-1 && n != 0 )        write(1,str,n);    while ((n = read(fd2,str,100)) != -1 && n != 0)        write(1,str,n);}