Linux开发C语言应用程序，编译出现以下四条warning：

warning: implicit declaration of function ‘strcmp’ [-Wimplicit-function-declaration]
    17 | if (argc != 3 || strcmp(argv[1], “–help”) == 0) {
         | ^~~~~~
warning: implicit declaration of function ‘read’; did you mean ‘fread’? [-Wimplicit-function-declaration]
    33 | while ((numRead = read(inputFd, buf, BUF_SIZE)) > 0) {
         | ^~~~
         | fread
warning: implicit declaration of function ‘write’; did you mean ‘fwrite’? [-Wimplicit-function-declaration]
    34 | if (write(outputFd, buf, numRead) != numRead) {
         | ^~~~~
         | fwrite
warning: implicit declaration of function ‘close’; did you mean ‘pclose’? [-Wimplicit-function-declaration]
    42 | if (close(inputFd) == -1) {
         | ^~~~~
         | pclose

当xxx是strcmp时，解决方法是：引入#include <string.h>。

当xxx是read/write/close时，解决方法是：引入#include <fcntl.h>和#include <unistd.h>。

当xxx是usleep时，解决方法是：引入#include <unistd.h>。