我正在执行一项任务，我们正在开发自己的 RPC 客户端。在编译我的服务器部分时，我收到以下警告：
implicit declaration of function 'read'
implicit declaration of function 'write'
我知道如果我要在我的 main 之后创建一个函数，我通常会收到此警告，例如：
int main() {
doSomething();
}
void doSomething() {
...
}
在上述情况下，它应该抱怨我创建的函数“doSomething”。
那么为什么我的编译器会抱怨一个系统调用是隐式声明的，当它出现在一个在 main 之前声明的函数中时？下面是出现系统调用的函数。
void Open(int connfd) {
/*Get message size*/
unsigned char temp[4] = { 0 };
int n = read(connfd, temp, 4);
if(n < 0) {/*On error*/
perror("Read error");
exit(1);
}/*End if*/
unsigned int msgSize = temp[0] +
(temp[1] * 256) +
(temp[2] * 256 * 2) +
(temp[3] * 256 * 3);
printf("msgSize = %d\n", msgSize);
/*Allocate memory for message*/
char * msg = malloc(msgSize);
if(msg == NULL) {
perror("Allocation error");
exit(1);
}/*End if*/
msg = memset(msg, 0, msgSize);
/*Read entire message from client*/
n = read(connfd, msg, msgSize);
if(n < 0) {/*On error*/
perror("Read error");
exit(1);
}/*End if*/
/*Extract pathname from message - NULL terminated*/
char * pathname = malloc(strlen(msg) + 1);
if(pathname == NULL) {
perror("Allocation error");
exit(1);
}/*End if*/
pathname = memset(pathname, 0, strlen(msg) + 1);
pathname = memcpy(pathname, msg, strlen(msg));
/*Extract flags from message*/
int i;
for(i = 0; i < sizeof(int); i++) {
temp[i] = msg[strlen(pathname) + 1 + i];
}/*End for i*/
unsigned int flags = temp[0] +
(temp[1] * 256) +
(temp[2] * 256 * 2) +
(temp[3] * 256 * 3);
/*Extract mode from message*/
for(i = 0; i < sizeof(mode_t); i++) {
temp[i] = msg[strlen(pathname) + 1 + sizeof(int) + 1 + i];
}/*End for i*/
mode_t mode = temp[0] +
(temp[1] * 256) +
(temp[2] * 256 * 2) +
(temp[3] * 256 * 3);
free(msg);/*Free msg since it is no longer needed*/
/*Open pathname*/
umask(0);
int fd = open(pathname, flags, mode);
free(pathname);/*Free pathname since it is no longer needed*/
/*Prepare response*/
char * response = malloc(sizeof(int) * 2);
if(response == NULL) {
perror("Allocation error");
exit(1);
}/*End if*/
response = memset(response, 0, sizeof(int) * 2);
/*Build return message*/
memcpy(&response[0], &fd, sizeof(fd));
memcpy(&response[4], &errno, sizeof(fd));
/*Can't guarante socket will accept all we try to write, cope*/
int num, put;
int left = sizeof(int) * 2; put = 0;
while(left > 0) {
if((num = write(connfd, response + put, left)) < 0) {
perror("inet_wstream: write");
exit(1);
} else {
left -= num;
put += num;
}/*End else*/
}/*End while*/
free(response);/*Free response since it is no longer needed*/
return;
}/*End Open*/