readn函数功能：在网络编程的读取数据中，通常会需要用到一个读指定字节才返回的函数，linux系统调用中没有给出，需要自己封装。
readn实现代码：
int readn(int fd, void *vptr, size_t n){ size_t nleft = n; //readn函数还需要读的字节数 ssize_t nread = 0; //read函数读到的字节数 unsigned char *ptr = (char *)vptr; //指向缓冲区的指针
while (nleft > 0) { nread = read(fd, ptr, nleft); if (-1 == nread) { if (EINTR == errno) nread = 0; else return -1; } else if (0 == nread) { break; } nleft -= nread; ptr += nread; } return n - nleft;}
writen函数功能：读满n个字节才返回
writen代码实现：
int writen(int fd, const void *vptr, size_t n){ size_t nleft = n; //writen函数还需要写的字节数 ssize_t nwrite = 0; //write函数本次向fd写的字节数 const char* ptr = vptr; //指向缓冲区的指针
while (nleft > 0) { if ((nwrite = write(fd, ptr, nleft)) <= 0) { if (nwrite < 0 && EINTR == errno) nwrite = 0; else return -1; } nleft -= nwrite; ptr += nwrite; } return n;}
readline函数功能：读到'\n'或者读满缓冲区才返回
readline函数实现：
static ssize_t readch(int fd, char *ptr){ static int count = 0; static char* read_ptr = 0; static char read_buf[1024*4] = {0};
if (count <= 0) { again: count = read(fd, read_buf, sizeof(read_buf)); if (-1 == count) if (EINTR == errno) goto again; else return -1; else if (0 == count) return 0; read_ptr = read_buf; } count--; *ptr = *read_ptr++; return 1;}
ssize_t readline(int fd, void *vptr, size_t maxlen){ ssize_t i = 0; ssize_t ret = 0; char ch = ''; char* ptr = NULL;
ptr = (char *)vptr;
for (i = 1; i < maxlen; ++i) { ret = readch(fd, &ch); if (1 == ret) { *ptr++ = ch; if ('\n' == ch) break; } else if (0 == ret) { *ptr = 0; return i-1; } else return -1; } *ptr = 0; return i;}
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