I/O重定向函数dup2

从标准输入拷贝一个文本文件到标准输出，使用dup2函数重定向到把一个磁盘文本文件拷贝到另一个磁盘文件的结尾处。

#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> #include <stdio.h> #include <errno.h> #include <stdlib.h> #define MAXBUF 20 #define DEF_MODE S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH #define DEF_UMASK S_IWGRP|S_IWOTH ssize_t rio_readn(int fd, void *usrbuf, size_t n); ssize_t rio_writen(int fd, void *usrbuf, size_t n); int main(int argc, char **argv) { int n,fd1,fd2; char buf[MAXBUF]; fd1=open("hello.txt",O_RDONLY,0); umask(DEF_UMASK); fd2 = open("bar.txt", O_WRONLY|O_APPEND|O_CREAT, DEF_MODE); dup2(fd1,0); dup2(fd2,1); while((n=rio_readn(STDIN_FILENO,buf,MAXBUF))!= 0) rio_writen(STDOUT_FILENO,buf,n); exit(0); } ssize_t rio_readn(int fd, void *usrbuf, size_t n) { size_t nleft = n; ssize_t nread; char *bufp = usrbuf; while (nleft > 0) { if ((nread = read(fd, bufp, nleft)) < 0) { if (errno == EINTR) /* Interrupted by sig handler return */ nread = 0; /* and call read() again */ else return -1; /* errno set by read() */ } else if (nread == 0) break; /* EOF */ nleft -= nread; bufp += nread; } return (n - nleft); /* Return >= 0 */ } ssize_t rio_writen(int fd, void *usrbuf, size_t n) { size_t nleft = n; ssize_t nwritten; char *bufp = usrbuf; while (nleft > 0){ if ((nwritten = write(fd, bufp, nleft)) <= 0){ if (errno == EINTR) /* Interrupted by sig handler return */ nwritten = 0; /* and call write() again */ else return -1; /* errno set by write() */ } nleft -= nwritten; bufp += nwritten; } return n; }