linux的系统调用open, read函数（文件编程）使用demo
1.引言
为了学习linux系统下的app开发，记载了学习文件编程的笔记
2.open函数
功能
打开一个文件
头文件
#include<sys/stat.h> #include<fcntl.h>
函数形式
int open(const char* pathname, int flags, mode_t mode);
返回值
如果调用成功，则返回文件描述符号，标识文件资源，后续会使用。 如果调用出错，则会返回-1
参数
pathname：打开的文件名（含路径）。
flags: 文件访问模式的bit mask。
mode: 文件权限模式
3. 使用案例
copy文件的案例
文件如下，名字:copy_file.c
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <errno.h>#define BUFFER_SIZE 1024 int main(int argc,char *argv[])
{ int from_fd,to_fd; int bytes_read,bytes_write; char buffer[BUFFER_SIZE]; char *ptr; //参数防卫，命令选项必须输入3个参数if(argc!=3) { fprintf(stderr,"Usage:%s fromfile tofile/n/a",argv[0]); exit(1); } //以只读的方式打开第一个参数（文件路径）if((from_fd=open(argv[1],O_RDONLY))==-1) { //打开失败fprintf(stderr,"Open %s Error:%s/n",argv[1],strerror(errno)); exit(1); } // Constant Octal value Permission bit
// S_ISUID 04000 Set-user-ID
// S_ISGID 02000 Set-group-ID
// S_ISVTX 01000 Sticky
// S_IRUSR 0400 User-read
// S_IWUSR 0200 User-write
// S_IXUSR 0100 User-execute
// S_IRGRP 040 Group-read
// S_IWGRP 020 Group-write
// S_IXGRP 010 Group-execute
// S_IROTH 04 Other-read
// S_IWOTH 02 Other-write
// S_IXOTH 01 Other-execute// Flag describtion
// O_RDONLY Open for reading only v3
// O_WRONLY Open for writing only v3
// O_RDWR Open for reading and writing v3
// O_CLOEXEC Set the close-on-exec flag (since Linux 2.6.23) v4
// O_CREAT Create file if it doesn’t already exist v3
// O_DIRECT File I/O bypasses buffer cache
// O_DIRECTORY Fail if pathname is not a directory v4
// O_EXCL With O_CREAT: create file exclusively v3
// O_LARGEFILE Used on 32-bit systems to open large files
// O_NOATIME Don’t update file last access time on read() (since Linux 2.6.8)
// O_NOCTTY Don’t let pathname become the controlling terminal v3
// O_NOFOLLOW Don’t dereference symbolic links v4
// O_TRUNC Truncate existing file to zero length v3
// O_APPEND Writes are always appended to end of file v3
// O_ASYNC Generate a signal when I/O is possible
// O_DSYNC Provide synchronized I/O data integrity (since Linux 2.6.33) v3
// O_NONBLOCK Open in nonblocking mode v3
// O_SYNC Make file writes synchronousif((to_fd=open(argv[2],O_WRONLY|O_CREAT,S_IRUSR|S_IWUSR))==-1) { fprintf(stderr,"Open %s Error:%s/n",argv[2],strerror(errno)); exit(1); } while(bytes_read=read(from_fd, buffer, BUFFER_SIZE)) { if((bytes_read==-1)&&(errno!=EINTR)) {break;} else if(bytes_read > 0) { ptr = buffer; while(bytes_write=write(to_fd,ptr,bytes_read)) { if((bytes_write==-1)&&(errno!=EINTR))break; else if(bytes_write==bytes_read)break; else if(bytes_write>0) { ptr+=bytes_write; bytes_read-=bytes_write; } } if(bytes_write==-1)break; } } close(from_fd); close(to_fd); exit(0);
}
编译用Makefile
TARGET := app
#src file
SRC := copy_file.call:$(TARGET)@echo "make successfull"$(TARGET): $(SRC)@echo $(SRC)gcc $^ -I. -o $@clean:rm $(TARGET).PHONY:all,clean
ubuntu系统下使用gcc编译通过，运行实例如下
终端命令：./app copy_file.c a.c
结果展示命令： ll
total 41
drwxrwxrwx 1 root root 4096 May 25 23:05 ./
drwxrwxrwx 1 root root 0 Mar 8 21:31 ../
-rwxrwxrwx 1 root root 3165 May 25 23:05 a.c* --->（复制成功的新文件）
-rwxrwxrwx 1 root root 17056 May 25 23:04 app*
-rwxrwxrwx 1 root root 3165 May 25 23:04 copy_file.c*