Linux文件操作总结（系统调用和标准IO库函数）

一、系统调用

❑ open: Open a file or device

❑ read: Read from an open file or device

❑ write: Write to a file or device

❑ close: Close the file or device

❑ ioctl: Pass control information to a device driver

函数原型：#include <unistd.h>

#include <fcntl.h>

#include <sys/types.h>

#include <sys/stat.h>

int open(const char *path, int oflags);

int open(const char *path, int oflags, mode_t mode);

(mode=O_RDONLY  |  O_WRONLY  |  O_RDWR)

size_t write(int fildes, const void *buf, size_t nbytes);

size_t read(int fildes, void *buf, size_t nbytes);

int close(int fildes);

int ioctl(int fildes, int cmd, ...);

off_t lseek(int fildes, off_t offset, int whence);

此处whence有三个参数可选：SEEK_SET、SEEK_CUR、SEEK_END

SEEK_SET：是一个绝对值，相对文件头的offset位置；

SEEK_CUR：是相对于当前文件读写指针的offset相对位置；

SEEK_END：是相对于文件末尾的offset相对位置。

❑ SEEK_SET: offset is an absolute position

❑ SEEK_CUR: offset is relative to the current position

❑ SEEK_END: offset is relative to the end of the file

int fstat(int fildes, struct stat *buf);

int stat(const char *path, struct stat *buf);

int lstat(const char *path, struct stat *buf);

二、标准IO库函数

❑ fopen, fclose

❑ fread, fwrite

❑ fflush

❑ fseek

❑ fgetc, getc, getchar

❑ fputc, putc, putchar

❑ fgets, gets

❑ printf, fprintf, and sprintf

❑ scanf, fscanf, and sscanf

函数原型：

#include <stdio.h>

FILE *fopen(const char *filename, const char *mode);

❑ “r” or “rb”: Open for reading only

❑ “w” or “wb”: Open for writing, truncate to zero length

❑ “a” or “ab”: Open for writing, append to end of file，追加方式，将要添加的内容追加在已有的部分后面。

❑ “r+” or “rb+” or “r+b”: Open for update (reading and writing)，如果想要打开一个文件，先读取里面的内容，再更新原来的内容，可以用此方式。

❑ “w+” or “wb+” or “w+b”: Open for update, truncate to zero length，如果想要打开一个文件，先写此文件，再读此文件，可以用此方式。

❑ “a+” or “ab+” or “a+b”: Open for update, append to end of file，打开一个文件，可读可写，写的话，是以追加方式写在文件末尾的。

size_t fread(void *ptr, size_t size, size_t nitems, FILE *stream);

size_t fwrite (const void *ptr, size_t size, size_t nitems, FILE *stream);

int fclose(FILE *stream);

int fflush(FILE *stream);

int fseek(FILE *stream, long int offset, int whence);

此处whence有三个参数可选：SEEK_SET、SEEK_CUR、SEEK_END

SEEK_SET：是一个绝对值，相对文件头的offset位置；

SEEK_CUR：是相对于当前文件读写指针的offset相对位置；

SEEK_END：是相对于文件末尾的offset相对位置。

❑ SEEK_SET: offset is an absolute position

❑ SEEK_CUR: offset is relative to the current position

❑ SEEK_END: offset is relative to the end of the file

int fgetc(FILE *stream);

int getc(FILE *stream);

int getchar();

int fputc(int c, FILE *stream);

int putc(int c, FILE *stream);

int putchar(int c);

char *fgets(char *s, int n, FILE *stream);

char *gets(char *s);

int printf(const char *format, ...);

int sprintf(char *s, const char *format, ...);

int fprintf(FILE *stream, const char *format, ...);

int scanf(const char *format, ...);

int fscanf(FILE *stream, const char *format, ...);

int sscanf(const char *s, const char *format, ...);

三、例程

1.文件拷贝（系统调用方式） 

#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
int main()
{
char c;
int in, out;
in = open(“file.in”, O_RDONLY);
out = open(“file.out”, O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);
while(read(in,&c,1) == 1)
write(out,&c,1);
exit(0);
}

#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
int main()
{
char block[1024];
int in, out;
int nread;
in = open(“file.in”, O_RDONLY);
out = open(“file.out”, O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);
while((nread = read(in,block,sizeof(block))) > 0)
write(out,block,nread);
exit(0);
}

前者要比后者效率低很多很多，因为系统调用极其浪费CPU资源，应尽量减少系统调用的次数。

2.文件拷贝（IO库函数方式）

#include <stdio.h>
#include <stdlib.h>
int main()
{
int c;
FILE *in, *out;
in = fopen(“file.in”,”r”);
out = fopen(“file.out”,”w”);
while((c = fgetc(in)) != EOF)
fputc(c,out);
exit(0);
}

#include <stdio.h>
#include <stdlib.h>

int main()
{
char buf[1024];
int c;
FILE *in, *out;
in = fopen("file.in","r");
out = fopen("file.out","w");
while((c = fread(buf,1,1024,in)) != 0)
fwrite(buf,1,c,out);
exit(0);
}