Introduction to File I/O

• A set of functions defined by POSIX (Portable Operating System Interface)
• No buffering mechanism is provided and system calls are made for each read/write operation
• The core concept is the file descriptor
• Accessing various types of files

File Descriptor

• One file descriptor for each open file
• The file descriptor is a non-negative integer, and Linux assigns a file descriptor to each open file in the program
• File descriptors are allocated from 0, increasing in order
• File IO operations are done using file descriptors

Note: Open file systems in each program will be assigned file descriptors individually, without interacting with each other

File I/O differs from standard I/O

Standard I/O	File I/O
ANSIC	POSIX
Buffered (reduces system calls)	No buffer (system calls are required to read and write files)
Stream (FILE structure) Open File	A file descriptor represents an open file

open File

The open function creates or opens a file:

#include <fcntl.h>
int open(const char *path,int oflag,...);
//Parameter 1: Open file path parameter 2: Open mode

• Return file descriptor on success; return EOF on error
• Use two parameters when opening a file
• Permissions for the new file specified by the third parameter when creating the file
• Only device files can be opened

Example 1:

Open 1.txt in a write-only manner.Create if the file does not exist and empty if it does exist.

int fd;
if((fd=open("1.txt",O_WRONLY|O_CREAT|O_TRUNC,0666))<0
{
	perror("open");
	return -1;
}

Example 2:

Open file 1.txt read-write.Create if file does not exist and error if file exists:

int fd;
if((fd=open("1.txt",O_RDWR|O_CREAT|O_EXCL,0666))<0)
{
	if(errno==EEXIST)
	{
		perror("exist error");
	}
	else
	{
		perror("other error");
	}
}

close file

The close function closes an open file:

#include <unistd.h>
int close(int fd);

• Return 0 on success; EOF on error
• Close all open files automatically at program end
• File cannot be manipulated after it has been closed

read File

The read function reads data from a file:

#include <unistd.h>
ssize_t read(int fd,void *buf,size_t count);

• Returns the actual number of bytes read on success; returns EOF on error
• Return 0 when reading to the end of the file
• buf is the buffer to receive data

Example

Read content from specified files (text files) and size them

#include <stdio.h>
#include <unistd.h>
int main(int argc,char *argv[])
{
	int fd,n,total=0;
	char buff[64];
	if(argc<2){
		prinff("Usage:%s<file>\n",argv[0];
		return -1;
	}
	if((fd=open(argv[1],O_RDONLY))<0){
		perror("open";
		return -1;
	}
	while((n=read(fd,buf,64))>0){//Read 64 bytes into buf and save the return value to n
		total+=n;//The value of total is the size of the file
	}
	printf("size:%d\n",total);
	return 0;
}

write to file

The write function is used to write data to a file:

#include <unistd.h>
ssize_t write(int fd,void *buf,size_t count);//buf writes; count writes size

• Returns the actual number of bytes written on success; returns EOF on error
• buf is the buffer to send data
• count should not exceed buf size

Example

Write keyboard input to a file until quit ends:

#include <stdio.h>
#include <unistd.h>
#include <string.h>
int main(int argc,char *argv[])
{
	int fd;
	char buf[20];
	//Open file as write-only, empty if file does not exist to create, and empty if file exists
	if((fd =open(argv[1],O_WRONLY|OCREAT|O_TRUNC,0666))<0{
		perror("open");
		return -1;
	}
	while(fgets(buf,20,stdin)!=NULL){
		if(strcmp(buf,"quit\n")==0)break;
		write(fd,buf,strlen(buf));
	}

Locate file (lseek)

The lseek function is used to locate files:

#include <unistd.h>
off_t lseek(int fd,off_t offset,intt whence);

• Return the read and write location of the current file on success; return EOF on error
• The offset and whence parameters are exactly the same as flook

Access directory (opendir/readdir)

The opendir function opens a directory file:

#include <dirent.h>
DIR *opendir(const char *name);

• DIR is the structure type used to describe an open directory file
• Return directory stream pointer on success; NULL on error

The readdir function reads the contents of the directory stream:

#include <dirent.h>
struct sirent *readdir(DIR *dirp);

• struct dirent is the type of structure used to describe a catalog item in a catalog stream
• Include char d_Member such as name[256]
• Return to next directory item in directory stream dirp on success
• Error or return to NULL at end

Close directory

closedir closes a directory file:

#include <dirent.h>
int closedir(DIR *drip);

Example:

Print the names of all files in the specified directory:

#include <stdio.h>
#include <dirent.h>
int main(int argc,char *argv[])
{
	DIR *drip;
	struct dirent *dp;
	if(argc<2){
		printf("Usage :%s<directory>\n"argc[0]);return -1;
	}
	if((dirp=opendir(argv[1]))==NULL){
		perror("opendir");
		return -1;
	}
	while((dp=readdir(dirp))!=NULL){
		printf("%s\n",dp->d_name);
	}
	closedir(dirp);
	return 0;
}

Modify File Properties (chmod/fchmod)

The chmod/fchmod function is used to modify file access rights:

#include <sys/stat.h>
int chmod(const char *path,mode_t mode);
int fchmod(int fd,mode_t mode);

• Return 0 on success; EOF on error
• root and file owner can modify file access

Get file properties (stat/lstat/fstat)

#include <stdio.h>
int stat(const char *path,struct stat *buf);
int lstat(const char *path,struct stat *buf);
int fstat(int fd,struct stat *buf);

• Return 0 on success and EOF on error;
• If path is a symbolic link stat, it gets the properties of the target file; lstat gets the properties of the link file

Strct stat is the type of structure that holds file attributes:

Structural type	Effect
mode_t st_mode	Types and access rights
uid_t st_uid	Owner id
uid_t st_gid	User id
off_t st_size	file size
time_t st_mtime	Last Modified Time

St_The model uses the macros provided by the system to determine the file type:
Through (st_Mode&0170000) The calculated values match the following

file type	calculated value
S_ISREG(st_mode) Normal file	0100000
S_ISDIR(st_mode) catalog file	0040000
S_ISCHR(st_mode)	0020000
S_ISBLK(st_mode)	0060000

Example

Get and display file properties:

#include <stdio.h>
#include <unistd.h>
#include <time.h>
#include <sys/types.h>
#include <sys/stat.h>
int main(int argc,char *argv[])
{
	struct stat buf;
	struct tm *tp;//Pointer to get local time
	int n;
	if(argc<2)
	{
		printf("Usage:%s <file>\n",argv[0]);
		return -1;
	}
	if(lstat(argv[1],&buf)<0)
	{
		perror("lstat");
		return -1;
	}
	switch(buf.st_mode&S_IFMT)
	{
		case S_IFRGE:
			printf("-");
			break;
		case S_IFDIR:
			printf("d");//Is a directory file
			break;
	}
	for(n=8;n>=0;n--)
	{
		if(buf.st_mode&(1<<n))
		{
			switch(n%3)
			{
			case 2:
				printf("r");
			case 1:
				printf("w");
				break;
			case 0:
				printf("x");
				break;
			}
		}
		else
		{
			printf("-");			
		}
	}
	printf("%lu",buf.st_size);
	tp=localtime(buf.st_mtime);//Convert to Local Time
	printf("%d-%02d-%02d",tp->tm_year+1900,tp->tm_mon+1,tp->tm_mday);//Years, months, days
	printf("%s\n",argc[1]);
	return 0;
}