Files in linux programming

File IO programming under Linux

Linux system call refers to a set of «special interfaces» provided by the operating system to user programs through which user programs can obtain special services provided by the operating system.
To better protect kernel space, programs are divided into kernel space and user space, which run at different levels and are logically isolated from each other.In Linux, user programs cannot access services provided by the kernel directly, and they must use the services provided by the kernel through system calls.
The user programming interface (API) in Linux follows the most popular application programming interface standard in UNIX, POSIX.These system call programming interfaces are mainly implemented through the C library (libc).
Expiration portal:
The most complete summary of Linux common commands ever (super detailed!Ultra-comprehensive) Collection of this one is enough
Are you clear about these operations of standard IO on Linux (with detailed descriptions and routines)

Article Directory

• Introduction to File I/O
• File Descriptor
• File I/O differs from standard I/O
• open File
• close file
• read File
• write to file
• Locate file (lseek)
• Access directory (opendir/readdir)
• Close directory
• Modify File Properties (chmod/fchmod)
• Get file properties (stat/lstat/fstat)

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 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:

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

Example 2:

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:

• 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 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

#include #include int main(int argc,char *argv[])  int fd,n,total=0; char buff[64]; if(argc2) prinff("Usage:%s\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 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

#include #include #include 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 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 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 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 int closedir(DIR *drip); 

Example:

#include #include int main(int argc,char *argv[])  DIR *drip; struct dirent *dp; if(argc2) printf("Usage :%s\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  int chmod(const char *path,mode_t mode); int fchmod(int fd,mode_t mode); 

Get file properties (stat/lstat/fstat)

#include 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

#include #include #include #include  #include  int main(int argc,char *argv[])  struct stat buf; struct tm *tp;//Pointer to get local time int n; if(argc2)  printf("Usage:%s \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))  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; > 



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Posted by locomotive on Tue, 16 Jun 2020 18:34:54 -0700

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