The C programming language (version 2) notes / 8 UNIX system interface / 8.3 open, create, close, unlink

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8.3 open、creat、close、unlink

In addition to the default standard input 、 Standard output and standard error files , All other files must be opened explicitly before reading or writing
system call open and creat Used to realize this function

open With the first 7 This chapter discusses fopen be similar
The difference is , The former returns a file descriptor , It's just a int Type value , The latter returns a file pointer
If an error occurs ,open Will return -1

#include <fcntl.h>

int fd; 
int open(char *name, int flags, int perms); 
fd = open(name, flags, perms);

And fopen equally , Parameters name Is a string containing the file name
The second parameter flags It's a int Type value , It explains how to open the file , The main values are as follows ：

O_RDONLY     Open the file read-only 
O_WRONLY     Open the file in write only mode 
O_RDWR       Open the file read-write 

stay System V UNIX In the system , These constants are in the header file <fcntl.h> In the definition of
And in the Berkeley（BSD） In version <sys/file.h> In the definition of

have access to fd = open(name, O_RDONLY, 0); Open a file to read

In the discussion of this chapter ,open Parameters of perms The value of is always 0

If you use open Open a file that doesn't exist , Will result in an error
have access to creat The system calls to create a new file or overwrite an existing old file , As shown below ：

int creat(char *name, int perms); 
fd = creat(name, perms);

If creat Successfully created the file , It will return a file descriptor , Otherwise return to -1
If this file already exists ,creat The length of this file will be truncated to 0, So as to discard the existing content
Use creat Creating an existing file does not cause an error

If the file to be created does not exist , be creat With the parameters perms Create a file with the specified permissions
stay UNIX File system , One for each file 9 Bit permission information
They control the of the file respectively owner 、 Owner group and The other members Reading of files 、 Write and perform access
therefore , Through one 3 The octal number of bits can easily explain different permissions
for example ,0755 Of a document owner You can read it 、 Write and perform operations , and Owner group and The other members Only read and execute operations

Here is a simplified UNIX Program cp explain creat Usage of
This program copies one file to another
This version we wrote can only copy one file , Directory is not allowed as the second parameter , also , The permissions of the target file are not obtained by copying , But redefined

#include <stdio.h>
#include <fcntl.h>
#include "syscalls.h"
#define PERMS 0666 /* RW for owner, group, others */

void error(char *, ...);

/* cp: copy f1 to f2 */ 
main(int argc, char *argv[]) 
{
     
    int f1, f2, n; 
    char buf[BUFSIZ];
    
    if (argc != 3) 
        error("Usage: cp from to"); 
    if ((f1 = open(argv[1], O_RDONLY, 0)) == -1) 
        error("cp: can't open %s", argv[1]); 
    if ((f2 = creat(argv[2], PERMS)) == -1) 
        error("cp: can't create %s, mode %03o", argv[2], PERMS); 
    while ((n = read(f1, buf, BUFSIZ)) > 0) 
        if (write(f2, buf, n) != n) 
            error("cp: write error on file %s", argv[2]); 
    return 0; 
}

The output file created by the program has fixed permissions 0666
utilize 8.6 To be discussed in section stat system call , You can get a schema of an existing file , And assign this pattern to its copy

Be careful , function error It's like a function printf, Variable length parameter table can be brought when calling
Pass below error The implementation of the function shows how to use printf Another member of the function family vprintf
Standard library functions vprintf Function and printf Function similar to , The difference is , It replaces the variable length parameter table with a parameter
And this parameter is set by calling va_start Macro initialization
Again ,vfprintf and vsprintf The functions are associated with fprintf and sprintf Function similar to

#include <stdio.h> 
#include <stdarg.h>

/* error: print an error message and die */ 
void error(char *fmt, ...) 
{
     
    va_list args; 
    va_start(args, fmt); 
    fprintf(stderr, "error: "); 
    vprintf(stderr, fmt, args); 
    fprintf(stderr, "\n"); 
    va_end(args); 
    exit(1); 
}

There is a limit to the number of files a program can open at the same time （ Usually it is 20）
Accordingly , If a program needs to process many files at the same time , Then it must reuse the file descriptor
function close(int fd) Used to disconnect file descriptors from open files , And release this file descriptor , For other documents
close Functions and... In the standard library fclose The function corresponds to , But it doesn't need cleaning （flush） buffer
If the program passes exit Function to exit or return from the main program , All open files will be closed

function unlink(char *name) Will file name Remove... From the file system , It corresponds to standard library functions remove

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