Linux C files and directories 3 files read and write

File read/write

File read and write refers to the reading of information from a file or writing information to a file. Linux file reads can be implemented using the Read function, and file writes can be implemented using the Write function. when a file is written, it is only manipulated in the buffer of the file and may not be written to the file immediately. You need to use the sync or Fsync function to write the buffer's data to a file .

File Write operations:

function write can write a string to an already opened file, this function is used as follows:

ssize_t Write (int fd, void *buf, size_t count);

Parameters:

FD: The file number of the file that has been opened.

BUF: the string to write.

Count: An integer that requires the number of characters to be written. Represents the number of bytes that need to be written to the content.

return value:

[[email protected] exercise]$ cat write.c
#include <stdio.h>
#include <unistd.h>
# Include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h
#include <errno.h>
int main (void)
{
int fd;
Char path[] = "Txt1.txt";
Char s[]= "Hello". .";
extern int errno;
FD = open (path, o_wronly| O_creat| O_trunc, 0766);
if (fd! =-1)
{
printf ("opened file%s. \ n", path);
}
Else
{
printf ("Can ' t Open file%s.\n", path);                          
printf ("errno:%d\n", errno),                 &N Bsp        //print error number
printf ("ERR:%s\n", Strerror (errno));             Print the information corresponding to the error number.
}
Write (fd, S, sizeof (s)),
Close (FD);
printf ("done\n");
return 0;
}

[Email protected] exercise]$./write
Opened file Txt1.txt.
Done

Reading the file function read

The function read reads a string from an open file.

ssize_t Read (int fd, void *buf, size_t count);

Parameter: FD: Represents the number of the file that has been opened.

BUF: is a null pointer, and the read content is returned to the string that the pointer points to.

Count: Indicates the number of characters that need to be read.

Return value: Returns the number of characters read to. A return value of 0 indicates that the end of the file has been reached or that no content is readable in the file.

FD = open (path, o_rdonly);
if (FD! =-1)
{
printf ("opened file%s. \ n", path);
}
Else
{
printf ("Can ' t Open file%s.\n", path);
printf ("errno:%d\n", errno);
printf ("ERR:%s\n", Strerror (errno));
}
if (size = read (fd, str, sizeof (STR))) < 0)
{
printf ("ERR:%s", strerror (size)); If there is a mistake, print the error message by error number.
}
Else
{
printf ("%s\n", str);
printf ("%d\n", size);
}
Close (FD);
return 0;
}

Result

　　

Opened file Txt1.txt.
Hello ...
10

Linux C files and directories 3 files read and write