O_APPEND drapeau et lseek

En effet, O_APPEND seulement affecte le comportement de write, mais pas celle de read. Quelle que soit la façon dont la position actuelle d'un fichier est modifié par lseek, write toujours append-only.

Lorsque vous open un fichier avec O_RDWR | O_APPEND, read vont commencer à partir du début du fichier.

Dans le manuel de open (man 2 open),

O_APPEND
Le fichier est ouvert en mode ajout. Avant chaque écrire(2), l'offset du fichier est positionné à la fin du fichier.

Dans le manuel de write (man 2 write),

Si O_APPEND drapeau de l'état du dossier de drapeaux est définie, le fichier offset
doit être réglé à la fin du fichier avant chaque écrire.

Dans le noyau Linux fs/ext4 syscall écrire -> vfs_write -> ext4_file_write_iter,
le ext4_file_write_iter appellera ext4_write_checks

alors appel generic_write_checks

vous trouverez l'endroit où le réglage de la pos = file.size

/* FIXME: this is for backwards compatibility with 2.4 */
if (iocb->ki_flags & IOCB_APPEND)
    iocb->ki_pos = i_size_read(inode);
pos = iocb->ki_pos;

La démonstration suivante peut le vérifier.

cat open_append.cc
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <string>
#include <iostream>
int main(int argc, char *argv[]) {
std::string path = "./test.txt";
std::string content = "hello_world";
std::string read_buf(content.size(), 0x0);
struct stat st_buf;
ssize_t bytes_read = -1;
ssize_t bytes_write = -1;
int ret = -1;
off_t cur_off = -1;
int fd = ::open(path.c_str(), O_CREAT | O_RDWR | O_TRUNC, 0644);
if (fd < 0) {
std::cerr << "open err path " << path
<< " errno " << errno << std::endl;
return -1;
}
std::cout << "open ok path " << path
<< " fd " << fd << std::endl;
//Step 1 write some data into an empty file
bytes_write = ::write(fd, content.data(), content.size());
if (bytes_write < 0) {
std::cerr << "write err fd " << fd
<< " errno " << errno << std::endl;
goto out;
}
std::cout << "write ok fd " << fd
<< " data " << content
<< " nbytes " << bytes_write << std::endl;
::close(fd);
//Step 2 open the file again with O_APPEND
fd = -1;
fd = ::open(path.c_str(), O_CREAT | O_RDWR | O_APPEND, 0644);
if (fd < 0) {
std::cerr << "open again err path " << path
<< " errno " << errno << std::endl;
return -1;
}
std::cout << "open again ok path " << path
<< " fd " << fd << std::endl;
//Step 3 the current position of the file NOT affected by O_APPEND
cur_off = ::lseek(fd, 0, SEEK_CUR);
if (cur_off < 0) {
std::cerr << "lseek err SEEK_CUR fd " << fd
<< " errno " << errno << std::endl;
goto out;
}
//cur_off expected to be 0
std::cout << "lseek ok SEEK_CUR fd " << fd
<< " cur_off " << cur_off << std::endl;
//Step 4  the read will start from the beginning of the file
bytes_read = read(fd, (char*)read_buf.data(), content.size());
if (bytes_read < 0) {
std::cerr << "read err fd " << fd
<< " errno " << errno << std::endl;
goto out;
}
std::cout << "read ok fd " << fd
<< " data " << read_buf
<< " nbytes " << bytes_read << std::endl;
//Step 5 change the position to the half of the file size
cur_off = ::lseek(fd, content.size() / 2, SEEK_SET);
if (cur_off < 0) {
std::cerr << "lseek err SEEK_SET fd " << fd
<< " errno " << errno << std::endl;
goto out;
}
//cur_off expected to be content.size() /2
std::cout << "lseek ok SEEK_SET fd " << fd
<< " cur_off " << cur_off << std::endl;
//Step 6 write will append data from the end of the file
//the current position is ignored
bytes_write = ::write(fd, content.data(), content.size());
if (bytes_write < 0) {
std::cerr << "append write err fd " << fd
<< " errno " << errno << std::endl;
goto out;
}
std::cout << "append write ok fd " << fd
<< " append data " << content
<< " append nbytes " << bytes_write << std::endl;
//Step 7 the file size is double content.size()
memset((void*)&st_buf, 0x0, sizeof(struct stat));
ret = lstat(path.c_str(), &st_buf);
if (ret < 0) {
std::cerr << "lstat err path " << path
<< " errno " << errno << std::endl;
goto out;
}
std::cout << "lstat ok path " << path
<< " st_size " << st_buf.st_size << std::endl;
ret = 0;
out:
if (fd >= 0) {
close(fd);
}
return ret;
}

Résultat de sortie

open ok path ./test.txt fd 3
write ok fd 3 data hello_world nbytes 11
open again ok path ./test.txt fd 3
lseek ok SEEK_CUR fd 3 cur_off 0
read ok fd 3 data hello_world nbytes 11
lseek ok SEEK_SET fd 3 cur_off 5
append write ok fd 3 append data hello_world append nbytes 11
lstat ok path ./test.txt st_size 22

OriginalL'auteur York Tsai