I have this peice of code that should read from a pseudo terminal and print to another pseudo terminal (stdin):
// Declare & define a file descriptor.
int fd;
int status;
char string[] = "XXXX";
char buffer[8];
// Establish connection between a file and the file descriptor.
fd = open("/dev/pts/4", O_RDWR);
// Check if connection was established.
if(fd == -1){
printf("Assigning failed: %s\n", strerror(errno));
}
else{
fputs("Assigning successful!\n", stdout);
// Set some flags for the connection.
fcntl(fd, F_SETFL, 0);
}
// Read from the connection.
status = read(fd, buffer, 8);
if (status == -1){
printf("Read failed: %s\n", strerror(errno));
}
else{
printf("I read this: %s\n", buffer);
}
return 0;
I don't know exactly what fcntl(fd, F_SETFL, 0); does, but untill I used it I wasn't able to read from the /dev/pts/4. After I used it for the first time I could read normaly, even if I commented out this line of code.
I tried to explain it to myself reading POSIX...
F_SETFL flag is defined in the POSIX as:
Set the file status flags, defined in <fcntl.h>, for the file
description associated with fildes from the corresponding bits in the
third argument, arg, taken as type int. Bits corresponding to the file
access mode and the file creation flags, as defined in <fcntl.h>, that
are set in arg shall be ignored. If any bits in arg other than those
mentioned here are changed by the application, the result is
unspecified. If fildes does not support non-blocking operations, it is
unspecified whether the O_NONBLOCK flag will be ignored.
So it sets file status flags asociated with the 1st argument (file descriptor) of fnctl() to 0? I found an explanation about the status flags in the POSIX:
The <fcntl.h> header shall define the following symbolic constants for
use as file status flags for open(), openat(), and fcntl(). The values
shall be suitable for use in #if preprocessing directives.
O_APPEND Set append mode. O_DSYNC Write according
to synchronized I/O data integrity completion. O_NONBLOCK
Non-blocking mode. O_RSYNC Synchronized read I/O
operations. O_SYNC Write according to synchronized I/O
file integrity completion.
So does fcntl(fd, F_SETFL, 0); set all those to 0? Are all of those flags a single bit? How do I only set one of them to 1 or 0? And how do I know which bit is assigned to which flag?
It's a bitmask. Each flag is a bit (or several bits) in the integer you pass to fcntl. Each symbolic constant expands to a number in which only the corresponding bit(s) is set, and so you can set several flags by ORing them together. For instance, if bit 2 is the append flag, then O_APPEND would be 0b100 == 4; if bit 3 is the nonblock flag, then O_NONBLOCK would be 0b1000 == 8, and so on. But by using the symbolic constants, you don't need to know which bits they actually are, which is good since they may vary between OSes.
In fcntl(fd, F_SETFL, 0), the number 0 has no bits set, so all the flags are cleared. To set append and clear the rest, you could do fcntl(fd, F_SETFL, O_APPEND);. To set both append and nonblock, OR their constants together: fcntl(fd, F_SETFL, O_APPEND | O_NONBLOCK);
With similar bit operations, you can operate on the flags returned by fl = fcntl(fd, F_GETFL):
int fl = fcntl(fd, F_GETFL);
if (fl & O_APPEND) { ... } // test if in append mode
fcntl(fd, F_SETFL, fl | O_NONBLOCK); // turn on nonblock and keep everything else the same
fcntl(fd, F_SETFL, fl & ~O_DSYNC); // turn off dsync and keep everything else the same
There's some more examples here.
I am starting serial port programming in Linux. After reading several examples on the web, I don't understand exact effect of fcntl(fd, F_SETFL, 0)? It is clearing bits, but what flags does it affect? What does it set and or clear?
Take one by one
1) Function call used
fcntl() - It perform the operation on file descriptor passed in argument.
2) 2nd argument in call
F_SETFL (int)
Set the file status flags to the value specified by arg. File access
mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags (i.e.,
O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored. On Linux this
command can change only the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME,
and O_NONBLOCK flags.
3) 3rd argument in call
It is 0 means, It set file status flag to zero.
As Jean-Baptiste Yunès said in comment.
file access mode and file creation flags are ignored. This command
reset every other flags: no append, no async, no direct, no atime, and
no nonblocking
So finally
fcntl(fd, F_SETFL, 0)
This call will set opened file desciptor's file status flag to value 0.
But idealy this way we should not change file status flag.
Best way is to first get the current file status flag using F_GETFL and then just change required bit in that.
See example:
If you want to modify the file status flags, you should get the current flags with F_GETFL and modify the value. Don’t assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag O_NONBLOCK without altering any other flags:
/* Set the O_NONBLOCK flag of desc if value is nonzero,
or clear the flag if value is 0.
Return 0 on success, or -1 on error with errno set. */
int
set_nonblock_flag (int desc, int value)
{
int oldflags = fcntl (desc, F_GETFL, 0);
/* If reading the flags failed, return error indication now. */
if (oldflags == -1)
return -1;
/* Set just the flag we want to set. */
if (value != 0)
oldflags |= O_NONBLOCK;
else
oldflags &= ~O_NONBLOCK;
/* Store modified flag word in the descriptor. */
return fcntl (desc, F_SETFL, oldflags);
}
per the man page for fcntl()
F_SETFL (int)
Set the file status flags to the value specified by arg. File
access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation
flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are
ignored. On Linux this command can change only the O_APPEND,
O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags. It is not
possible to change the O_DSYNC and O_SYNC flags; see BUGS,
below.
This is from the man page.
While trying to use fcntl() with command F_GETFL and F_SETFL, I got some questions:
Why the flag returned by fcntl(fd, F_GETFL) only include a subset of bits of what I set when open file? Does it only show the ones that are modifiable?
When use fcntl(fd, F_SETFL, flag), how should I pass the flag param, do I need to read flag via fcntl(fd, F_GETFL) first, then modify it and pass it? Or internally it just do a bit & operation with the new param?
Where can I find a full list of the 32 (or less) bits of open file flags?
Code - [dup_fd_share.c]:
// prove duplicated fd shared file offset and open file status,
// TLPI exercise 5.5
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#define BUF_SIZE 100
void fd_share() {
char *fp = "/tmp/fd_share.txt";
char *buf = "abc\n";
int write_size = 4;
int fd, fd2;
off_t cur, cur2;
int open_flag, open_flag2;
// open
int flag = O_RDWR | O_CREAT | O_TRUNC | O_APPEND;
printf("file flag param: %x\n", flag);
fd = open(fp, flag, 0644);
// dup
fd2 = dup(fd);
// initial offset
cur = lseek(fd, 0, SEEK_CUR);
printf("fd[%d] offset: %ld\n", fd, cur);
cur2= lseek(fd2, 0, SEEK_CUR);
printf("fd[%d] offset: %ld\n", fd2, cur2);
// write, offset change,
write(fd, buf, 4);
printf("write %d chars\n", write_size);
// new offset
cur = lseek(fd, 0, SEEK_CUR);
printf("fd[%d] offset: %ld\n", fd, cur);
cur2= lseek(fd2, 0, SEEK_CUR);
printf("fd[%d] offset: %ld\n", fd2, cur2);
// get original open file flag,
open_flag = fcntl(fd, F_GETFL);
printf("fd[%d] open flag: %x\n", fd, open_flag);
open_flag2 = fcntl(fd2, F_GETFL);
printf("fd[%d] open flag: %x\n", fd2, open_flag2);
// change open file flag,
open_flag &= ~O_APPEND;
if(fcntl(fd, F_SETFL, open_flag) == -1) {
printf("failed to set flag\n");
return;
}
printf("change open file flag, remove %s\n", "O_APPEND");
open_flag = fcntl(fd, F_GETFL);
printf("fd[%d] open flag: %x\n", fd, open_flag);
open_flag2 = fcntl(fd2, F_GETFL);
printf("fd[%d] open flag: %x\n", fd2, open_flag2);
close(fd);
}
int main(int argc, char *argv[]) {
fd_share();
return 0;
}
Output:
file flag param: 642
fd[3] offset: 0
fd[4] offset: 0
write 4 chars
fd[3] offset: 4
fd[4] offset: 4
fd[3] open flag: 402
fd[4] open flag: 402
change open file flag, remove O_APPEND
fd[3] open flag: 2
fd[4] open flag: 2
You asked:
Why the flag returned by fcntl(fd, F_GETFL) only include a subset of bits of what I set when open file? Does it only show the ones that are modifiable?
No; it only shows the ones that are "remembered" by the system, such as O_RDWR. These can really be called "flags". Some of the other bits ORed into the oflag parameter are more like "imperative instructions" to the open system call: for example, O_CREAT says "please create this file if it doesn't exist" and O_TRUNC says "please truncate it", neither of which are "flags". A file that was truncated on creation is indistinguishable from a file that was not truncated on creation: they're both just "files". So after open is done creating or truncating the file, it doesn't bother to "remember" that history. It only "remembers" important things, like whether the file is open for reading or writing.
Edited to add: These different kinds of flags have semi-official names. O_RDWR is the "access mode" (remembered, non-modifiable); O_APPEND is an "operating mode" (remembered, usually modifiable); O_TRUNC is an "open-time flag" (pertains to the open operation itself, not to the file descriptor; therefore not remembered). Notice that the "access mode" is not modifiable — you can't use fcntl to turn a read-only fd into a write-only fd.
When use fcntl(fd, F_SETFL, flag), how should I pass the flag param, do I need to read flag via fcntl(fd, F_GETFL) first, then modify it and pass it? Or internally it just do a bit & operation with the new param?
F_SETFL overwrites the flags with exactly what you pass in (although it will ignore your puny attempts to set bits-that-aren't-really-flags, such as O_TRUNC). IF you want to set a specific flag and leave the other flags as-is, then you must F_GETFL the old flags, | the new flag in, and then F_SETFL the result. This must be done as two separate system calls; there is no atomic or thread-safe way to accomplish it as far as I know.
Where can I find a full list of the 32 (or less) bits of open file flags?
In fcntl.h or its documentation (man fcntl). For example, on my MacBook the man page says:
The flags for the F_GETFL and F_SETFL commands are as follows:
O_NONBLOCK Non-blocking I/O; if no data is available to a read call, or if a write operation would block, the read or write
call returns -1 with the error EAGAIN.
O_APPEND Force each write to append at the end of file; corresponds to the O_APPEND flag of open(2).
O_ASYNC Enable the SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be
read.
In other words, there are exactly three bits you can set (or unset) on OS X. Whereas on Linux, the man page says this:
On Linux this command can change only the O_APPEND, O_ASYNC,
O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
Incidentally, some Linux filesystems have the concept of an "append-only file" at the filesystem level; if you open one of those files and then try to clear the resulting descriptor's O_APPEND flag, you'll get an EPERM error. The "append-only"-ness of a file can be controlled at the filesystem level using the chattr utility.
Here's a more systematic version of your test program. It might not be of interest to you, but I learned something by writing it, so I'm leaving it here. :)
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int main() {
int fd = open("/tmp/fd_share.txt", O_RDWR | O_CREAT | O_TRUNC | O_APPEND, 0644);
// append to empty file
write(fd, "aaaaaaaaaa", 10);
off_t cur = lseek(fd, 1, SEEK_SET);
printf("offset after being set to 1: %ld\n", (long)cur);
// append
write(fd, "bbbbbbbb", 8);
cur = lseek(fd, 0, SEEK_CUR);
printf("offset after appending bbbbbbbb: %ld\n", (long)cur);
cur = lseek(fd, 2, SEEK_SET);
printf("offset after being set to 2: %ld\n", (long)cur);
// now toggle "append mode" to FALSE
int open_flag = fcntl(fd, F_GETFL);
if (fcntl(fd, F_SETFL, open_flag & ~O_APPEND) == -1) {
printf("failed to set flag\n");
return 0;
}
cur = lseek(fd, 0, SEEK_CUR);
printf("offset after unsetting O_APPEND: %ld\n", (long)cur);
cur = lseek(fd, 3, SEEK_SET);
printf("offset after being set to 3: %ld\n", (long)cur);
// write without appending
write(fd, "cccc", 4);
cur = lseek(fd, 0, SEEK_CUR);
printf("offset after writing cccc: %ld\n", (long)cur);
// now toggle "append mode" to TRUE
open_flag = fcntl(fd, F_GETFL);
if (fcntl(fd, F_SETFL, open_flag | O_APPEND) == -1) {
printf("failed to set flag\n");
return 0;
}
cur = lseek(fd, 0, SEEK_CUR);
printf("offset after unsetting O_APPEND: %ld\n", (long)cur);
// append
write(fd, "dd", 2);
cur = lseek(fd, 0, SEEK_CUR);
printf("offset after appending dd: %ld\n", (long)cur);
close(fd);
}
The output of this program on my MacBook (as it should be on any POSIX system AFAIK) is:
offset after being set to 1: 1
offset after appending bbbbbbbb: 18
offset after being set to 2: 2
offset after unsetting O_APPEND: 2
offset after being set to 3: 3
offset after writing cccc: 7
offset after unsetting O_APPEND: 7
offset after appending dd: 20
1) The return of fcnl is a code that described if the function succceded and how:
RETURN VALUE
For a successful call, the return value depends on the operation:
F_DUPFD The new descriptor.
F_GETFD Value of file descriptor flags.
F_GETFL Value of file status flags.
F_GETLEASE
Type of lease held on file descriptor.
F_GETOWN Value of descriptor owner.
F_GETSIG Value of signal sent when read or write becomes possible, or
zero for traditional SIGIO behavior.
F_GETPIPE_SZ, F_SETPIPE_SZ
The pipe capacity.
F_GET_SEALS
A bit mask identifying the seals that have been set for the
inode referred to by fd.
All other commands
Zero.
On error, -1 is returned, and errno is set appropriately.
ERRORS
EACCES or EAGAIN
Operation is prohibited by locks held by other processes.
EAGAIN The operation is prohibited because the file has been memory-
mapped by another process.
EBADF fd is not an open file descriptor
EBADF cmd is F_SETLK or F_SETLKW and the file descriptor open mode
doesn't match with the type of lock requested.
EBUSY cmd is F_SETPIPE_SZ and the new pipe capacity specified in arg
is smaller than the amount of buffer space currently used to
store data in the pipe.
EBUSY cmd is F_ADD_SEALS, arg includes F_SEAL_WRITE, and there
exists a writable, shared mapping on the file referred to by
fd.
EDEADLK
It was detected that the specified F_SETLKW command would
cause a deadlock.
EFAULT lock is outside your accessible address space.
EINTR cmd is F_SETLKW or F_OFD_SETLKW and the operation was
interrupted by a signal; see signal(7).
EINTR cmd is F_GETLK, F_SETLK, F_OFD_GETLK, or F_OFD_SETLK, and the
operation was interrupted by a signal before the lock was
checked or acquired. Most likely when locking a remote file
(e.g., locking over NFS), but can sometimes happen locally.
EINVAL The value specified in cmd is not recognized by this kernel.
EINVAL cmd is F_ADD_SEALS and arg includes an unrecognized sealing
bit.
EINVAL cmd is F_ADD_SEALS or F_GET_SEALS and the filesystem
containing the inode referred to by fd does not support
sealing.
EINVAL cmd is F_DUPFD and arg is negative or is greater than the
maximum allowable value (see the discussion of RLIMIT_NOFILE
in getrlimit(2)).
EINVAL cmd is F_SETSIG and arg is not an allowable signal number.
EINVAL cmd is F_OFD_SETLK, F_OFD_SETLKW, or F_OFD_GETLK, and l_pid
was not specified as zero.
EMFILE cmd is F_DUPFD and the process already has the maximum number
of file descriptors open.
ENOLCK Too many segment locks open, lock table is full, or a remote
locking protocol failed (e.g., locking over NFS).
ENOTDIR
F_NOTIFY was specified in cmd, but fd does not refer to a
directory.
EPERM Attempted to clear the O_APPEND flag on a file that has the
append-only attribute set.
EPERM cmd was F_ADD_SEALS, but fd was not open for writing or the
current set of seals on the file already includes F_SEAL_SEAL.
2) Flags to be set is your choice: :
F_SETFL (int)
Set the file status flags to the value specified by arg. File
access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation
flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are
ignored. On Linux this command can change only the O_APPEND,
O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags. It is not
possible to change the O_DSYNC and O_SYNC flags; see BUGS,
below.
3) HERE you have a complete description.
#include<apue.h>
#include<unistd.h>
#include<fcntl.h>
#include<string.h>
int main(int argc , char *argv[])
{
int fd = open("test.txt",O_WRONLY | O_CREAT | O_TRUNC);
int pid,status;
const char *str_for_parent = "str_for_parent\n";
const char *str_for_child = "str_for_child\n";
if ((pid = fork()) < 0)
err_sys("fork error");
else if (pid == 0)
{
write(fd,str_for_child,strlen(str_for_child)+1);
_exit(0);
}
wait(&status);
write(fd,str_for_parent,strlen(str_for_parent)+1);
return 0;
}
The test.txt is created by open().But it's permission(---------x) is different with those files(-rw-rw-r--) created by touch or any other softwares in my system.my umask is 0002
open actually takes an (optional) third parameter:
int open(const char *pathname, int flags, mode_t mode);
The mode of the new file is set to the AND of mode and the inverse of your umask (ie, mode & ~umask). Since you're not passing mode, it's getting initialized to some garbage, and you're getting the wrong mode as a result.
Generally if you're passing O_CREAT, you should pass a mode of 0666, unless you have some specific reason to use some other mode (eg, if you want to make sure the file is not readable by other users regardless of what the umask is set to).
If you supply the O_CREAT flag to open(), you must also supply a third mode parameter, which is combined with the current umask to set the created file's permission bits.
Unless you are specifically creating an executable file, you should almost always use 0666 as this parameter:
int fd = open("test.txt",O_WRONLY | O_CREAT | O_TRUNC, 0666);
This should give you the same result that you see from touch. Note that the leading zero in C indicates an octal constant.
I'm working on a networking assignment and we are tasked with creating a remote file access server with a protocol we were given. My difficulties come in the lack of information I can find that explains the process of calculating the bits for the oflag argument in open().
I receive a message from a client to open a file and in the message I parse characters for the flags to use in oflag. Specifically they are:
R - O_RDONLY
W - O_WRONLY
RW - O_RDWR
A - O_APPEND
C - O_CREAT
T - O_TRUNC
E - O_EXCL
I went around Google and searched bitwise operations, enumeration flags, bit flags, calculating bit flags, etc. and couldn't find something that was useful in figuring out how to create the bits for oflag. Maybe I just didn't know what I was looking for and overlooked useful information?
Could someone please:
Point me in the direction/provide links to documentation/example of how to calculate the bits/# I ought to put into oflags given my parsed characters or
Show me the enumeration types for the flags and the ordering they ought to go in
Thanks a bunch for you help and if I wasn't clear on my problem or what I am trying to do, just let me know and I will clarify ASAP.
The O_... flags are numbers each with a different single bit set. For example on my system they are defined in fcntl.h as
#define O_RDONLY 00
#define O_WRONLY 01
#define O_RDWR 02
#define O_CREAT 0100 /* not fcntl */
#define O_EXCL 0200 /* not fcntl */
#define O_NOCTTY 0400 /* not fcntl */
#define O_TRUNC 01000 /* not fcntl */
#define O_APPEND 02000
You use | (logical OR) to combine the flags and pass in a single number to open with all the bits set for each option you want. So e.g. open("file", O_RDWR | O_CREAT).
You can compute an int and pass that into open too if you want.
int flags = 0;
if (...)
flags |= O_RDWR;
...
open('file', flags);
The usual expression would be something like O_RDWR | O_CREAT. And note that O_RDWR is exactly O_RDONLY | O_WRONLY
You can do something like this:
char *flags = "r";
int oflag = 0;
if (strchr(flags,'r')) oflag |= O_RDONLY;
and so on for the rest of the flags.