I have a small problem with my code in the following. I call it in my class from within a state machine this->write_file(this->d_filename);. The case in the loop gets hit a couple of times, however I only have one line of entries in the CSV file I want to produce.
I'm not sure why this is. I open the file with this->open(filename) in my write function. It returns the file-descriptor. The file is opened with O_TRUNK, and if ((d_new_fp = fdopen(fd, d_is_binary ? "wba" : "w")) == NULL). While the aba refers to write, binary and append. Therefore I expect more than one line.
The fprintf statement writes my data. It also has a \n.
fprintf(d_new_fp, "%s, %d %d\n", this->d_packet, this->d_lqi, this->d_lqi_sample_count);
I simply can't figure out why my file doesn't grow.
Best,
Marius
inline bool
cogra_ieee_802_15_4_sink::open(const char *filename)
{
gruel::scoped_lock guard(d_mutex); // hold mutex for duration of this function
// we use the open system call to get access to the O_LARGEFILE flag.
int fd;
if ((fd = ::open(filename, O_WRONLY | O_CREAT | O_TRUNC | OUR_O_LARGEFILE,
0664)) < 0)
{
perror(filename);
return false;
}
if (d_new_fp)
{ // if we've already got a new one open, close it
fclose(d_new_fp);
d_new_fp = 0;
}
if ((d_new_fp = fdopen(fd, d_is_binary ? "wba" : "w")) == NULL)
{
perror(filename);
::close(fd);
}
d_updated = true;
return d_new_fp != 0;
}
inline void
cogra_ieee_802_15_4_sink::close()
{
gruel::scoped_lock guard(d_mutex); // hold mutex for duration of this function
if (d_new_fp)
{
fclose(d_new_fp);
d_new_fp = 0;
}
d_updated = true;
}
inline void
cogra_ieee_802_15_4_sink::write_file(const char* filename)
{
if (this->open(filename))
{
fprintf(d_new_fp, "%s, %d %d\n", this->d_packet, this->d_lqi,
this->d_lqi_sample_count);
if (true)
{
fprintf(stderr, "Writing file %x\n", this->d_packet);
}
}
}
Description for O_TRUNC from man open:
If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified.
The file is opened in each call to write_file(), removing anything that was previously written. Replace O_TRUNC with O_APPEND.
Related
I have a following scenario: Someone creates a pseudo terminal via opening /dev/ptmx. New terminal is created and named for example /dev/pts/2. Then, in my program I open /dev/pts/2 for reading. But I also open other devices for reading and use select() function to wait for any incoming data. The select have also some timeout specified for performing other stuff when no data arrives for too long. After successful select i read data using read() function and then print it on the screen.
I encountered an issue if the pseudo terminal is closed by the one who created it. In this case select function ends immediately indicating success as well as read ends indicating "no data" by returning zero. The issue imho is that neither select nor read returns error in such case. How should I handle this to detect that the terminal is no longer existing?
Status processData()
{
fd_set readFileDescriptorSet; // defined somewhere else
int maxFileDescriptor; // defined somewhere else
struct timeval timeout; // defined somewhere else
int ret = select(maxFileDescriptor + 1, &readFileDescriptorSet, nullptr, nullptr, &timeout);
if (!ret) // timeout
return Status::success();
if (ret < 0) // error from select()
return Status::error("select error");
ssize_t rd;
char buff[10];
do {
rd = read(interfaces.serialPort.getFileDescriptor(), buff, sizeof(buff) - 1);
if (rd > 0) { // some data has been read
buff[rd] = '\0';
std::cout << buff;
}
} while (rd > 0);
if (rd < 0) // error from read()
return Status::error("read error");
return Status::success();
}
While the way I open the pseudo terminal is following:
Status internalOpen(std::string fileName)
{
close();
fileDescriptor = ::open(fileName.c_str(), O_RDWR | O_NOCTTY | O_NONBLOCK);
if (fileDescriptor == -1)
return Status::error("Terminal::internalOpen::open('" + fileName + "')");
struct termios attributes;
if (tcgetattr(fileDescriptor, &attributes))
return Status::error("Terminal::internalOpen::tcgetattr()");
setAttributes(attributes);
if (tcsetattr(fileDescriptor, TCSANOW, &attributes))
return Status::error("Terminal::internalOpen::tcsetattr()");
return Status::success();
}
void setAttributes(struct termios &attributes)
{
cfmakeraw(&attributes);
cfsetspeed(&attributes, Config::baudRate);
attributes.c_iflag &= ~(IXOFF | IXANY);
attributes.c_oflag &= ~(ONLCR);
attributes.c_lflag &= ~(ECHOE);
attributes.c_cflag &= ~(CSTOPB | CRTSCTS);
attributes.c_cflag |= CREAD | CLOCAL;
attributes.c_cc[VMIN] = 0;
attributes.c_cc[VTIME] = 0;
}
After select() returns indicating that there's something to be read, the shown code loops repeatedly trying to read() from the non-blocking file descriptor until it is 0:
do {
rd = read( ...
} while (rd > 0);
That's certainly reasonable. Except that the closed connection results in the very first read() returning 0, which the shown logic cannot discriminate.
All that's really needed here is to keep track of whether anything has been read, prior read() returning 0. But if read() returned 0 right off the bat, your goose is cooked.
Additionally, there a few other improvements will make things more robust.
After select() returns, actually check if the file descriptor's bit remains set in the readFileDescriptorSet. The shown logic simply assumes that it is, by checking for all other possibilities. Still, this is somewhat fragile. It's easy to forget this assumption if something tangentially related gets modified (i.e., another fle descriptor gets thrown into the mix).
Use poll() instead of select(), and explicitly check for POLLHUP|POLLRDHUP in revents. The file descriptor closure condition is more explicitly called out, in the poll() interface.
I'm writing unit tests for a function that may lock a file (using fcntl(fd, F_SETLK, ...)) under some conditions.
I want my unit test to be able to EXPECT that the file is or is not locked at certain points. But I can't find any way to test that. I've tried using F_GETLK, but it will only tell you if the lock would not be able to be placed. Since a given process can re-lock the same file as often as it wants, F_GETLK is returning F_UNLCK, indicating the file is unlocked.
For instance, if I run the following little program:
int main(int argc, char** argv) {
int fd = open("/tmp/my_test_file", O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
if (fd < 0) {
return EXIT_FAILURE;
}
// Initial lock
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0; // Lock entire file.
if (fcntl(fd, F_SETLK, &lock) < 0) {
return EXIT_FAILURE;
}
// Test lock:
lock.l_type = F_WRLCK;
lock.l_pid = 0;
if (fcntl(fd, F_GETLK, &lock) < 0) {
return EXIT_FAILURE;
}
switch (lock.l_type) {
case F_WRLCK:
std::cout << lock.l_pid << " is holding the lock\n";
break;
case F_UNLCK:
std::cout << "File is unlocked\n";
break;
default:
std::cout << "Unexpected " << lock.l_type << "\n";
break;
}
return EXIT_SUCCESS;
}
It will print:
File is unlocked
So, is there a way for a process to test if it is holding an fcntl file lock?
Also, are there other kinds of (Linux-portable!) file locks I could be using that would solve my problem?
Well I am not aware of any "already available library” for this, but on implementation level I would suggest you to have a log file which keeps track of that.
You can simply create a file named “log”, mmap(2) it as MAP_SHARED into each process accessing the file and whenever a file lock succeeds, write the current process’ pid at the end of that log file maintaining offset to the end with CAS. That will help you analyse order of locks.
Maybe simply by opening file in append more and writing to the end, the pid of current process.
Or maybe a quicker way to do so such a test is to create a fifo file by mkfifo(2) and write to that file.
The function fopen("file-name",a); will return a pointer to the end of the file. If the file exist it is opened, otherwise a new file is created.
Is it possible to use the append mode and open the file only if it already exist? (and return a NULL pointer otherwise).
Thanks in advance
To avoid race conditions, opening and checking for existence should be done in one system call. In POSIX this can be done with open as it will not create the file if the flag O_CREAT is not provided.
int fd;
FILE *fp = NULL;
fd = open ("file-name", O_APPEND);
if (fd >= 0) {
/* successfully opened the file, now get a FILE datastructure */
fp = fdopen (fd, "a")
}
open may fail for other reasons too. If you do not want to ignore all of them, you will have to check errno.
int fd;
FILE *fp = NULL;
do {
fd = open ("file-name", O_APPEND);
/* retry if open was interrupted by a signal */
} while (fd < 0 && errno == EINTR);
if (fd >= 0) {
/* successfully opened the file, now get a FILE datastructure */
fp = fdopen (fd, "a")
} else if (errno != ENOENT) { /* ignore if the file does not exist */
perror ("open file-name"); /* report any other error */
exit (EXIT_FAILURE)
}
First check if the file already exists. A simple code to do that might be like this:
int exists(const char *fname)
{
FILE *file;
if ((file = fopen(fname, "r")))
{
fclose(file);
return 1;
}
return 0;
}
It will return 0 if file doesn't exist...
and use it like this:
if(exists("somefile")){file=fopen("somefile","a");}
I have this method which supposed to get a buffer and write some content to a file:
void writeTasksToDevice()
{
TaskInfo *task;
unsigned int i = lastTaskWritten;
printf("writing elihsa\n");
outputFile.write(" Elisha2", 7);
//pthread_mutex_lock(&fileMutex);
for(; i < (*writingTasks).size(); i++)
{
task = (*writingTasks).at(i);
if(NULL == task)
{
printf("ERROR!!! in writeTasksToDevice - there's a null task in taskQueue. By "
" design that should NEVER happen\n");
exit(-1);
}
if(true == task->wasItWritten)
{
//continue;
}
else // we've found a task to write!
{
printf("trying to write buffer to file\n");
printf("buffer = %s, length = %d\n", task->buffer, task->length);<====PRINT HERE IS OK< PRINTING WHAT IS WANTED
outputFile.write(task->buffer, task->length); <===SHOULD WRITE HERE
printf("done writing file\n");
}
}
//pthread_mutex_unlock(&fileMutex);
// TODO: check if we should go to sleep and wait for new tasks
// and then go to sleep
}
the buffer content is:
task->buffer: elishaefla
task->length: 10
i opened the stream in another init function using:
outputFile.open(fileName, ios :: app);
if(NULL == outputFile)
{
//print error;
return -1;
}
but at the end, the file content is empty, nothing is being written.
any idea why?
You did not provide enough information to answer the question with certainty, but here are some of the issues you might be facing:
You did not flush the buffer of the ofstream
You did not close the file that you are trying to open later on (if I'm correct, outputFile is a global variable, so it is not closed automatically until the end of the program)
I wrote a UNIX daemon (targeting Debian, but it shouldn't matter) and I wanted to provide some way of creating a ".pid" file, (a file which contains the process identifier of the daemon).
I searched for a way of opening a file only if it doesn't exist, but couldn't find one.
Basically, I could do something like:
if (fileexists())
{
//fail...
}
else
{
//create it with fopen() or similar
}
But as it stands, this code does not perform the task in a atomic fashion, and doing so would be dangerous, because another process might create the file during my test, and the file creation.
Do you guys have any idea on how to do that?
Thank you.
P.S: Bonus point for a solution which only involves std::streams.
man 2 open:
O_EXCL Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and pathname already exists, then open()
will fail. The behavior of O_EXCL is undefined if O_CREAT is not specified.
so, you could call fd = open(name, O_CREAT | O_EXCL, 0644); /* Open() is atomic. (for a reason) */
UPDATE: and you should of course OR one of the O_RDONLY, O_WRONLY, or O_RDWR flags into the flags argument.
I learned about proper daemonizing here (back in the day):
http://www.enderunix.org/docs/eng/daemon.php
It is a good read. I have since improved the locking code to eliminate race conditions on platforms that allow advisory file locking with specific regions specified.
Here is a relevant snippet from a project that I was involved in:
static int zfsfuse_do_locking(int in_child)
{
/* Ignores errors since the directory might already exist */
mkdir(LOCKDIR, 0700);
if (!in_child)
{
ASSERT(lock_fd == -1);
/*
* before the fork, we create the file, truncating it, and locking the
* first byte
*/
lock_fd = creat(LOCKFILE, S_IRUSR | S_IWUSR);
if(lock_fd == -1)
return -1;
/*
* only if we /could/ lock all of the file,
* we shall lock just the first byte; this way
* we can let the daemon child process lock the
* remainder of the file after forking
*/
if (0==lockf(lock_fd, F_TEST, 0))
return lockf(lock_fd, F_TLOCK, 1);
else
return -1;
} else
{
ASSERT(lock_fd != -1);
/*
* after the fork, we instead try to lock only the region /after/ the
* first byte; the file /must/ already exist. Only in this way can we
* prevent races with locking before or after the daemonization
*/
lock_fd = open(LOCKFILE, O_WRONLY);
if(lock_fd == -1)
return -1;
ASSERT(-1 == lockf(lock_fd, F_TEST, 0)); /* assert that parent still has the lock on the first byte */
if (-1 == lseek(lock_fd, 1, SEEK_SET))
{
perror("lseek");
return -1;
}
return lockf(lock_fd, F_TLOCK, 0);
}
}
void do_daemon(const char *pidfile)
{
chdir("/");
if (pidfile) {
struct stat dummy;
if (0 == stat(pidfile, &dummy)) {
cmn_err(CE_WARN, "%s already exists; aborting.", pidfile);
exit(1);
}
}
/*
* info gleaned from the web, notably
* http://www.enderunix.org/docs/eng/daemon.php
*
* and
*
* http://sourceware.org/git/?p=glibc.git;a=blob;f=misc/daemon.c;h=7597ce9996d5fde1c4ba622e7881cf6e821a12b4;hb=HEAD
*/
{
int forkres, devnull;
if(getppid()==1)
return; /* already a daemon */
forkres=fork();
if (forkres<0)
{ /* fork error */
cmn_err(CE_WARN, "Cannot fork (%s)", strerror(errno));
exit(1);
}
if (forkres>0)
{
int i;
/* parent */
for (i=getdtablesize();i>=0;--i)
if ((lock_fd!=i) && (ioctl_fd!=i)) /* except for the lockfile and the comm socket */
close(i); /* close all descriptors */
/* allow for airtight lockfile semantics... */
struct timeval tv;
tv.tv_sec = 0;
tv.tv_usec = 200000; /* 0.2 seconds */
select(0, NULL, NULL, NULL, &tv);
VERIFY(0 == close(lock_fd));
lock_fd == -1;
exit(0);
}
/* child (daemon) continues */
setsid(); /* obtain a new process group */
VERIFY(0 == chdir("/")); /* change working directory */
umask(027); /* set newly created file permissions */
devnull=open("/dev/null",O_RDWR); /* handle standard I/O */
ASSERT(-1 != devnull);
dup2(devnull, 0); /* stdin */
dup2(devnull, 1); /* stdout */
dup2(devnull, 2); /* stderr */
if (devnull>2)
close(devnull);
/*
* contrary to recommendation, do _not_ ignore SIGCHLD:
* it will break exec-ing subprocesses, e.g. for kstat mount and
* (presumably) nfs sharing!
*
* this will lead to really bad performance too
*/
signal(SIGTSTP,SIG_IGN); /* ignore tty signals */
signal(SIGTTOU,SIG_IGN);
signal(SIGTTIN,SIG_IGN);
}
if (0 != zfsfuse_do_locking(1))
{
cmn_err(CE_WARN, "Unexpected locking conflict (%s: %s)", strerror(errno), LOCKFILE);
exit(1);
}
if (pidfile) {
FILE *f = fopen(pidfile, "w");
if (!f) {
cmn_err(CE_WARN, "Error opening %s.", pidfile);
exit(1);
}
if (fprintf(f, "%d\n", getpid()) < 0) {
unlink(pidfile);
exit(1);
}
if (fclose(f) != 0) {
unlink(pidfile);
exit(1);
}
}
}
See also http://gitweb.zfs-fuse.net/?p=sehe;a=blob;f=src/zfs-fuse/util.c;h=7c9816cc895db4f65b94592eebf96d05cd2c369a;hb=refs/heads/maint
The only way I can think of is to use system level locks. See this: C++ how to check if file is in use - multi-threaded multi-process system
One way to approach this problem is to open the file for appending. If the function succeeds and the position is at 0 then you can be fairly certain this is a new file. Could still be an empty file but that scenario may not be important.
FILE* pFile = fopen(theFilePath, "a+");
if (pFile && gfetpos(pFile) == 0) {
// Either file didn't previously exist or it did and was empty
} else if (pFile) {
fclose(pFile);
}
It would appear that there's no way to do it strictly using streams.
You can, instead, use open (as mentioned above by wildplasser) and if that succeeds, proceed to open the same file as a stream. Of course, if all you're writing to the file is a PID, it is unclear why you wouldn't just write it using C-style write().
O_EXCL only excludes other processes that are attempting to open the same file using O_EXCL. This, of course, means that you never have a perfect guarantee, but if the file name/location is somewhere nobody else is likely to be opening (other than folks you know are using O_EXCL) you should be OK.