I'd like to receive (and later process) write(1) and wall(1) messages using a (Unix 98-style) pseudo tty on Linux.
I already have the following minimal implementation:
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <utempter.h>
#define BUF_LENGTH 1024
int
main (void)
{
FILE *lf;
int masterfd, slavefd;
char *slave_name = NULL;
char buf[BUF_LENGTH];
size_t nbytes = sizeof(buf);
ssize_t bytes_read;
int exit_code = EXIT_SUCESS;
if ((masterfd = posix_openpt (O_RDWR | O_NOCTTY)) == -1
|| grantpt (masterfd) == -1
|| unlockpt (masterfd) == -1
|| (slave_name = ptsname (masterfd)) == NULL)
exit (EXIT_FAILURE);
if (!(lf = fopen("term.log","w")))
exit (EXIT_FAILURE);
addToUtmp (slave_name, NULL, masterfd);
for (;;)
{
bytes_read = read(masterfd, buf, nbytes);
if (bytes_read <= 0)
break
fwrite (buf, 1, bytes_read, lf);
}
if (bytes_read < 0)
{
fprintf (stderr, "error reading from master pty: %s\n", strerror (errno));
exit_code = EXIT_FAILURE;
}
fclose (lf);
if (slavefd >= 0)
close (slavefd);
if (masterfd >= 0)
{
removeLineFromUtmp (slave_name, masterfd);
close (masterfd);
}
exit (exit_code);
}
The problem is now that it only works for reading the first message, then read gives me a EIO error. Why is that?
It looks like this happens simply when the last slave file descriptor is closed. Considering write(1) and wall(1) will have the only file descriptor to the slave, you get EIO as soon as those finish writing.
The easiest way to keep this from happening is by keeping a file descriptor around. Right after your ptsname call, do an open(slave_name, O_RDRW).
(Curiously, you already have a slavefd variable, and the code to clean it up. Are you testing us? :p)
Related
The following code is supposed to read the file "rules.txt" and write it to a device line by line.
The flow should be:
Read line from rules.txt
Echo it to device
The following code always end in segfault because of readline and I have no idea why:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <ctype.h>
#include <termios.h>
#include <sys/types.h>
#include <sys/mman.h>
#define BUFFER_LENGTH 256
int main()
{
char *line;
size_t len = BUFFER_LENGTH;
int fd = open("./rules.txt", O_RDONLY);
if(fd == -1)
{ perror("open failed"); return 0; }
FILE* fout = fopen("/sys/class/Rule_Table_Class/Rule_Table_Class_Rule_Table_Device/sysfs_att", "w+");
if(fout == NULL)
{ close(fd); perror("fopen failed, log.txt is busy!"); return 0; }
while (1)
{
line = (char*) malloc(len*sizeof(char));
if(line==NULL){
perror("malloc failed!"); return 0;
}
int bytesRead = getline(&line, &len, fd);
if (bytesRead == -1)
{
perror("Failed to read the message from the device.");
return errno;
}
sprintf(line,"%s","lala");
printf("line = %s", line);
}
fclose(fout);
close(fd);
return 0;
}
EDIT:
I corrected the code and still get a segfault. Here is the corrected code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <ctype.h>
#include <termios.h>
#include <sys/types.h>
#include <sys/mman.h>
#define BUFFER_LENGTH 256
int main()
{
FILE * fp;
char * line = NULL;
size_t len = 0;
ssize_t read;
fp = fopen("./rules.txt", "r");
if(fp == NULL)
{ perror("open failed"); return 0; }
FILE* fout = fopen("/sys/class/Rule_Table_Class/Rule_Table_Class_Rule_Table_Device/sysfs_att", "w+");
if(fout == NULL)
{ perror("fopen failed!"); return 0; }
while (1)
{
ssize_t bytesRead = getline(&line, &len, fp);
if (bytesRead == -1)
{
return 0;
}
printf("line = %s", line);
fprintf(line,"%s",fout);
}
fclose(fout);
fclose(fp);
return 0;
}
First of all, the proper core getline() loop is
/* FILE *in = fopen(..., "r"); */
char *line = NULL;
size_t size = 0;
while (1) {
ssize_t len = getline(&line, &size, in);
if (len < 0)
break;
/* You have 'len' chars at 'line', with line[len] == '\0'. */
}
so, it is not getline() that causes the segfault, it is your fprintf(line, "%s", fout); that should be either fprintf(fout, "%s", line); or just fputs(line, fout);, or fwrite(line, 1, bytesRead, fout); since the line can contain embedded NUL bytes that fprintf() and fputs() consider an end of string mark.
If we modify the code so that the source and target file names are taken as command-line arguments (with - denoting standard input or standard output), this is what I'd personally like to see:
/* SPDX-License-Identifier: CC0-1.0 */
/* This tells the GNU C library to expose POSIX features, including getline(). */
#define _POSIX_C_SOURCE 200809L
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
int main(int argc, char *argv[])
{
if (argc != 3 || !strcmp(argv[1], "-h") || !strcmp(argv[1], "--help")) {
const char *self = (argc > 0 && argv && argv[0] && argv[0][0]) ? argv[0] : "(this)";
fprintf(stderr, "\n");
fprintf(stderr, "Usage: %s [ -h | --help ]\n", self);
fprintf(stderr, " %s SOURCE TARGET\n", self);
fprintf(stderr, "\n");
fprintf(stderr, "This copies all content from SOURCE to TARGET, line by line.\n");
fprintf(stderr, "Use '-' for standard input source or standard output target.\n");
fprintf(stderr, "\n");
return EXIT_SUCCESS;
}
const char *srcpath = argv[1];
const char *dstpath = argv[2];
/* If the path is "-", set it to NULL. */
if (srcpath && !strcmp(srcpath, "-"))
srcpath = NULL;
if (dstpath && !strcmp(dstpath, "-"))
dstpath = NULL;
FILE *src, *dst;
/* Open source for reading. If srcpath is NULL, use stdin. */
if (srcpath) {
src = fopen(srcpath, "r");
if (!src) {
fprintf(stderr, "%s: %s.\n", srcpath, strerror(errno));
return EXIT_FAILURE;
}
} else {
src = stdin;
}
/* Open target for writing. If dstpath is NULL, use stdout. */
if (dstpath) {
dst = fopen(dstpath, "w");
if (!dst) {
fprintf(stderr, "%s: %s.\n", dstpath, strerror(errno));
fclose(src);
return EXIT_FAILURE;
}
} else {
dst = stdout;
}
char *line = NULL;
size_t size = 0;
unsigned long linenum = 0;
while (1) {
ssize_t len = getline(&line, &size, src);
if (len < 0)
break;
linenum++;
if (fwrite(line, 1, len, dst) != (size_t)len) {
fprintf(stderr, "%s: Write error on line %lu.\n", dstpath ? dstpath : "(standard output)", linenum);
fclose(dst);
fclose(src);
return EXIT_FAILURE;
}
}
/* Technically, we don't need to release dynamically allocated (non-shared) memory,
because we're just about to exit, and the OS will automagically do that for us.
We can do this at any point we want during the loop, too. */
free(line);
line = NULL;
size = 0;
/* We do not know why getline() returned -1. Check if an error occurred, and whether we're at end of input. */
if (ferror(src) || !feof(src)) {
fprintf(stderr, "%s: Read error on line %lu.\n", srcpath ? srcpath : "(standard input)", linenum);
fclose(dst);
fclose(src);
return EXIT_FAILURE;
}
/* Check if any write errors have occurred. */
if (ferror(dst)) {
fprintf(stderr, "%s: Write error on line %lu.\n", dstpath ? dstpath : "(standard output)", linenum);
fclose(dst);
fclose(src);
}
/* Read errors should not occur at close time, but it costs very little for us to test anyway. */
if (fclose(src)) {
fprintf(stderr, "%s: Read error on line %lu.\n", srcpath ? srcpath : "(standard input)", linenum);
fclose(dst);
return EXIT_FAILURE;
}
/* Write errors can occur at close time, if the output has been buffered, or the target is on
a remote filesystem. Again, it costs us very little to check. */
if (fclose(dst)) {
fprintf(stderr, "%s: Write error on line %lu.\n", dstpath ? dstpath : "(standard output)", linenum);
return EXIT_FAILURE;
}
/* No errors; target is an identical copy of the source file. */
fprintf(stderr, "%lu lines copied successfully.\n", linenum);
return EXIT_SUCCESS;
}
The SPDX-License-Identifier is a common way to indicate the license of the code. I use CC0-1.0, which basically means "use as you wish, just don't blame the author for any issues: no guarantees, no warranties."
The #define _POSIX_C_SOURCE 200809L tells the GNU C library that we want it to expose POSIX.1-2008 features, which include getline(). If you look at man 3 getline, you'll see in the Synopsis section that glibc 2.10 and later require _POSIX_C_SOURCE to be defined to at least 200809L.
Most of the code is to print the usage, and getting the source and target file names from the command line, and handling - as a special name, "standard stream", so that it can be used even in pipes by specifying - as the input and/or output file name.
The getline() loop uses fwrite() to write the line to the target file. This way, if the input contains embedded NUL bytes (\0), the target will still be identical to the source file.
After the loop, we discard the line buffer, although since the program is just about to exit, we could omit that (since the OS will release all dynamically allocated (non-shared) memory when we exit anyway).
I like code that checks with ferror(src) || !feof(src) if an error occurred in src or src did not reach end of input; and that checks the return value of fclose(), in case a delayed (write) error is reported. Granted, fclose() should never fail for a read-only file, and fclose() should only fail for files we've written to in specific circumstances, but it costs very little to check, and this way the user running the program will be told if the program detected loss of data.
I believe it is morally reprehensible to ignore checking for such errors (especially "because they occur so rarely"), since such tests are the only way the human user can know whether the operation was successful, or if some odd problem occurred. It is up to the human to investigate any problems, and up to our programs to report detectable issues.
The getline function takes a FILE * as a parameter, not an int. Replace the following line:
int fd = open("./rules.txt", O_RDONLY);
By;
FILE *fin = fopen("./rules.txt", "r");
Fix the error checking in the following lines accordingly, like you did with fout.
Then replace the line:
int bytesRead = getline(&line, &len, fd);
Instead it should now use fin:
ssize_t bytesRead = getline(&line, &len, fin);
Note that getline returns ssize_t, not int.
You are also never writing to fout, but I guess you're still working on this code.
Make sure to enable compiler warnings, because your compiler would surely have warned you for using an int parameter where a FILE * was expected.
I've read some of the warnings against using the sysctl() call in C, and it seems if I cannot use sysctl() safely, the only other way I can find to make the needed change would be to use soemething like:
system("echo fs.inotify.max_user_watches=NEW_MAX_DIRECTORIES >> /etc/sysctl.conf");
system("sysctl -p");
(of course, this assumes ensuring the binary is running as root. However, I would rather NOT have to shell out using system calls.
Can someone point me in the correct and safe of using sysctl()?
here is a snippet of the code I am using.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <getopt.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <string.h>
#include <errno.h>
int main ()
{
int ret;
const char *LOGNAME="iNotifyMonitor";
logger(INFO, "================================================");
ret = startDaemon();
daemonRunning = ret;
if (ret == 0)
{
daemonRunning = 1;
FIRST_RUN = 0;
}
if(ret)
{
syslog(LOG_USER | LOG_ERR, "Error starting iNotifyMonitor");
logger(ERR, "Unable to start iNotifyMonitor");
closelog();
return EXIT_FAILURE;
}
signal(SIGINT, signalHandler);
signal(SIGHUP, signalHandler);
char *log_file_name = malloc(sizeof(char *) * sizeof(char *));
sprintf(log_file_name, "%s%s", INM_LOG_DIR, INM_LOG_FILE);
/* Try to open log file to this daemon */
if (INM_OPEN_LOG && INM_LOG_FILE)
{
log_stream = fopen(concatString(INM_LOG_DIR, INM_LOG_FILE), "a+");
if (log_stream == NULL)
{
char *errMsg;
sprintf(errMsg, "Cannot open log file %s, error: %s", concatString(INM_LOG_DIR, INM_LOG_FILE), strerror(errno));
log_stream = stdout;
}
}
else
{
log_stream = stdout;
}
while (daemonRunning == 1)
{
if (ret < 0)
{
logger(LOG_ERR, "Can not write to log stream: %s, error: %s", (log_stream == stdout) ? "stdout" : log_file_name, strerror(errno));
break;
}
ret = fflush(log_stream);
if (ret != 0)
{
logger(LOG_ERR, "Can not fflush() log stream: %s, error: %s",
(log_stream == stdout) ? "stdout" : log_file_name, strerror(errno));
break;
}
int curcount =countDirectory("/home/darrinw/Development/CrossRoads/");
directoryCount = curcount;
if(directoryCounrt > INM_MAX_DIRECTORIES)
{
int newVal = roundUp(directoryCount, 32768);
// call to sysctl() to modify fs.inotify.max_users_watches=newVal
}
sleep(INM_SCAN_INTERVAL);
}
My understanding is that the modern recommended approach to access sysctl variables is via the pseudo-files in /proc/sys. So just open /proc/sys/fs/inotify/max_user_watches and write there.
int fd = open("/proc/sys/fs/inotify/max_user_watches", O_WRONLY);
dprintf(fd, "%d", NEW_MAX_DIRECTORIES);
close(fd);
Error checking left as an exercise.
Modifying /etc/sysctl.conf would make the setting persist across reboots (assuming your distribution uses the file this way, I am not sure if all of them do). That's kind of rude to do automatically; probably better to use the documentation to advise the system administrator to do it themselves if it's needed.
I have seen device file can be accessed directly in Linux and I want to have a try. I have a free disk partition without any file system. My test code is below.
I expect to get output read data: 199 when I run the program at the second time. But actually, I get output read data: 0 twice. No errors emerge during the program. I have no idea where is wrong.
Thanks for your time.
Test Code:
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
int main(){
int num = 0;
int fd = open("/dev/sda6", O_RDWR);
if(fd == -1){
fprintf(stderr, "open device failed, errno : %s(%d) \n",
strerror(errno), errno);
return 1;
}
ssize_t ret = read(fd, &num, sizeof(int));
if(ret != sizeof(int)){
fprintf(stderr, "read fails, errno : %s(%d) \n",
strerror(errno), errno);
return 1;
}
printf("read data: %d\n", num);
num = 199;
ret = write(fd, &num, sizeof(int));
if(ret != sizeof(int)){
fprintf(stderr, "write fails, errno : %s(%d) \n",
strerror(errno), errno);
return 1;
}
close(fd);
return 0;
}
The read and write start reading/writing at the implicit file offset stored in the descriptor, and increment it by the number of bytes read/written. Therefore, you would now read bytes 0 .. 3, and then write bytes 4 .. 7.
Instead of read and write and messing with lseek et al, use the POSIX standard pread and pwrite that do not use the implicit file offset in the descriptor but take explicit file offsets from the beginning of a file in the call.
#include <unistd.h>
ssize_t pread(int fd, void *buf, size_t count, off_t offset);
ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);
...
ssize_t ret = pread(fd, &num, sizeof(int), 0);
ssize_t ret = pwrite(fd, &num, sizeof(int), 0);
You do not seek in your program, so what it does:
Read the first 4 bytes of the device, then write the second 4 bytes.
Try
lseek(fd,0,SEEK_SET);
before write if you want to write at the beginning of fole.
I am trying to write a program on how to read a file 10 bytes per time using read, however, I do not know how to go about it. How should I modify this code to read 10bytes per time. Thanks!!!!
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/time.h>
int main (int argc, char *argv[])
{
printf("I am here1\n");
int fd, readd = 0;
char* buf[1024];
printf("I am here2\n");
fd =open("text.txt", O_RDWR);
if (fd == -1)
{
perror("open failed");
exit(1);
}
else
{
printf("I am here3\n");
if(("text.txt",buf, 1024)<0)
printf("read error\n");
else
{
printf("I am here3\n");
/*******************************
* I suspect this should be the place I make the modification
*******************************/
if(read("text.txt",buf, 1024)<0)
printf("read error\n");
else
{
printf("I am here4\n");
printf("\nN: %c",buf);
if(write(fd,buf,readd) != readd)
printf("write error\n");
}
}
return 0;
}
The final parameter of read() is the maximum size of the data you wish to read so, to try and read ten bytes at a time, you would need:
read (fd, buf, 10)
You'll notice I've also changed the first parameter to the file descriptor rather than the file name string.
Now, you'll probably want that in a loop since you'll want to do something with the data, and you also need to check the return value since it can give you less than what you asked for.
A good example for doing this would be:
int copyTenAtATime (char *infile, char *outfile) {
// Buffer details (size and data).
int sz;
char buff[10];
// Try open input and output.
int ifd = open (infile, O_RDWR);
int ofd = open (outfile, O_WRONLY|O_CREAT);
// Do nothing unless both opened okay.
if ((ifd >= 0) && (ofd >= 0)) {
// Read chunk, stopping on error or end of file.
while ((sz = read (ifd, buff, sizeof (buff))) > 0) {
// Write chunk, flagging error if not all written.
if (write (ofd, buff, sz) != sz) {
sz = -1;
break;
}
}
}
// Finished or errored here, close files that were opened.
if (ifd >= 0) close (ifd);
if (ofd >= 0) close (ofd);
// Return zero if all okay, otherwise error indicator.
return (sz == 0) ? 0 : -1;
}
change the value in read,
read(fd,buf,10);
From man of read
ssize_t read(int fd, void *buf, size_t count);
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
if(read("text.txt",buf, 1024)<0)// this will give you the error.
First argument must be an file descriptor.
I'm writing a program that when run from two separate bash sessions as two separate processes, opens a named pipe between the two to allow strings to be sent from one to the other.
When the process is first executed from one terminal, it checks stat(fname, buf) == -1 to see if a file at path fname exists and if not, creates it. The process then assumes that since it was the one to make the FIFO, it is the one that will be sending messages through it and continues accordingly.
After that occurs, the program can then be run from another terminal that should determine that it will be the receiver of messages through the pipe by checking stat(fname, buf) == -1. The condition should return false now, and stat(fname, buf) itself should return 0 because there exists a file at fname now.
But for reasons I am unable to discern, when the second process is run, stat(fname, buf) still returns -1. The variable errno is set to EFAULT. The man page for stat() only decribes EFAULT as "Bad address." Any help determining why the error occurs or what is meant by "Bad address." would be greaty appreciated.
I've verified that the file is indeed created by the first process as intended. The first process waits at the line pipe = open(fname, O_WRONLY); because it can't continue until the other end of pipe is opened.
Edit: The following is a self-contained implementation of my code. I have confirmed that it compiles and experiences the problem I described here.
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#define MAX_LINE 80
#define oops(m,x) { perror(m); exit(x); }
int main(int argc, char const *argv[]) {
char line[MAX_LINE];
int pipe, pitcher, catcher, initPitcher, quit;
struct stat* buf;
char* fname = "/tmp/absFIFOO";
initPitcher = catcher = pitcher = quit = 0;
while (!quit) {
if (((!pitcher && !catcher && stat(fname, buf) == -1) || pitcher) && !quit) {
// Then file does not exist
if (errno == ENOENT) {
// printf("We're in the file does not exist part\n");
if (!pitcher && !catcher) {
// Then this must be the first time we're running the program. This process will take care of the unlink().
initPitcher = 1;
int stat;
if (stat = mkfifo(fname, 0600) < 0)
oops("Cannot make FIFO", stat);
}
pitcher = 1;
// open a named pipe
pipe = open(fname, O_WRONLY);
printf("Enter line: ");
fgets(line, MAX_LINE, stdin);
if (!strcmp(line, "quit\n")) {
quit = 1;
}
// actually write out the data and close the pipe
write(pipe, line, strlen(line));
close(pipe);
}
} else if (((!pitcher && !catcher) || catcher) && !quit) {
// The first condition is just a check to see if this is the first time we've run the program. We could check if stat(...) == 0, but that would be unnecessary
catcher = 1;
pipe = open("/tmp/absFIFO", O_RDONLY);
// set the mode to blocking (note '~')
int flags;
flags &= ~O_NONBLOCK;
fcntl(pipe, F_SETFL, flags); //what does this do?
// read the data from the pipe
read(pipe, line, MAX_LINE);
if (!strcmp(line, "quit\n")) {
quit = 1;
}
printf("Received line: %s\n", line);
// close the pipe
close(pipe);
}
}
if (initPitcher)
unlink(fname);
return 0;
}
You have this piece of code:
struct stat* buf;
...
if (((!pitcher && !catcher && stat(fname, buf) == -1)
When you call stat(), buf isn't initalized and there's no telling what it points to.
You must allocate some storage for it, so stat() has a valid place to store the result.
The easiest thing is to just allocate it on the stack:
struct stat buf;
...
if (((!pitcher && !catcher && stat(fname, &buf) == -1)
You have not shown your code, but EFAULT means 'bad address'. This indicates that you have not properly allocated (or passed) your buffer for stat or the filename (fname).
buf isn't initialised anywhere. What exactly do you expect to happen?