I am trying to understand an observation on behavior of select() when used on stdin, when it is receiving data from a pipe.
Basically I had a simple C program using the following code:
hello.c:
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <termios.h>
int main(int argc, char *argv[])
{
int flags, opt;
int nsecs, tfnd;
fd_set rfds;
struct timeval tv;
int retval;
int stdin_fileno_p1 = STDIN_FILENO+1;
char c;
int n;
/* Turn off canonical processing on stdin*/
static struct termios oldt, newt;
tcgetattr( STDIN_FILENO, &oldt);
newt = oldt;
newt.c_lflag &= ~(ICANON);
tcsetattr( STDIN_FILENO, TCSANOW, &newt);
while (1)
{
FD_ZERO(&rfds);
FD_SET(STDIN_FILENO, &rfds);
tv.tv_sec = 0;
tv.tv_usec = 0;
retval = select(stdin_fileno_p1, &rfds, NULL, NULL, &tv);
if ( retval && (retval!=-1) )
{
n = read(STDIN_FILENO, &c, 1);
write(STDOUT_FILENO, &c, 1);
}
else printf("No Data\n");
usleep(100000);
}
tcsetattr( STDIN_FILENO, TCSANOW, &oldt);
}
If I ran the program as follows I could see characters echoing when I type keys on while the program is running. When keys are not pressed, it displays "No Data" as expected.
./hello
However, if use the program as follows, the program never gets to a state where is displays "No Data". Instead last character "c" is repeatedly displayed.
echo -n abc | ./hello
I'm a bit puzzled by this observation, and would be grateful if you could help me to understand the observed behavior.
The problem is that your program does not detect an end-of-file condition when it reads from the STDIN_FILENO descriptor. After echo has written the c character it will close its end of the pipe, which will cause the select in your program to return immediately and your read to return 0 as an indication that no more data will ever be available from that descriptor. Your program doesn't detect that condition. Instead it just calls write with whatever character was left in the buffer by the last successful read and then repeats the loop.
To fix, do if (n==0) break; after the read.
Related
How can we determine whether a socket is ready to read/write in socket programming.
On Linux, use select() or poll().
On Windows, you can use WSAPoll() or select(), both from winsock2.
Mac OS X also has select() and poll().
#include <sys/select.h>
int select(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
select() and pselect() allow a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered ready if it is possible to perform the corresponding I/O operation (e.g., read(2)) without blocking. – https://linux.die.net/man/3/fd_set
#include <poll.h>
int poll(struct pollfd *fds, nfds_t nfds, int timeout);
poll() performs a similar task to select(2): it waits for one of a set of file descriptors to become ready to perform I/O.
– https://linux.die.net/man/2/poll
Example of select usage:
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void)
{
fd_set rfds;
struct timeval tv;
int retval;
/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(0, &rfds);
/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;
retval = select(1, &rfds, NULL, NULL, &tv);
/* Don't rely on the value of tv now! */
if (retval == -1)
perror("select()");
else if (retval)
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf("No data within five seconds.\n");
exit(EXIT_SUCCESS);
}
Explanation of the above code:
FD_ZERO initializes the rfds set. FD_SET(0, &rfds) adds fd 0 (stdin) to the set. FD_ISSET can be used to check whether a specific file descriptor is ready after select returns.
The select call in this example waits until rfds has input or until 5 seconds passes. The two NULLs in the select call are where file descriptor sets (fd_sets) to be checked for ready to write status and exceptions, respectively, would be passed. The tv argument is the number of seconds and microseconds to wait. The first argument to select, nfds, is the highest numbered file descriptor in any of the three sets (read, write, exceptions sets) plus one.
Example of poll usage (from man7.org):
/* poll_input.c
Licensed under GNU General Public License v2 or later.
*/
#include <poll.h>
#include <fcntl.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
int
main(int argc, char *argv[])
{
int nfds, num_open_fds;
struct pollfd *pfds;
if (argc < 2) {
fprintf(stderr, "Usage: %s file...\n", argv[0]);
exit(EXIT_FAILURE);
}
num_open_fds = nfds = argc - 1;
pfds = calloc(nfds, sizeof(struct pollfd));
if (pfds == NULL)
errExit("malloc");
/* Open each file on command line, and add it 'pfds' array. */
for (int j = 0; j < nfds; j++) {
pfds[j].fd = open(argv[j + 1], O_RDONLY);
if (pfds[j].fd == -1)
errExit("open");
printf("Opened \"%s\" on fd %d\n", argv[j + 1], pfds[j].fd);
pfds[j].events = POLLIN;
}
/* Keep calling poll() as long as at least one file descriptor is
open. */
while (num_open_fds > 0) {
int ready;
printf("About to poll()\n");
ready = poll(pfds, nfds, -1);
if (ready == -1)
errExit("poll");
printf("Ready: %d\n", ready);
/* Deal with array returned by poll(). */
for (int j = 0; j < nfds; j++) {
char buf[10];
if (pfds[j].revents != 0) {
printf(" fd=%d; events: %s%s%s\n", pfds[j].fd,
(pfds[j].revents & POLLIN) ? "POLLIN " : "",
(pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
(pfds[j].revents & POLLERR) ? "POLLERR " : "");
if (pfds[j].revents & POLLIN) {
ssize_t s = read(pfds[j].fd, buf, sizeof(buf));
if (s == -1)
errExit("read");
printf(" read %zd bytes: %.*s\n",
s, (int) s, buf);
} else { /* POLLERR | POLLHUP */
printf(" closing fd %d\n", pfds[j].fd);
if (close(pfds[j].fd) == -1)
errExit("close");
num_open_fds--;
}
}
}
}
printf("All file descriptors closed; bye\n");
exit(EXIT_SUCCESS);
}
Explanation of above code:
This code is a bit more complex than the previous example.
argc is the number of arguments. argv is the array of arguments given to the program. argc[0] is usually the name of the program. If argc is less than 2 (which means only one argument was given), the program outputs a usage message and exits with a failure code.
pfds = calloc(nfds, sizeof(struct pollfd)); allocates memory for an array of struct pollfd which is nfds elements long and zeroes the memory. Then there is a NULL check; if pfds is NULL, that means calloc failed (usually because the program ran out of memory), so the program prints the error with perror and exits.
The for loop opens each filename specified in argv and assigns it to corresponding elements of the pfd array. Then sets .events on each element to POLLIN to tell poll to check each file descriptor for whether it is ready to read
The while loop is where the actual call to poll() happens. The array of struct pollfds, pfds, the number of fds, nfds, and a timeout of -1 is passed to poll. Then the return value is checked for error (-1 is what poll return when there is an error) and if there is an error, the program prints an error message and exits. Then the number of ready file descriptors is printed.
In the second for loop inside the while loop, the program iterates over the array of pollfds and checks the .revents field of each structure. If that field is nonzero, an event occurred on the corresponding file descriptor. The program prints the file descriptor, and the event, which can be POLLIN (ready for input), POLLHUP (hang up), or POLLERR (error condition). If the event was POLLIN, the file is ready to be read.
The program then reads 10 bytes into buf. If an error happens when reading, the program prints an error and exits. Otherwise, the program prints the number of bytes read and the contents of the buffer buf.
In case of error or hang up (POLLERR, POLLHUP) the program closes the file descriptor and decrements num_open_fds.
Finally the program says that all file descriptors are closed and exits with EXIT_SUCCESS.
I want do a simple program, where a father process create some child processes; before child pause(), they notification father process.
Child processes run correctly, but father wait on select, otherwise child have written on socket; where is the mistake?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <errno.h>
typedef struct{
pid_t pid;
int sockfd;
}Child;
void err_exit(char* str)
{
perror(str);
exit(EXIT_FAILURE);
}
int convert_int(char* str)
{
int v;
char*p;
errno = 0;
v = strtol(str,&p,0);
if(errno != 0 || *p != '\0')
err_exit("errno");
return v;
}
void child_job(pid_t pid,int sockfd)
{
int v = write(sockfd,"1",1);
if(v == -1)
err_exit("write");
printf("process %d in pause()\n",pid);
pause();
}
int main(int argc, char* argv[])
{
int nsel;
fd_set masterset;
int n_child,i;
int sockfd[2];
pid_t pid;
Child* c = NULL;
if(argc != 2)
err_exit("usage: <awake2> #children\n");
FD_ZERO(&masterset);
n_child = convert_int(argv[1]);
c = malloc(n_child*sizeof(Child));
if(c == NULL)
err_exit("malloc");
for(i = 0; i <n_child; i++){
if ((socketpair(AF_LOCAL, SOCK_STREAM, 0, sockfd)) < 0) { //create socket between child and father
perror("errore in socketpair");
exit(1);
}
if ((pid = fork()) > 0) {
if (close(sockfd[1]) == -1) { //father process closes sockfd[1]
perror("errore in close");
exit(1);
}
c[i].pid = pid;
c[i].sockfd = sockfd[0];
FD_SET(c[i].sockfd, &masterset);
}
else if(!pid)
child_job(getpid(),c[i].sockfd);
}
for(;;){
if ((nsel = select(n_child+1, &masterset, NULL, NULL, NULL)) < 0) {
perror("errore in bind");
exit(1);
}
int i;
for(i = 0; i <n_child; i++){
if(FD_ISSET(c[i].sockfd, &masterset)) {
printf("changed fd\n");
}
}
}
}
One thing that's wrong is you're passing c[i].sockfd to child_job(). In the parent process, it was set to the first socket fd in the pair, but child_job() is called in the child process, where c never gets set to anything. You're passing the original contents of the malloc memory. Change that to child_job(getpid(), sockfd[1]); and you'll be getting closer.
Another thing is that the first argument to select is probably too low. n_child is the number of children, but you need to pass a number here that's greater than the highest file descriptor in your set. For example, run the program with the argument 1 so it creates 1 child. It is likely to start out with file descriptors 0, 1, and 2 open, so the socket pair will be file descriptors 3 and 4. The 3 goes into the fd_set, but the first argument to select is 1+1=2. select ignores your fd 3 because it's above the limit.
To fix that, create a new variable int maxfd; near your fd_set, initialize it to -1 when you FD_ZERO the set, and after every call to FD_SET, update it:
if( [whatever fd you just gave to FD_SET] > maxfd)
maxfd = [whatever fd you just gave to FD_SET];
and call select with maxfd+1 as the first argument.
(Or maybe switch to poll)
That should get you far enough that your first select call works. After that, you'll find more problems.
The fd_set you pass to select will be modified (that's why you can do FD_ISSET tests on it afterward). If you go back to the top of the loop and pass it again without reinitializing it, select will not be looking at all the file descriptors any more, just the ones that were ready in the first call. To fix this, make a second fd_set and copy the master into it just before the select call, and never pass the master to select. (Or you can rebuild the set from scratch each time by scanning the child table.)
If you get a readable fd from select, you should read it before calling select again, otherwise you're just in a "eat CPU calling select over and over" loop.
Trying to use perl API functions in C program. Couldn't find the function to do regular expression match. Wish there is a function like regexmatch in the following program.
#include <EXTERN.h> /* from the Perl distribution */
#include <perl.h> /* from the Perl distribution */
#include <sys/time.h>
typedef unsigned long ulong;
static PerlInterpreter *my_perl; /*** The Perl interpreter ***/
int main(int argc, char **argv, char **env) {
int numOfArgs = 0;
PERL_SYS_INIT3(&numOfArgs, NULL, NULL);
my_perl = perl_alloc();
perl_construct(my_perl);
SV* str = newSVpv(argv[1], strlen(argv[1]));
if (regexmatch(str, "/hi (\S+)/")) {
printf("found a match\n");
}
return 0;
}
I know it's possible to use pcre library, just wonder if it's possible to get it from perl library here (libperl.so.5.14.2 on ubuntu 12.04)
Thanks!
UPDATE 1:
Did some google search and got the following simple program compiling. But when I ran the program as ./a.out ping pin, it gave "Segmentation fault" in the "pregcomp" function. Not sure why.
#include <EXTERN.h> /* from the Perl distribution */
#include <perl.h> /* from the Perl distribution */
#include <sys/time.h>
#include <embed.h>
typedef unsigned long ulong;
static PerlInterpreter *my_perl; /*** The Perl interpreter ***/
struct REGEXP * const engine;
int main(int argc, char **argv, char **env) {
int numOfArgs = 0;
PERL_SYS_INIT3(&numOfArgs, NULL, NULL);
my_perl = perl_alloc();
perl_construct(my_perl);
SV* reStr = newSVpv(argv[2], strlen(argv[2]));
printf("compiling regexp\n");
REGEXP * const compiled_regex = pregcomp(reStr, 0);
printf("execing regexp\n");
int len = strlen(argv[1]);
pregexec(compiled_regex, argv[1], argv[1] + len, argv[1], 5, NULL, 0);
return 0;
}
Don't mess with Perl's private internals. Call a Perl sub that uses the match operator.
Say you previously compiled the following in your interpreter (using eval_pv),
sub regex_match { $_[0] =~ $_[1] }
Then you can call
static bool regex_match_sv(SV* str, SV* re) {
dSP;
bool matched;
ENTER;
SAVETMPS;
PUSHMARK(SP);
XPUSHs(str);
XPUSHs(re);
PUTBACK;
call_pv("regex_match", G_SCALAR);
SPAGAIN;
matched = SvTRUE(POPs);
PUTBACK;
FREETMPS;
LEAVE;
return matched;
}
As of title.
The program will wait for the first event, and then go into an infinite loop - why doesn't it just process one event at a time?
#include <stdio.h>
#include <stdlib.h>
#include <sys/inotify.h>
#include <unistd.h>
int main (int argc, char **argv)
{
int id, wd;
int a;
struct inotify_event e;
id = inotify_init ();
wd = inotify_add_watch (id, "/home/andrea/Downloads", IN_CREATE);
puts ("waiting...");
while (read (id, &e, sizeof (struct inotify_event)))
{
printf ("created %s\n", e.name);
puts ("waiting...");
}
return 0;
}
Firstly, the events reported by inotify aren't of the size inotify_event, since there is an additional name reported as well. Use ioctl with FIONREAD to get the amount of bytes available for reading.
int avail;
ioctl(id, FIONREAD, &avail);
Secondly, you used blocking I/O. If you instead use inotify_init1(O_NONBLOCK) to initialise inotify, read() will immediately return and set errno to EAGAIN if no data is available. Of course, this is optional if you first used FIONREAD to check if there is data available in the first place.
I am writing a program that allocates huge chunks of memory using mmap and then accesses random memory locations to read and write into it.
I just tried out the following code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
int main() {
int fd,len=1024*1024;
fd=open("hello",O_READ);
char*addr=mmap(0,len,PROT_READ+PROT_WRITE,MAP_SHARED,fd,0);
for(fd=0;fd<len;fd++)
putchar(addr[fd]);
if (addr==MAP_FAILED) {perror("mmap"); exit(1);}
printf("mmap returned %p, which seems readable and writable\n",addr);
munmap(addr,len);
return 0;
}
But I cannot execute this program, is there anything wrong with my code?
First of all, the code won't even compile on my debian box. O_READ isn't a correct flag for open() as far as I know.
Then, you first use fd as a file descriptor and the you use it as a counter in your for loop.
I don't understand what you're trying to do, but I think you misunderstood something about mmap.
mmap is used to map a file into the memory, this way you can read / write to the created memory mapping instead of using functions to access the file.
Here's a short program that open a file, map it the the memory and print the returner pointer :
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main() {
int fd;
int result;
int len = 1024 * 1024;
fd = open("hello",O_RDWR | O_CREAT | O_TRUNC, (mode_t) 0600);
// stretch the file to the wanted length, writting something at the end is mandatory
result = lseek(fd, len - 1, SEEK_SET);
if(result == -1) { perror("lseek"); exit(1); }
result = write(fd, "", 1);
if(result == -1) { perror("write"); exit(1); }
char*addr = mmap(0, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (addr==MAP_FAILED) { perror("mmap"); exit(1); }
printf("mmap returned %p, which seems readable and writable\n",addr);
result = munmap(addr, len);
if (result == -1) { perror("munmap"); exit(1); }
close(fd);
return 0;
}
I left out the for loop, since I didn't understood its purpose. Since you create a file and you want to map it on a given length, we have to "stretch" the file to the given length too.
Hope this helps.