Can I use standard input for interprocess communication? I wrote the following gnu c code as an experiment, but the program hangs waiting for input after printing the character defined as val. Neither a newline nor fflush in the sending process seem to alleviate the problem.
#include <unistd.h>
#include <stdio.h>
int main(void) {
char val = '!';
int proc = fork();
if (proc < 0)
return -1;
if (proc == 0) {
write(0, &val, 1);
return 0;
}
else {
char ch[2] = { 0 };
read(0, ch, 1);
printf("%s\n", ch);
return 0;
}
return -2;
}
You can use pipe for IPC. Now if you want to use STDIN_FILENO and STDOUT_FILENO it would look like this:
#include <unistd.h>
#include <stdio.h>
int main(void) {
char val = '!';
int filedes[2];
pipe(filedes);
int proc = fork();
if (proc < 0)
return -1;
if (proc == 0) {
close(1);
dup(filedes[1]);
close(filedes[0]);
close(filedes[1]);
write(1, &val, 1);
return 0;
}
else {
char ch[2] = { 0 };
close(0);
dup(filedes[0]);
close(filedes[0]);
close(filedes[1]);
read(0, ch, 1);
printf("%s\n", ch);
return 0;
}
return -2;
}
Combination close(x) and dup(filedes[x]) closes STDOUT/STDIN makes copy of filedes[x] into first available descriptor, what you just closed. As suggested by Jonathan example is now closing both filedes ends and without any doubts is using STDIN/STDOUT.
Related
I have an exercise where I need to interact with a C program through pipe.
I have the following source, which I can't modify.
#include <stdio.h>
#include <stdlib.h>
int main()
{
int number;
int answer;
number = rand() % 100;
printf("Print the double of the number %d\n", number);
scanf("%d", &answer);
if(number * 2 == answer)
printf("Success\n");
else
printf("Error\n");
}
I tried to interact with this program with this code
#include <unistd.h>
#include <stdio.h>
int main(int argc, char **argv, char **env)
{
int STDIN_PIPE[2];
int STDOUT_PIPE[2];
pipe(STDIN_PIPE);
pipe(STDOUT_PIPE);
pid_t pid = fork();
if(pid == 0)
{
char *path = "/path/to/binary";
char *args[2];
args[0] = path;
args[1] = NULL;
close(STDIN_PIPE[1]);
close(STDOUT_PIPE[0]);
dup2(STDIN_PIPE[0], STDIN_FILENO);
dup2(STDOUT_PIPE[1], STDOUT_FILENO);
execve(path, args, env);
}
else
{
char buf[128];
close(STDIN_PIPE[0]);
close(STDOUT_PIPE[1]);
while(read(STDOUT_PIPE[0], buf, 1))
write(1, buf, 1);
}
}
But when I run it, it falls in an infinite loop without printing nothing.
I have fixed a number of issues in your code, added a lot of error checks and completed it so that the end goal is reached.
In the child process, srand() must be called to initialize the random number generator or you always get the same value.
The in the child process, you must flush(stdout) after printing the question so that it is really written to the pipe.
And finally, scanf() return value must be checked.
In the main process, I added a lot of error checks. And I write a readLine function to - guess what - read a line from the pipe. A line is terminated by the end-of-line character \n.
There is still room for some enhancements...
I tested my code using Visual Studio Code configured for gcc and running under Ubuntu 20.04.
Here is the child process source:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
int number;
int answer;
time_t t;
srand((unsigned)time(&t));
number = rand() % 100;
printf("Print the double of the number %d\n", number);
fflush(stdout);
int n = scanf("%d", &answer);
if (n != 1) {
printf("Invalid input\n");
return 1;
}
if ((number * 2) == answer) {
printf("Success\n");
return 0;
}
printf("Error %d is not 2 * %d\n", answer, number);
return 1;
}
And here is the main process source:
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
int readLine(int fd, char *buf, int bufSize);
int main(int argc, char **argv, char **env)
{
int STDIN_PIPE[2];
int STDOUT_PIPE[2];
if (pipe(STDIN_PIPE))
{
perror("pipe(STDIN_PIPE)");
return 1;
}
if (pipe(STDOUT_PIPE)) {
perror("pipe(STDOUT_PIPE)");
return 1;
}
pid_t pid = fork();
if (pid == 0)
{
char *path = "../Child/Child"; // Path to child process, adapt to your environment
char *args[2];
args[0] = path;
args[1] = NULL;
if (dup2(STDIN_PIPE[0], STDIN_FILENO) == -1) {
perror("dup2(STDIN) failed");
return 1;
}
if (dup2(STDOUT_PIPE[1], STDOUT_FILENO) == -1) {
perror("dup2(STDIN) failed");
return 1;
}
// Close all pipe ends
close(STDIN_PIPE[0]); // Close read end of STDIN_PIPE
close(STDIN_PIPE[1]); // Write end of STDIN_PIPE
close(STDOUT_PIPE[0]); // Read end of STDOUT_PIPE
close(STDOUT_PIPE[1]); // Close write end of STDOUT_PIPE
if (execve(path, args, env) == -1) {
perror("execve failed");
return 1;
}
}
else
{
char buf[128];
int bufSize = sizeof(buf) / sizeof(buf[0]);
int i;
// Read the question asked by child process
if (readLine(STDOUT_PIPE[0], buf, bufSize) < 0) {
printf("readLine failed.\n");
return 1;
}
// We receive something like "Print the double of the number 83"
printf("Child process question is \"%s\".\n", buf);
// Extract the number at end of string
i = strlen(buf) - 1;
while ((i >= 0) && isdigit(buf[i]))
i--;
int value = atoi(buf + i + 1);
// Write our answer to write end of STDIN_PIPE
char answer[128];
int answerSize = sizeof(answer) / sizeof(answer[0]);
int answerLen = snprintf(answer, answerSize, "%d\n", value * 2);
printf("Our answer is \"%d\".\n", value * 2);
if (write(STDIN_PIPE[1], answer, answerLen) != answerLen) {
printf("write failed.\n");
return 1;
}
// Read the response (success or failure) sent by child process
if (readLine(STDOUT_PIPE[0], buf, bufSize) < 0) {
printf("readLine failed.\n");
return 1;
}
if (strcasecmp(buf, "Success") == 0)
printf("Child process returned success.\n");
else
printf("Child process returned failure.\n");
// Close all pipe ends
close(STDIN_PIPE[0]); // Close read end of STDIN_PIPE
close(STDIN_PIPE[1]); // Write end of STDIN_PIPE
close(STDOUT_PIPE[0]); // Read end of STDOUT_PIPE
close(STDOUT_PIPE[1]); // Close write end of STDOUT_PIPE
}
return 0;
}
// Read a line from file descriptor
// A line is any characters until \n is received or EOF
// \n is not kept
// Return the number of characters read or <0 if error:
// -1 => Input buffer overflow
// -2 => read() failed and errno has the error
int readLine(int fd, char *buf, int bufSize)
{
int i = 0;
while (1)
{
// Check if enough room in the buffer
if (i >= bufSize) {
printf("Input buffer overflow\n");
return -1;
}
// Read one character from the pipe
ssize_t n = read(fd, buf + i, 1);
if (n == -1)
{
perror("read() failed");
return -2;
}
if (n == 0)
{
// EOF received, that's OK
return i;
}
// NUL terminate the buffer
buf[i + 1] = 0;
// Check for end of line character
if (buf[i] == '\n') {
buf[i] = 0; // Remove ending \n
return i;
}
i++;
}
}
I have two programs: Program "Vanilla", and program "verB".
My instructions are that the main process will deal with I\O from the user, and the child will call execve() and run the "Vanilla" process. To accomplish this, I have to use dup2() to replace stdin\stdout on both pipes. (The Vanilla program should use fgets() to read from the pipe).
Inside the "Vanilla" program I read two strings from the user until ctrl+D is pressed, Vanilla calls "xorMethod()" which is doing something (not relevant what) and returns a result.
When I run the "verB" program on Linux(), I only get the "Please insert first, the second string" and then nothing happens and the program stops running.
I want that the parent will continue getting two strings until ctrl+D is pressed, and print the result that he got from his child on the screen.
Vanilla.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include "Vanila.h"
#include "xor.h"
#define MAXLEN 80
int main()
{
char s1[MAXLEN + 1];
char s2[MAXLEN + 1];
while (!feof(stdin))
{
if (readString(s1) == -1)
break;
if (readString(s2) == -1)
break;
fflush(stdin);
int res = xorMethod(s1, s2);
printf("%s xor %s = %d", s1, s2, res);
}
return 1;
}
int readString(char * string)
{
if ((fgets(string, MAXLEN + 1, stdin) < 0 || feof(stdin)))
return -1;
string[strcspn(string, "\n")] = 0;
return 1;
}
verB.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include "Vanila.h"
#define MAXLEN 80
int readStr(char * string);
int main()
{
int pipeToChild[2];
int pipeToParent[2];
char * argv[] = { "./Vanilla",NULL };
if (pipe(pipeToChild) == -1)
return -1;
if (pipe(pipeToParent) == -1)
return -1;
pid_t pid = fork();
if (pid == -1)
return -1;
if (pid == 0) //CHILD proccess
{
close(pipeToChild[0]);
close(pipeToParent[1]);
dup2(pipeToChild[0], fileno(stdin));
dup2(pipeToParent[1], fileno(stdout));
execve(argv[0], argv, NULL);
}
else
{
char string1[MAXLEN + 1];
char string2[MAXLEN + 1];
char result[MAXLEN + 1];
close(pipeToChild[0]);
close(pipeToParent[1]);
while (!feof(stdin))
{
printf("Please insert first string : ");
if (readStr(string1) == -1)
return -1;
printf("Please insert second string : ");
if (readStr(string2) == -1)
return -1;
write(pipeToChild[1], string1, strlen(string1));
write(pipeToChild[1], string2, strlen(string2));
read(pipeToParent[0], &result, MAXLEN);
printf("%s\n", result);
}
wait(NULL);
}
return 1;
}
int readStr(char * string)
{
if ((fgets(string, MAXLEN + 1, stdin) < 0 || feof(stdin)))
return -1;
string[strcspn(string, "\n")] = 0;
return 1;
}
You close the wrong ends of the pipes in the child process. You close the read end of pipeToChild and then dup2 the standard input stream, so you child program will have a closed standard input stream. You should close the write end of pipeToChild and the read end of pipeToParent in the child process, and the other way around in the main process:
if (pid == 0) //CHILD proccess
{
close(pipeToChild[1]);
close(pipeToParent[0]);
/* ... */
else //PARENT
{
close(pipeToChild[0]);
close(pipeToParent[1]);
Basically I have a parent process that forks a child and feeds it it's stdin through a pipe. The child process can terminate in one of two cases:
the write end of the pipe is closed by the parent, meaning it reached the end of stdin thus receiving an EOF,
or it receives a certain input through the pipe(-1 in this case) and exits
My parent code looks roughly like this:
close(pi[0]); // close input end
signal(SIGPIPE, SIG_IGN); // do not handle SIGPIPE
char buffer;
int ok = 1;
while(ok && read(STDIN_FILENO, &buffer, 1) > 0) {
int b_written = write(pi[1], &buffer, 1);
if(b_written == -1) {
if(errno == EPIPE) ok = 0;
else perror("pipe write"); // some other error
}
}
As you can see, I check whether the read end of a pipe is closed by checking for errno == EPIPE. However this means that the read loop does one extra iteration before closing. How could I possibly poll to see if the pipe is closed without necessarily writing something to it?
This snippet will check if the other end of a writable pipe is closed using poll(2). This works on Linux -- I'm not sure about other OSes or what POSIX says.
#include <poll.h>
#include <stdbool.h>
#include <stdio.h>
#include <unistd.h>
bool is_pipe_closed(int fd) {
struct pollfd pfd = {
.fd = fd,
.events = POLLOUT,
};
if (poll(&pfd, 1, 1) < 0) {
return false;
}
return pfd.revents & POLLERR;
}
The child could send a signal, such as SIGUSR1 when it detects it has finished. Parent could set a flag to when it receives SIGUSR1 signal, and check this flag before trying to read input. But I am not absolutely sure SIGUSR1 could not be received after checking the flag ans before reading input from stdin). So I prefer to use a control pipe, each time child know it will be able to read one more data it write a 1 in this control pipe. The result could be something like that:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/wait.h>
#define STOP_VALUE 100
#define SIZE_STDIN_BUFFER 1024
static char can_read_more = 1;
static int handle_child(int *p_child_input_stream, int *p_control_stream)
{
int pipefd[2][2];
pid_t fk;
if (pipe(pipefd[0]) < 0) // Pipe to read input from
{
perror("pipe");
return -1;
}
if (pipe(pipefd[1]) < 0) // Pipe to notifiate parent input can be processed
{
perror("pipe");
close(pipefd[0][0]);
close(pipefd[0][1]);
return -1;
}
if ((fk = fork()) < 0)
{
perror("fork");
close(pipefd[0][0]);
close(pipefd[0][1]);
close(pipefd[1][0]);
close(pipefd[1][1]);
return -1;
}
if (fk == 0)
{
close(pipefd[0][1]);
close(pipefd[1][0]);
write(pipefd[1][1], &can_read_more, sizeof(char)); // sizeof(char) == 1
ssize_t nb_read = 0;
char buffer;
while (nb_read >= 0)
{
nb_read = read(pipefd[0][0], &buffer, sizeof(char));
if (nb_read > 0)
{
printf("0x%02x\n", (unsigned int) buffer);
if (buffer == STOP_VALUE)
{
nb_read = -1;
}
else
{
write(pipefd[1][1], &can_read_more, sizeof(char));
}
}
}
close(pipefd[0][0]);
close(pipefd[1][1]);
exit(0);
}
close(pipefd[0][0]);
close(pipefd[1][1]);
*p_child_input_stream = pipefd[0][1];
*p_control_stream = pipefd[1][0];
return 0;
}
int main()
{
int child_input_stream;
int control_stream;
if (handle_child(&child_input_stream, &control_stream) < 0)
{
return 1;
}
char stdin_buffer[SIZE_STDIN_BUFFER];
char buffer;
int ok = 1;
int child_available_input = 0;
while(ok)
{
while (child_available_input <= 0 && ok)
{
ssize_t nb_control = read(control_stream, &buffer, sizeof(char));
if (nb_control > 0)
{
child_available_input += buffer;
}
else
{
fprintf(stderr, "End of child reading its input detected.\n");
ok = 0;
}
}
if (ok)
{
if (fgets(stdin_buffer, SIZE_STDIN_BUFFER, stdin) == NULL)
{
ok = 0;
}
else
{
if (stdin_buffer[strlen(stdin_buffer) - 1] == '\n')
{
stdin_buffer[strlen(stdin_buffer) - 1] = '\0';
}
char dummy;
int input;
if (sscanf(stdin_buffer, "%d%c", &input, &dummy) == 1)
{
buffer = (char) input;
write(child_input_stream, &buffer, sizeof(char));
child_available_input--;
}
}
}
}
return 0;
}
I'm trying to write a C program that grabs command output and then i'll be passing that to another program.
I'm having an issue, I cant work out how to get the command output and store it. Below is a sample of what I have
if(fork() == 0){
execl("/bin/ls", "ls", "-1", (char *)0);
/* do something with the output here */
}
else{
//*other stuff goes here*
}
so basically im wondering if there is any way i can get the output from the "execl" and pass it to some thing else (e.g. via storing it in some kind of buffer).
Suggestions would be great.
You have to create a pipe from the parent process to the child, using pipe().
Then you must redirect standard ouput (STDOUT_FILENO) and error output (STDERR_FILENO) using dup or dup2 to the pipe, and in the parent process, read from the pipe.
It should work.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define die(e) do { fprintf(stderr, "%s\n", e); exit(EXIT_FAILURE); } while (0);
int main() {
int link[2];
pid_t pid;
char foo[4096];
if (pipe(link)==-1)
die("pipe");
if ((pid = fork()) == -1)
die("fork");
if(pid == 0) {
dup2 (link[1], STDOUT_FILENO);
close(link[0]);
close(link[1]);
execl("/bin/ls", "ls", "-1", (char *)0);
die("execl");
} else {
close(link[1]);
int nbytes = read(link[0], foo, sizeof(foo));
printf("Output: (%.*s)\n", nbytes, foo);
wait(NULL);
}
return 0;
}
Open a pipe, and change stdout to match that pipe.
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int pipes[2];
pipe(pipes); // Create the pipes
dup2(pipes[1],1); // Set the pipe up to standard output
After that, anything which goes to stdout,(such as through printf), comes out pipe[0].
FILE *input = fdopen(pipes[0],"r");
Now you can read the output like a normal file descriptor. For more details, look at this
Thanks Jonathan Leffler, and i optimize the above code for it can't read all response for one time.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#define die(e) do { fprintf(stderr, "%s\n", e); exit(EXIT_FAILURE); } while (0);
int main() {
int link[2];
pid_t pid;
char foo[4096 + 1];
memset(foo, 0, 4096);
if (pipe(link)==-1)
die("pipe");
if ((pid = fork()) == -1)
die("fork");
if(pid == 0) {
dup2 (link[1], STDOUT_FILENO);
close(link[0]);
close(link[1]);
execl("/bin/ls", "ls", "-1", (char *)0);
die("execl");
} else {
close(link[1]);
int nbytes = 0;
std::string totalStr;
while(0 != (nbytes = read(link[0], foo, sizeof(foo)))) {
totalStr = totalStr + foo;
printf("Output: (%.*s)\n", nbytes, foo);
memset(foo, 0, 4096);
}
wait(NULL);
}
return 0;
}
If you want the output in a string (char *), here's an option (for Linux at least):
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/uio.h>
#include <sys/wait.h>
#include <unistd.h>
char* qx(char** cmd, int inc_stderr) {
int stdout_fds[2];
pipe(stdout_fds);
int stderr_fds[2];
if (!inc_stderr) {
pipe(stderr_fds);
}
const pid_t pid = fork();
if (!pid) {
close(stdout_fds[0]);
dup2(stdout_fds[1], 1);
if (inc_stderr) {
dup2(stdout_fds[1], 2);
}
close(stdout_fds[1]);
if (!inc_stderr) {
close(stderr_fds[0]);
dup2(stderr_fds[1], 2);
close(stderr_fds[1]);
}
execvp(*cmd, cmd);
exit(0);
}
close(stdout_fds[1]);
const int buf_size = 4096;
char* out = malloc(buf_size);
int out_size = buf_size;
int i = 0;
do {
const ssize_t r = read(stdout_fds[0], &out[i], buf_size);
if (r > 0) {
i += r;
}
if (out_size - i <= 4096) {
out_size *= 2;
out = realloc(out, out_size);
}
} while (errno == EAGAIN || errno == EINTR);
close(stdout_fds[0]);
if (!inc_stderr) {
close(stderr_fds[1]);
do {
const ssize_t r = read(stderr_fds[0], &out[i], buf_size);
if (r > 0) {
i += r;
}
if (out_size - i <= 4096) {
out_size *= 2;
out = realloc(out, out_size);
}
} while (errno == EAGAIN || errno == EINTR);
close(stderr_fds[0]);
}
int r, status;
do {
r = waitpid(pid, &status, 0);
} while (r == -1 && errno == EINTR);
out[i] = 0;
return out;
}
int main() {
char* argv[3];
argv[0] = "ls";
argv[1] = "-la";
argv[2] = NULL;
char* out = qx(argv, 0);
printf("%s", out);
free(out);
}
I have something that I've been staring at for most of the night and can't figure out. I'm writing code in C that is supposed to use pipes to pass a byte back and forth, allowing me to switch between a parent and child process that will take turns writing a string to a file. Here's my code:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
int fd[2];
int fd2[2];
char token = 'a';
int file = open("output.txt", O_RDWR|O_CREAT|O_TRUNC, S_IRUSR|S_IWUSR);
if (pipe(fd) == -1 || pipe(fd) == -1)
{
printf("Pipe failed");
return(-1);
}
pipe(fd2);
int pid = fork();
int i;
int j;
write(fd[1], token, 1);
if (pid) // Parent enters here
{
for (i = 0; i < 100;)
{
if (read(fd[0], token, 1) != -1)
{
write(file, "ppppppp", 7);
i++;
write(fd2[1], token, 1);
}
//usleep(500000);
}
wait();
}
else if (pid == 0) // Child enters here
{
for (j = 0; j < 100;)
{
if (read(fd2[0], token, 1) != -1)
{
write(file, "ccccc", 5);
j++;
write(fd[1], token, 1);
}
//usleep(500000);
}
}
else // Error creating child
{
exit (-1);
}
close(file);
return 0;
}
I know the writing to a file works when I don't use the pipes, but now I'm getting an infinite loop and I don't know what the problem is.
I figured it out! Funny how small things make all the difference.