I'm using execl() to run a certain program off of a forked child. Here's what I've got:
int setup(int* arg1, int* arg2) {
int fd[2], ret, pid;
// create the pipe
ret = pipe(fd);
if (ret == -1) {
fprintf(stderr, "Pipe Failed");
exit(-1);
}
pid = fork();
if (pid < 0) {
fprintf(stderr, "Fork Failed");
exit(-1);
} else if (pid == 0) {
dup2(fd[0],0);
close(fd[0]);
close(fd[1]);
int uhoh = execl("prog", "prog", arg1, arg2, NULL);
fprintf(stderr, "%d %d\n", uhoh, getpid());
} else {
close(fd[0]);
return fd[1];
}
}
The goal here is to create a child, attach the read end of a pipe to it's STDIN, then replace it with the prog program. Stuff wasn't working so I added the fprintf on STDERR to see what the return value was and it's constantly -1.
My current working directory looks like this:
stuff/
main.c
main
prog.c
prog
So should this execl() call be working? Would it be returning -1 if maybe there was a problem with prog?
Related
Problem - when calling ls -l | grep etc, stuck on grep (grep child process does not exit)
trying to run "ls | grep r" with "execvp()" suggests that
need to close file descriptors
wait outside of the forking loop
IMO I have performed both of above but the problem still exists.
Any opinion is welcome, thanks!
Note that below is a hard-coded version for 2 pipes only
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
int main()
{
int i = 0;
int pfd[2];
if (pipe(pfd) != 0)
{
printf("Error creating pipe\n");
exit(errno);
}
char **ptr = get_pipes(); // pipes as array of strings
char *command = *ptr;
while (command != NULL)
{
if (i == 2)
break; // hard code to ignore all commands after 2nd pipe
char **args = parse_cmd(command); // this parses a space-separated command as arguments
pid_t pid = fork();
if (pid == 0 && i == 0) // 1st pipe, 1st child
{
close(pfd[0]); // close pipe read end
dup2(pfd[1], 1); // set pipe write end to stdout
if (execvp(args[0], args) == -1)
{
fprintf(stderr, "%s: %s\n", args[0], strerror(errno));
exit(EXIT_FAILURE);
}
}
else if (pid == 0 && i == 1) // 2nd pipe, 2nd child
{
close(pfd[1]); // close pipe write end
dup2(pfd[0], 0); // set pipe read end to stdin
if (execvp(args[0], args) == -1)
{
fprintf(stderr, "'%s': %s\n", args[0], strerror(errno));
exit(EXIT_FAILURE);
}
// exit(EXIT_SUCCESS);
}
else if (pid > 0) // parent
{
printf("Parent pid: %d and child's pid is %d\n", (int)getpid(), (int)pid);
}
command = *++ptr;
i++;
}
pid_t zombie_pid;
int status;
do
{
zombie_pid = waitpid(-1, &status, 0);
printf("Child PID %d died with status %d\n", (int)zombie_pid, WEXITSTATUS(status));
} while (zombie_pid > 0);
}
Hi I'm having a bit of trouble with my pipe execute function, where I want a shell in C to be able to execute a pipe. arg1 is the input before the pipe and arg2 is the command after the pipe. I want the program to terminate after ctr -d but it seems to quit without it, the moment the code is executed. An example of my input is ls | wc, where arg1 = ls and arg2 = wc. Any help/ pointers will be greatly appreciated, thank you.
int executepipe (char ** arg1, char ** arg2) {
int fds[2];
int child=-1;
int status = pipe(fds);
if (status < 0)
{
printf("\npipe error");
return -1;
}
int pid =-1;
pid= fork();
while(1){
if (pid < 0) { //error!
perror("fork");
exit(1);
}
//child
if (pid == 0){// child process (command after the pipe)
//signal(SIGINT, SIG_DFL);
//signal(SIGQUIT, SIG_DFL);
close(fds[1]);//nothing more to be written
dup2(fds[0], 0);
execvp(arg2[0], arg2);
//if errors exist execv wouldn't have been invoked
perror("cannot execute command");
exit(1);
}
else { // parent process (command before the pipe)
close(fds[0]);
signal(SIGINT, SIG_DFL);
signal(SIGQUIT, SIG_DFL);
dup2(fds[1], 1);
close(fds[1]);
execvp(arg1[0], arg1);
//if errors exist execv wouldn't have been invoked
perror("cannot execute command");
exit(1);
}
if ( wait(&child) == -1 ){
perror("wait");}
}
return 0;
};
I have to create 2 child process and send data from the parent to the two, so I used the pipe.
If I just use 1 child process and 1 pipe, all works perfectly with fdopen, fscanf and fprintf.
Also, if I create 2 pipe and send data to a single process, still works perfectly.
But, if I create a second process and try to read from the second pipe, nothing happen.
for example:
int main() {
pid_t pid1, pid2;
int a[2];
pipe(a);
pid1 = fork();
if(pid1 == 0) {
char x,y;
FILE *stream;
stream = fdopen(a[0],"r");
fscanf(stream,"%c",&x);
printf("%c\n", x);
close(a[1]);
close(a[0]);
} else {
int b[2];
pipe(b);
pid2 = fork();
FILE *stream1, *stream2;
close(a[0]);
close(b[0]);
stream1 = fdopen(a[1],"w");
stream2 = fdopen(b[1],"w");
fprintf(stream1, "yo bella zio\n");
fprintf(stream2, "como estas\n");
fflush(stream1);
fflush(stream2);
close(a[1]);
close(b[1]);
waitpid (pid1, NULL, 0);
waitpid (pid2, NULL, 0);
if (pid2 == 0) {
FILE *stream;
close(b[1]);
close(a[1]);
close(a[0]);
stream = fdopen(b[0],"r");
fscanf(stream,"%c",&x);
printf("%c\n", x);
} else {
}
}
}
I really tried all combination. Declare all the pipe together, close or not close pipe. everything but nothing.
This code fixes the problems identified in my comment and some stray issues.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main(void)
{
pid_t pid1, pid2;
int a[2];
int b[2];
pipe(a);
pid1 = fork();
if (pid1 < 0)
{
fprintf(stderr, "failed to fork child 1 (%d: %s)\n", errno, strerror(errno));
exit(EXIT_FAILURE);
}
else if (pid1 == 0)
{
close(a[1]); // Must be closed before the loop
FILE *stream = fdopen(a[0], "r");
if (stream == NULL)
{
fprintf(stderr, "failed to create stream for reading (%d: %s)\n", errno, strerror(errno));
exit(EXIT_FAILURE);
}
int c;
while ((c = getc(stream)) != EOF)
putchar(c);
//char x;
//fscanf(stream, "%c", &x);
//printf("%c\n", x);
//close(a[0]); -- Bad idea once you've used fdopen() on the descriptor
printf("Child 1 done\n");
exit(0);
}
else
{
pipe(b);
pid2 = fork();
if (pid2 < 0)
{
fprintf(stderr, "failed to fork child 2 (%d: %s)\n", errno, strerror(errno));
exit(EXIT_FAILURE);
}
else if (pid2 == 0)
{
close(b[1]);
close(a[1]);
close(a[0]);
FILE *stream = fdopen(b[0], "r");
if (stream == NULL)
{
fprintf(stderr, "failed to create stream for reading (%d: %s)\n", errno, strerror(errno));
exit(EXIT_FAILURE);
}
int c;
while ((c = getc(stream)) != EOF)
putchar(c);
//char x;
//fscanf(stream, "%c", &x);
//printf("%c\n", x);
printf("Child 2 done\n");
exit(0);
}
}
close(a[0]);
close(b[0]);
FILE *stream1 = fdopen(a[1], "w");
if (stream1 == NULL)
{
fprintf(stderr, "failed to create stream for writing (%d: %s)\n", errno, strerror(errno));
exit(EXIT_FAILURE);
}
FILE *stream2 = fdopen(b[1], "w");
if (stream2 == NULL)
{
fprintf(stderr, "failed to create stream for writing (%d: %s)\n", errno, strerror(errno));
exit(EXIT_FAILURE);
}
fprintf(stream1, "yo bella zio\n");
fprintf(stream2, "como estas\n");
fflush(stream1); // Not necessary because fclose flushes the stream
fflush(stream2); // Not necessary because fclose flushes the stream
fclose(stream1); // Necessary because child won't get EOF until this is closed
fclose(stream2); // Necessary because child won't get EOF until this is closed
//close(a[1]); -- bad idea once you've used fdopen() on the descriptor
//close(b[1]); -- bad idea once you've used fdopen() on the descriptor
waitpid(pid1, NULL, 0);
waitpid(pid2, NULL, 0);
printf("All done!\n");
return 0;
}
Note that I changed the child processes so that (a) they explicitly exit in the code block, and (b) made their body into a loop so that all the data sent is printed. That required me to move the close(a[1]) in the first child; otherwise, the loop doesn't terminate because the o/s sees that child 1 has the descriptor open for writing.
When executed on a Mac running macOS 10.13.6 High Sierra (GCC 8.2.0 as the compiler), I get the output:
yo bella zio
Child 1 done
como estas
Child 2 done
All done!
I have an assignment for which I have to create C programm which has to do the following: The mother proccess has to send via pipe to the child proccess the result of the command ps hax -o user | sort | uniq -c. The child proccess has to take this result and write it to a file users.log.
This is the code I have written:
int main(int argc, char **argv)
{
int file_number;
pid_t pid, pid_2, pid_3, pid_4;
int n, pd[2],fd[2], zd[2];
char line[MAXLINE];
if ( pipe(pd) < 0 || pipe(fd) < 0 || pipe(zd) < 0)
{
perror("cannot create pipe."); // failure on create pipe
exit(1);
}
if ( (pid = fork()) < 0)
{
perror("cannot fork"); // failure on creating child proccess
exit(1);
}
else if ( pid > 0) // mother proccess
{
dup2(zd[1], STDOUT_FILENO);
close(zd[0]);
close(zd[1]);
if ( (pid_4 = fork()) < 0)
{
perror("cannot fork.");
exit(1);
}
else if (pid_4 > 0)
{
dup2(pd[1], STDOUT_FILENO);
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
execl("/bin/ps", "ps", "hax", "-o", "user", (char *)0);
}
if ( (pid_2 = fork()) < 0)
{
perror("cannot fork.");
exit(1);
}
else if (pid_2 == 0)
{
dup2(pd[0], 0);
dup2(fd[1], 1);
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
execl("/bin/sort", "sort", "", (char *)0);
}
if ( (pid_3 = fork()) < 0)
{
perror("cannot fork.");
exit(1);
}
else if (pid_3 == 0)
{
dup2(fd[0], 0);
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
execl("/bin/uniq", "uniq", "-c", (char *)0);
}
close(pd[0]);
close(pd[1]);
close(fd[0]);
close(fd[1]);
pid_t terminated;
int stat;
while ((terminated = wait(&stat)) > 0)
{
printf("Child %d terminated.\n", terminated);
}
}
else // child proccess
{
FILE *fptr = fopen("users.log", "a+");
if (fptr == NULL)
fprintf(stderr, "could not open file.\n");
file_number = fileno(fptr); // getting the file descriptor of fptr
dup2(zd[0], STDIN_FILENO);
close(zd[1]);
close(zd[0]);
n = read(zd[0], line, MAXLINE);
printf("Output: (%.*s)\n", n, line);
write(file_number, line, n);
}
return 0;
}
Running the code seems not to produce the desired result. Nothing is written to the file and I get this error:
Signal 17 (CHLD) caught by hax (procps-ng version 3.3.10).
hax:display.c:66: please report this bug
What am I missing here?
I am trying to execute "sudo conntrack -E -p udp -e NEW" command and then pipe the output of this command to "logger" command, but this doesnt work. anything obvious that is going wrong?
so parent is the "sudo conntrack...." which forks the child "logger ..."
void main () {
pid_t pid;
int status;
int j=0;
int exe_process;
FILE *prt1;
FILE *prt2;
int fd[2];
char *arg[]={ "sudo", "/usr/sbin/conntrack", "-E", "-p", "udp", "-e", "NEW", NULL };
char *arg1[]={ "/usr/bin/logger", "-t", "log-conntrack", "-p", "daemon.notice", NULL };
if (pipe(fd) < 0)
printf("pipe error\n");
if ((pid = fork()) < 0) /* fork a child process */
{
printf("ERROR: forking child process failed\n");
exit(1);
}
else if (pid > 0) /* for the parent process: */
{
printf("In parent process %d\n",getpid());
close(fd[0]);
if (execvp("/usr/sbin/conntrack", arg) < 0) /* execute the command */
{
printf("ERROR: exec failed\n");
exit(1);
}
prt1=fdopen(fd[1], "ab");
}
else /* for the child: */
{
printf("In parent child %d\n",getpid());
close(fd[1]);
prt2=fdopen(fd[0], "rb");
if (execvp("/usr/bin/logger", arg1) < 0) /* execute the command */
{
printf("ERROR: exec failed\n");
exit(1);
}
}
}
That's pretty much correct but the main problem is that you need dup() instead of what you are trying to do with fdopen. dup will redirect stdin/stdout the way you want.
else if (pid > 0) /* for the parent process: */
{
printf("In parent process %d\n",getpid());
close(fd[0]);
close(1);
dup(fd[1]);
if (execvp("/usr/sbin/conntrack", arg) < 0) /* execute the command */
{
printf("ERROR: exec failed\n");
exit(1);
}
//prt1=fdopen(fd[1], "ab");
}
And in the child:
close(0);
dup(fd[0]);