Pipes & exec & C - c

Question

Hi i have the following problem , I have to create a program that performs these Linux commands ls –la | sort | wc –l but in my code i just can read two of this command , can sameone help me with this??
int main(){
pid_t pids[3];
int dados[2],dados2[2],i;
if(pipe(dados) == -1 && pipe(dados2) == -1){
perror("pipe failed");
exit(1);
}
for(i=0;i<3;i++){
pids[i] = fork();
if(pids[i] == 0){
if(i==0){
close(dados[0]);
dup2(dados[1],1);
close(dados[1]);
execlp("ls","ls","-la",NULL);
perror("exec failed");
exit(-1);
}
if(i==1){
close(dados[1]);
dup2(dados[0],0);
close(dados[0]);
close(dados2[0]);
dup2(dados2[1],1);
close(dados2[1]);
execlp("sort","sort",NULL);
perror("exec failed");
exit(-1);
}else{
close(dados2[1]);
dup2(dados2[0],0);
close(dados2[0]);
printf("aaaaaaaaaaaaaaaaaa");
execlp("wc","wc","-l",NULL);
perror("exec failed");
exit(-1);
}
}
}
/* Pai tem de fechar a sua copia da extremidade de escrita
para a leitura do sort desbloquear */
close(dados[1]);
close(dados2[0]);
for(i=0;i<3;i++){
wait(NULL);
}
return 0;
}
i dont understand what miss in this

Answer 1

What catches my eye immediatly is
-if(pipe(dados) == -1 && pipe(dados2) == -1){
+if(pipe(dados) == -1 || pipe(dados2) == -1){
perror("pipe failed");
exit(1);
}
The parent pid just needs to read the output of the last command, so the parent neeeds to close all other pipe ends. The child processes needs to close stdin/stdout and dup2() the pipe descriptors to bind them to stdin/stdout and then exec(). With pipes it is really important that you close the not needed pipe end in your process. See http://linux.die.net/man/2/pipe
Currently i dont have an linux box beneith me, so i cant write and test linux code.
My summary for you is:
- parent reads from the last pipe (stdout of the last process in the chain)
- all other childs need to close() their stdin/stdout, dup2() the right pipeend, close all unneeded pipes (remember to close the dup2() source too, because it is a fd) and then exec.
Here is an example how i did create a pipe and rebind the fds.
// unset FD_CLOEXEC
set_close_on_exec(to_child,false);
set_close_on_exec(from_child, false);
// close and rebind the stdin/stdout/stderr
// dup the fd and recreate stdin/stdout with fd as the target
if (dup2(to_child, STDIN_FILENO) != 0 || dup2(from_child, STDOUT_FILENO) != 1) {
shared::perror("error duplicating socket for stdin/stdout");
exit(EXIT_FAILURE);
}
// close these FDs
close(to_child);
close(from_child);
// now we can exec the new sub process and talk to it through
// stdin/stdout/stderr
execlp(exe.c_str(), exe.c_str(), argv.c_str(), (char*)0);
// this should never be reached
shared::perror("error: executing the binary: " + exe);
exit(EXIT_FAILURE);
Remember that at pipe(int fds[2]) the index 1 is the write end of the pipe and index 0 is the read end of the pipe. So you need
Regards
Georg
Edit: I recommend you the book "The Linux Programming Interface: A Linux and UNIX System Programming Handbook" ISBN-13: 978-1593272203