Killing all descendants of a process - c

Question

I'm trying to create a shell wherein i need to implement pipelining..
The pipes are working fine untill all commands are correct else it either blocks or goes into infinite loop(when last cmd is wrong)..here's my pipelines function....
Code explanation:- I'm using two pipes for alternate pipes declared in command.Initially pipe1(fd1) in declared and when second pipe is found another pipe(fd2) is declared. When the third pipe is found pipe1(fd1 ) is redeclared and when fourth pipe is found pipe(fd2) is redeclared and this goes as long as new pipes are found.
num received by function is number of commands (actually number-1) separated by pipes.
Inside the loop then I have divided commands on five conditions
1>>(odd numbered cmd as i%2==0)starting command(i==0 and i%2==0)
2>>ending command(for I%2==0 and i==num)
3>>mid in pipe(i%2==0 and i!=num)
4>>(even numbered cmd as i%2!=0) mid in pipe(i%2!=0 and i!=num)
5>>ending even numbered cmd(i%2!=0 and i==num)
Heres my code :
void call_piping(int num){
int i,fd1[2],fd2[2],status;
pid_t pid;
//pipe(fd1);
//pipe(fd2);
for(i=0;i<=num;i++){
//printf("HH\n");
if(i%2==0){
if(pipe(fd1)==-1){
printf("Error creating pipe fd1\n");
return;
}
}
else{
if(pipe(fd2)==-1){
printf("Error creating pipe fd2\n");
return;
}
}
/*else{
close(fd1[1]);
}*/
if(i==0){
pid=fork();
if(pid==0){
close(fd1[0]);
close(1);
dup2(fd1[1],1);
if((execvp(argv[i][0],argv[i]))==-1){
printf("%s: Invalid command or Command not found\n",argv[i][0]);
close(fd1[1]);
return;
}
}
else{
waitpid(pid,&status,0);
close(fd1[1]);
//printf("Hello1");
}
}
else if(i%2==0 && i==num){
pid=fork();
if(pid==0){
close(0);
//close(1);
dup2(fd2[0],0);
if((execvp(argv[i][0],argv[i]))==-1){
printf("%s: Invalid command or Command not found\n",argv[i][0]);
close(fd2[0]);
return;
}
}
else{
waitpid(pid,&status,0);
//printf("Hello");
close(fd2[0]);
}
}
else if(i%2==0 && i!=num){
pid=fork();
if(pid==0){
close(0);
close(1);
dup2(fd2[0],0);
dup2(fd1[1],1);
if((execvp(argv[i][0],argv[i]))==-1){
printf("%s: Invalid command or Command not found\n",argv[i][0]);
close(fd1[1]);
//printf("Hell");
close(fd2[0]);
return;
}
}
else{
waitpid(pid,&status,0);
close(fd1[1]);
//printf("Hell");
close(fd2[0]);
}
}
else if(i%2!=0 && i==num){
pid=fork();
if(pid==0){
close(0);
dup2(fd1[0],0);
if((execvp(argv[i][0],argv[i]))==-1){
printf("%s: Invalid command or Command not found\n",argv[i][0]);
close(fd1[0]);
return;
}
}
else{
waitpid(pid,&status,0);
//printf("Hel");
close(fd1[0]);
}
}
else if(i%2!=0 && i!=num){
pid=fork();
if(pid==0){
close(0);
close(1);
dup2(fd1[0],0);
dup2(fd2[1],1);
if((execvp(argv[i][0],argv[i]))==-1){
printf("%s: Invalid command or Command not found\n",argv[i][0]);
close(fd2[1]);
close(fd1[0]);
return;
}
}
else{
waitpid(pid,&status,0);
//printf("He");
close(fd2[1]);
close(fd1[0]);
}
}
}
}
so basically it is error handling i'm having problem with...one solution that i thought was to all descendants processes created but dont know how to do that....

Answer 1

That is an extremely complex rat's nest of if statements. I've certainly not understood all the code; I've not even attempted to read it all. However, I don't think you're closing enough file descriptors quick enough.
For example, at the end, you have:
else{
waitpid(pid,&status,0);
//printf("He");
close(fd2[1]);
close(fd1[0]);
}
Diagnostic printing should always have a newline at the end; otherwise, you won't see the output in a timely manner. The two close() calls should be before the waitpid(); processes that wait for EOF on the pipe won't get EOF if the parent process still has it open for writing.
Early on in the code, you have:
if(pid==0){
close(fd1[0]);
close(1);
dup2(fd1[1],1);
if((execvp(argv[i][0],argv[i]))==-1){
printf("%s: Invalid command or Command not found\n",argv[i][0]);
close(fd1[1]);
return;
}
}
As a rule of thumb, if you use dup2() to associate a pipe file descriptor with either standard input or standard output, you should close both of the pipe file descriptors in that process. You also do not need to do close(1) before doing dup(fd[1], 1) — dup2() closes the target file descriptor anyway.
You should report errors on standard error, not standard output. The name should be self-explanatory.
You don't need to test the return value from execvp(); the function won't return unless it failed, so the test is redundant. It is also usually a good idea to exit from a failed exec*() call rather than return. If the exec*() fails and the code returns, you'll have two processes competing for standard input; this never leads to happiness.
The second fragment might be written like this (where, amongst other changes, I've added some spaces for improved readability; spaces are cheap!):
if (pid == 0)
{
dup2(fd1[1], 1);
close(fd1[0]);
close(fd1[1]);
execvp(argv[i][0], argv[i]);
fprintf(stderr, "%s: Invalid command or Command not found\n", argv[i][0]);
exit(1);
}