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I'm currently diving into creating a backgrounding a job in C with &. I need to implement a non-blocking waitpid in order for this to work. I know that. Also, I already am catching the condition if & is entered at the end of the command line. I'm just not sure how to exactly send the process with the end to be a background job and also implement it as executing while another prompt is prompting for the next command.
Anything would help at all, thank you.

 struct bgprocess{ int pid; struct bgprocess * next; struct bgprocess * prev; }; struct bgprocess * bgprocess1; bgprocess1 = malloc(sizeof(struct bgprocess)); bgprocess1->prev = NULL; bgprocess1->next = NULL; bgprocess1->pid = NULL; struct bgprocess * current; current = bgprocess1; do{ int bgreturn = 0; while (current != NULL){ if (waitpid(current->pid, &bgreturn, WNOHANG)){ printf("Child exited"); current->prev->next = current->next; current->next->prev = current->prev; current->prev = NULL; current->next = NULL; free(current); } current = current->next; } if (end){ int pid = fork(); if (pid < 0){ exit(1); } if (pid) { execvp(args[0], args); exit(0); } struct bgprocess * newNode; newNode = malloc(sizeof(struct bgprocess)); newNode->pid = pid; newNode->next = NULL; if (current->next == NULL){ current->next = newNode; } while (1){ if (current->next == NULL){ current->next = newNode; } current = current->next; } } } while (current != NULL); int bgreturn = 0; while (current != NULL){ if (waitpid(current->pid, &bgreturn,0)){ printf("Child exited"); current->prev->next = current->next; current->next->prev = current->prev; current->prev = NULL; current->next = NULL; free(current); } current = current->next; } }

Alright, so I've been working on this some more and I think I may be starting to understand. I still have a few syntax errors that I'm unaware of how to fix so I'll probably use gdb or something unless someone else can point them out. Am I going about it the right way or am I completely wrong?

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  • Are you looking to execute an external linux command from your C code in background ? Commented Oct 10, 2012 at 17:22
  • I'm looking to execute external commands on a solaris system from my own shell that is written in C. Commented Oct 10, 2012 at 17:23
  • possible duplicate of Waitpid equivalent with timeout? Commented Oct 10, 2012 at 17:24
  • Maybe you are interested into a stackoverflow.com/questions/1618756/… Commented Oct 10, 2012 at 17:25
  • did you tried something like that system("external command &"); ? Commented Oct 10, 2012 at 17:26

1 Answer 1

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Sounds like you're implementing a shell.

Just use fork to create a child process - which will be concurrent. From there you can use the exec* family to execute whatever executables you like, or simply run C code in the child while the parent goes back and prompts for more information (next command, etc.) Use wait with the WNOHANG option at the top of the loop to check for terminated children, and again (this time without WNOHANG) at the end for the rest of those children.

I must encourage you not to make this more complicated than it really is. Write out what you want in plain english (or your native language, or pseudocode), then merely translate that into C with the minimum amount of cleverness.

(pseudo)Code:

struct child { int pid; struct child * next; struct child * prev; } struct child * children = null; do { int return = 0; struct child * curr = children; while(curr != null){ if(waitpid(curr->pid, &return, WNOHANG)){ //Report child exited with return status 'return' //Remove child (linked list style) } curr = curr->next; } /* PROMPT, ETC */ if ( doInBackground ){ int pid = fork(); if(pid <0 )exit(); //error if(pid){ //Child execvp(processName, arrayOfArgs); //This should never get executed exit(); } //Add pid (linked list style, again) } }while(!exitCondition) int return = 0; struct child * curr = children; while(curr != null){ if(waitpid(curr->pid, &return, 0)){ //Report child exited with return status 'return' //Remove child (linked list style) } curr = curr->next; }
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7 Comments

Also, I have to make sure that there are no zombies left behind so that's why I believe that I need to use the waitpid.
fork returns a pid to the parent. Maintain a list of child PIDs, and when the shell exits, you can simply wait on each of them. At the top of the control loop, simply use wait with WNOHANG to check for any terminated processes to remove them from your list of active child PIDs.
Would it help if I posted the code I have so far so that you can get an idea of how I'm going about it? Cause I'm a little confused right now.
That will literally always help. But don't put it in the comments - edit your original question.
Alright, I placed the code in there. For some reason, let's say I type "ls &", it just keeps printing ls over and over again.
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