Wait for an unknown child process - c

Question

I need to execute n child processes, n being a parameter of the program, those n child process will execute one Linux command each, and when they end the program need to create another child process to substitute it.
The problem is that I don't really know how to wait for an unknown process. As each child can have different commands those commands can last different times, so the main process needs to be available to detect when one of the child process has ended and create another one in it's place.
I tried using a for loop with a pid_t array of n size which contains all the pids but it doesn't work. I also tried using wait but once a child process finishes the program stops. This is what I have:
int main(int argc, char *argv[]){
int opt;
int num_procesos=1;
int status;
pid_t pid;
if(argc > 3){
error(argv[0],EXIT_FAILURE);
}
while((opt = getopt(argc,argv,"p")) != -1){
switch(opt){
case 'p':
if(argv[2]==NULL){
error(argv[0],EXIT_FAILURE);
}
num_procesos = atoi(argv[2]);
if(num_procesos < 1 || num_procesos > 8){
fprintf(stderr,"Error: el número de procesos en ejecución tiene que estar entre 1 y 8.");
error(argv[0],EXIT_FAILURE);
}
break;
default:
num_procesos = 1;
break;
}
}
pid_t pids[num_procesos];
for(int i=0;i<num_procesos;i++){
switch(pid = fork()){
case -1:
perror("fork()");
exit(EXIT_FAILURE);
break;
case 0:
leer_y_ejecutar();
}
}
int j=0;
while(j==0){
for(int i=0;i<num_procesos;i++){
if(wait(&status)==0){
switch(pid = fork()){
case -1:
perror("fork()");
exit(EXIT_FAILURE);
break;
case 0:
leer_y_ejecutar();
}
}else{
perror("wait()");
exit(EXIT_FAILURE);
}
}
}
return EXIT_SUCCESS;
}
Don't mind about the function leer_y_ejecutar() it's just part of the rest of my code. If any test is necessary it can be substituted with a printf or something else.

Answer 1

From the wait(2) manual :
The wait() system call suspends execution of the calling thread until one of its children terminates. The call wait(&wstatus) is equivalent to waitpid(-1, &wstatus, 0);
...
waitpid(): on success, returns the process ID of the child whose state has changed; if WNOHANG was specified and one or more child(ren) specified by pid exist, but have not yet changed
state, then 0 is returned. On error, -1 is returned.\
The signature for waitpid is pid_t waitpid(pid_t pid, int *wstatus, int options);, so essentially, this piece of code :
while(j==0){
for(int i=0;i<num_procesos;i++){
// from here
if(wait(&status)==0){
switch(pid = fork()){
case -1:
perror("fork()");
exit(EXIT_FAILURE);
break;
case 0:
leer_y_ejecutar();
}
// to here
}else{
perror("wait()");
exit(EXIT_FAILURE);
}
}
}
Is never read, because wait(&status) == 0 cannot happen because WNOHANG has not been specified.
As a potential solution, I'd suggest changing it for wait(&status) > 0.