Calling "fork() execvp() _exit()" , and the process is not exiting - c

Question

So the brief summary is- I have a program which recursively searches for a file with a certain extension. Each time it finds one, it makes a copy, makes some changes to the copy, creates a patch (by using diff and execvp()) and delete the original file.
The problem I'm having is, after several hundred files, fork() returns with "Resource temporarily unavailable". I added a counter to see how many processes are still running when this failure happens, and it looks like none have been closed- the number of processes open is always the same as the number of files processed.
Now, I was under the impression that the flow should go something like this-
fork();//creates a child process
dostuff();//in the child process
_exit(1);//return control to the parent
but things appear not to be so simple. Perhaps somebody here may spot something obvious I'm missing in the code.
I have posted the 'cleanup' function which is responsible for the forking & patching- the rest is split over several files so hopefully this is enough.
(argument "name" is the original filename, and "newname" is the modified copy.)
void cleanup (char * name, char * newname)
{
if (pf)
{
pid_t patch_pid;
char * const diffargs[5] = {thisdiff, "-u", newname, name, NULL};
char * patchname = malloc(strlen(name) + 6);
strcpy(patchname, name);
strcat(patchname, ".patch");
if((patch_pid = fork()) < 0 )
{
printf("fork failed.\n%s\nfilecount: %ld\nopen forks: %d\n", strerror(errno), filecount, pcount);
exit(-1);
}
pcount++;
if (patch_pid == 0)
{
FILE *pfp;
if ((pfp = fopen(patchname, "w")) == NULL)
{
printf("Error opening file \"%s\" for writing.\n%s\n", patchname, strerror(errno));
exit(-1);
}
dup2(fileno(pfp), STDOUT_FILENO);
fclose(pfp);
execvp(diffargs[0], diffargs);
free(patchname);
if (remove(name) != 0)
{
printf("Error removing file %s\n%s\n", name, strerror(errno));
exit(-1);
}
if (rename(newname, name) != 0)
{
printf("Error renaming file %s\n%s\n", newname, strerror(errno));
exit(-1);
}
pcount--;
_exit(1);
}
}
else if (!df && !xf)
{
if (remove(name) != 0)
{
printf("Error removing file %s\n%s\n", name, strerror(errno));
exit(-1);
}
if (rename(newname, name) != 0)
{
printf("Error renaming file %s\n%s\n", newname, strerror(errno));
exit(-1);
}
}
}

Answer 1

Two suggestions:
Understand that exec*() replaces your process (if successful). Any code following it is unreachable.
Reap the exit status of exited processes with one of the wait*() functions; possibly in a signal handler for SIGCHLD.
Bonus suggestion: Read W. Richard Stevens' Advanced Programming in the Unix Environment; it's the Bible for tasks like these.

Answer 2

You probably want an else clause after the fork to handle the parent process as well. Something like the following might work;
void cleanup (char * name, char * newname)
{
if (pf)
{
/* SNIP - unchanged */
if (patch_pid == 0) /* child */
{
FILE *pfp;
if ((pfp = fopen(patchname, "w")) == NULL)
{
printf("Error opening file \"%s\" for writing.\n%s\n",
patchname, strerror(errno));
exit(EXIT_FAILURE);
}
dup2(fileno(pfp), STDOUT_FILENO);
fclose(pfp);
execvp(diffargs[0], diffargs);
perror("execvp");
exit(EXIT_FAILURE);
}
else /* parent */
{
pid_t rc;
int stat;
free(patchname);
rc = waitpid(patch_pid, &stat, 0);
if (rc < 0)
{
perror("waitpid");
/* do something appropriate here */
}
else
{
/* if you care about your children */
if (WIFEXITED(stat))
{
printf("%d exited with status %d\n",
(int)rc, WEXITSTATUS(stat));
}
else if (WIFSIGNALED(stat))
{
printf("%d terminated because of signal %d\n",
(int)rc, WTERMSIG(stat));
}
else if (WIFSTOPPED(stat))
{
printf("%d was STOPPED with signal %d\n",
(int)rc, WSTOPSIG(stat));
}
}
pcount--;
if (remove(name) != 0)
{
printf("Error removing file %s\n%s\n", name, strerror(errno));
exit(EXIT_FAILURE);
}
if (rename(newname, name) != 0)
{
printf("Error renaming file %s\n%s\n", newname, strerror(errno));
exit(EXIT_FAILURE);
}
}
}
}
I also high recommend reading Advanced Programming in the UNIX Environment by the late W. Richard Stevens. Actually, you should add UNIX Network Programming volume 1 & 2 to your list as well ;)
The other thing that I noticed is that you have a memory management error related to patchname. The malloc call isn't accounting for the extra NUL character at the end of the string.