mx_proc: Add mx_proc_get_parent

Add utility function to get the parent pid of a process from procfs.

mx_proc.c

@@ -450,3 +450,37 @@ struct mx_proc_info *mx_proc_tree_proc_info(struct mx_proc_tree *tree, pid_t pid
 
     return &(ptn->pinfo);
 }
+
+/*
+ * get parent pid of pid
+ * return -1 if pid is not found
+ */
+pid_t mx_proc_get_parent(pid_t pid) {
+    char statfilename[32];  // "/proc/2147483647/stat" = 22
+    FILE *statfile;
+    pid_t parent;
+    int n;
+    char linebuf[256];  // "2147483647 (max128chars) X 2147483647 " = 156
+
+    if (pid == 0)       // no need to try /proc/0/stat
+        return -1;
+
+    snprintf(statfilename, sizeof(statfilename), "/proc/%d/stat", pid);
+    statfile = fopen(statfilename, "r");
+    if (statfile==NULL)
+        return -1;
+    /*
+     * we can't just scanf(statfile, "%*d %*s %*c %d", &parent) because the comm might contain whitespace
+     * e.g. "579 (UVM global queu) S 2 0 0 0 -1 2129984...."
+     * all later fields are integer, so position at the last ")"
+     */
+    fgets(linebuf, sizeof(linebuf), statfile);
+    fclose(statfile);
+    char *c = strrchr(linebuf, ')');
+    if (c == NULL)
+        return -1;
+    n = sscanf(c, ") %*c %d", &parent);
+    if (n != 1)
+        return -1;
+    return parent;
+}

mx_proc.h

@@ -85,5 +85,5 @@ int mx_proc_tree(struct mx_proc_tree **newtree);
 int mx_proc_tree_free(struct mx_proc_tree **tree);
 
 struct mx_proc_info *mx_proc_tree_proc_info(struct mx_proc_tree *tree, pid_t pid);
-
+pid_t mx_proc_get_parent(pid_t pid);
 #endif