#15 Main process initialization synchronization when nmon works in "ralfmode".

[FairyFar]

1. Problem

When nmon works in ralfmode (using the parameter -p), the main process returns before the child process has completed initialization.

In some cases, for example in our docker test environment, the following command takes 20 seconds to return.

linux_bbbp("netstat -r", "/bin/netstat -r 2>/dev/null", WARNING);

So, there is no sampled data during this period, and it is difficult for callers to know the actual start time of sampling.

2. Solution

Set up a semaphore, then post semaphore after the child process is initialized. The main process returns and prints the PID after receiving the semaphore.

3. Patch

The patch for lmon16n.c:

--- lmon16n.c.orig  2022-02-26 18:35:41.345041000 +0800
+++ lmon16n.c   2022-02-26 21:10:57.619209565 +0800
@@ -69,6 +69,8 @@ static char *SccsId = "nmon " VERSION;
 #include <sys/time.h>
 #include <sys/socket.h>
 #include <sys/wait.h>
+#include <semaphore.h>
+#include <sys/mman.h>

 /* Windows moved here so they can be cleared when the screen mode changes */
 WINDOW *padwelcome = NULL;
@@ -4249,6 +4251,7 @@ int main(int argc, char **argv)
     long long tmpslab;
     char * slabstr;
     char truncated_command[257]; /* 256 +1 */
+    sem_t *nmon_proc_mm = NULL;


 #define MAXROWS 256
@@ -4689,14 +4692,23 @@ int main(int argc, char **argv)
        }
    }
    free(open_filename);
-   /* disconnect from terminal */
    fflush(NULL);
-   if (!debug && (childpid = fork()) != 0) {
-       if (ralfmode)
-       printf("%d\n", childpid);
-       exit(0);        /* parent returns OK */
-   }
+
    if (!debug) {
+       nmon_proc_mm = mmap(NULL, sizeof(sem_t), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANON, -1, 0);
+       sem_init(nmon_proc_mm, 1, 0);
+
+       /* disconnect from terminal */
+       if ((childpid = fork()) != 0) {
+       sem_wait(nmon_proc_mm);
+       sem_destroy(nmon_proc_mm);
+       munmap(nmon_proc_mm, sizeof(sem_t));
+
+       if (ralfmode)
+           printf("%d\n", childpid);
+       exit(0);            /* parent returns OK */
+       }
+
        close(0);
        if(using_stdout == 0)
        close(1);
@@ -5013,6 +5025,10 @@ int main(int argc, char **argv)
    linux_bbbp("lscfg-v", "/usr/sbin/lscfg -v 2>/dev/null", WARNING);
 #endif
    sleep(1);       /* to get the first stats to cover this one second and avoids divide by zero issues */
+
+   /* Post semaphore */
+   if (nmon_proc_mm != NULL)
+       sem_post(nmon_proc_mm);
     }
     /* To get the pointers setup */
     /* Was already done earlier, DONT'T switch back here to the old pointer! - switcher(); */

The patch for makefile:

--- makefile.orig   2022-02-26 20:57:54.000000000 +0800
+++ makefile    2022-02-26 21:05:26.558885815 +0800
@@ -1,6 +1,6 @@
 # NOTE: link the curent working copy of the code to lmon.c for compiling
 CFLAGS=-g -O3 -Wall
-LDFLAGS=-lncurses -lm
+LDFLAGS=-lncurses -lm -lpthread
 FILE=lmon.c

 #### POWER

[FairyFar]

Add patch for makefile.