Logging to a RAM Buffer

Default Debug Output

By default, when you enable debug output, that output goes to the system console and is mixed up with the normal console output. Normally, that is sufficient. However there are some cases where you might want to do things differently. For example, if there is time critical debug output that interferes with the operation of the device. Or, if you would like to separate the normal console output from the debug output.

One particularly troublesome case is network debug output to console in a Telnet session. Since the telnet session remaps the console output to the Telnet connection, the network debug output can generate infinite loops because the network operation generates debug output to the console, which generates more debug output, … and on and on.

With some creative configuration of the NuttX SYStem LOGging (SYSLOG) feature, these problems can all be eliminated.

The syslog Device

Debug output goes to the syslog device. As mentioned above, the default syslog device device is the system console. However there are many options to control the behavior of the syslog – too many in fact. There are so many options that you will probably have to perform experiments to get the syslog working as you would like it too.

The RAMLOG Device

The RAMLOG device is a special character device that can really be used for most any purpose. However, the RAMLOG device has some special attributes that make it ideal for use as a syslogging device.

  • It supports the syslog_putc interface needed for system logging
  • It behaves much like a pipe: It implements a queue. Writing to the RAMLOG device adds data to the head of the queue; reading from the RAMLOG device removes data from the tail of the queue.
  • It can be configured to return EOF when you try to read and there is nothing available in the RAMLOG.

Using the RAMLOG as the syslog Device

This Wiki page addresses the setup for one configuration: Using a RAMLOG as the syslog device. A RAMLOG is a circular buffer in memory. In this configuration, all debugout output goes to this circular buffer and can later be retrieved using the NSH dmesg command

Here is the summary of what I had to do to get the RAMLOG working as the syslog device. I use a simulation configuration, but for this feature this does not matter.

tools/configure.sh sim/nsh
make menuconfig

I added the following settings. First, these just give me some debug output to test against:


This configures the virtual file system to support the syslog device and is a necessary pre-condition for other settings:


These enables the RAMLOG and configure it for use as the syslog device

#CONFIG_SYSLOG_CHAR undefined, else duplicate output with syslog_putc()

Now when I run NuttX, I get output like this. The dmesg command now appears as an NSH command:

NuttShell (NSH) NuttX-7.1
nsh> help
help usage:  help [-v] [<cmd>]

  [           dd          free        mkdir       mw          sleep       
  ?           df          help        mkfatfs     ps          test        
  break       dmesg       hexdump     mkfifo      pwd         true        
  cat         echo        kill        mkrd        rm          umount      
  cd          exec        losetup     mh          rmdir       unset       
  cp          exit        ls          mount       set         usleep      
  cmp         false       mb          mv          sh          xd          

Builtin Apps:

The dmesg command dumps the contents and clears the RAMLOG:

nsh> dmesg
os_start: Entry
up_unblock_task: Unblocking TCB=52bc70
up_unblock_task: New Active Task TCB=52bc70
posix_spawn_exec: ERROR: exec failed: 22
cmd_mkrd: RAMDISK at 52d4f0
posix_spawn_exec: ERROR: exec failed: 22
mkfatfs_tryfat16: Too few or too many clusters for FAT16: 4081 < 983 < 1022
mkfatfs_clustersearch: Cannot format FAT16 at 1 sectors/cluster
mkfatfs_configfatfs: Sector size:          512 bytes
mkfatfs_configfatfs: Number of sectors:    1024 sectors
mkfatfs_configfatfs: FAT size:             12 bits
mkfatfs_configfatfs: Number FATs:          2
mkfatfs_configfatfs: Sectors per cluster:  1 sectors
mkfatfs_configfatfs: FS size:              3 sectors
mkfatfs_configfatfs:                       985 clusters
mkfatfs_configfatfs: Root directory slots: 512
mkfatfs_configfatfs: Volume ID:            00000000
mkfatfs_configfatfs: Volume Label:         "           "
posix_spawn_exec: ERROR: exec failed: 22
fat_mount: FAT12:
fat_mount:      HW  sector size:     512
fat_mount:          sectors:         1024
fat_mount:      FAT reserved:        1
fat_mount:          sectors:         1024
fat_mount:          start sector:    1
fat_mount:          root sector:     7
fat_mount:          root entries:    512
fat_mount:          data sector:     39
fat_mount:          FSINFO sector:   0
fat_mount:          Num FATs:        2
fat_mount:          FAT sectors:     3
fat_mount:          sectors/cluster: 1
fat_mount:          max clusters:    985
fat_mount:      FSI free count       -1
fat_mount:          next free        0
posix_spawn_exec: ERROR: exec failed: 22
posix_spawn_exec: ERROR: exec failed: 22


As mentioned, the dmesg command clears the RAMLOG. So when it is used again, only new debug output is shown:

nsh> dmesg
posix_spawn_exec: ERROR: exec failed: 22

As a side note, the posix_spawn_exec error will occur on each command in this configuration. That is because NSH first tries to execute a command from a file found in the file system on the PATH variable. You will not see this error in your system unless you have CONFIG_NSH_FILE_APPS=y defined in your configuration.