Lesson: Division By Zero In Source Scanners¶

The Problem¶

Scanner source files compute percentages without guarding against zero denominators:

# memory.source:17
Set XMemPct = (XMemTotal-XMemFree-XMemCache-XMemBuff)/XMemTotal

# cpu.source:17-19
Set XCpuTotDiff = XCpuUser+XCpuNice+XCpuSystem+XCpuIntr+XCpuIdle
                 - XCpuUser.pval-XCpuNice.pval-XCpuSystem.pval
                 - XCpuIntr.pval-XCpuIdle.pval

# battery.source:63-64
Set Level = XBatteryLeft/XBatteryTotal*100

On the first scanner tick, all values are still 0 (initial state or no data yet). The division yields 0/0 which produces either NaN, a crash, or an undefined value in zigshell-cairo-pango. The widget then displays NaN%, ---, or blanks until the next tick.

Root Cause¶

zigshell-cairo-pango initializes all scanner variables to 0. The expressions are evaluated immediately on the first scanner iteration. The division by zero happens before any real data arrives from /proc/stat, /proc/meminfo, or the battery sysfs interface.

In cpu.source, all .pval fields are 0 at first, and the current values are also 0 before /proc/stat populates them, so the denominator is 0 - 0 = 0.

In battery.source, XBatteryInit() (which discovers the battery) is never called (see separate lesson), so XBatteryTotal stays 0.

The Fix¶

Add a guard for zero denominators:

# memory.source
Set XMemPct = If(XMemTotal > 0,
  (XMemTotal-XMemFree-XMemCache-XMemBuff)/XMemTotal,
  0)

# battery.source
Set Level = If(XBatteryTotal > 0,
  XBatteryLeft/XBatteryTotal*100,
  0)

For cpu.source, the .pval fields need a similar guard:

# cpu.source
Set XCpuTotDiff = If(XCpuUser.pval > 0,
  XCpuUser+XCpuNice+XCpuSystem+XCpuIntr+XCpuIdle
    - XCpuUser.pval-XCpuNice.pval-XCpuSystem.pval
    - XCpuIntr.pval-XCpuIdle.pval,
  0)

Verification¶

# Find all divisions in source files
grep -rn '/' dotfiles/ocws/*.source | grep -v '//' | grep -v 'If.*> 0'

Where This Applies¶

File	Line	Expression
`memory.source`	17	`(XMemTotal-...)/XMemTotal`
`cpu.source`	17-19	Difference of .pval fields (all 0 on first tick)
`battery.source`	63-64	`XBatteryLeft/XBatteryTotal*100`

Pattern To Remember¶

Every division in a scanner expression needs a guard: If(denominator != 0, numerator/denominator, fallback). Variables start at 0, and the first tick runs before any real data arrives.