diff --git a/src/main.rs b/src/main.rs
index 98e129b..e182db2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -9,6 +9,7 @@ mod jobs;
 mod nvidia;
 mod process;
 mod procfs;
+mod procfsapi;
 mod ps;
 mod slurm;
 mod util;
diff --git a/src/process.rs b/src/process.rs
index 75d9f32..42fda57 100644
--- a/src/process.rs
+++ b/src/process.rs
@@ -3,7 +3,7 @@ use crate::command::{self, CmdError};
 use crate::util;
 use crate::TIMEOUT_SECONDS;
 
-#[derive(PartialEq)]
+#[derive(PartialEq, Debug)]
 pub struct Process {
     pub pid: usize,
     pub uid: usize,
diff --git a/src/procfs.rs b/src/procfs.rs
index be83d7c..9248428 100644
--- a/src/procfs.rs
+++ b/src/procfs.rs
@@ -1,16 +1,9 @@
 /// Collect CPU process information without GPU information, from files in /proc.
-extern crate libc;
-extern crate page_size;
-extern crate users;
 
 use crate::process;
+use crate::procfsapi;
 
 use std::collections::HashMap;
-use std::fs;
-use std::os::linux::fs::MetadataExt;
-use std::path;
-use std::time::{SystemTime, UNIX_EPOCH};
-use users::{get_user_by_uid, uid_t};
 
 /// Obtain process information via /proc and return a vector of structures with all the information
 /// we need.  In the returned vector, pids uniquely tag the records.
@@ -20,50 +13,49 @@ use users::{get_user_by_uid, uid_t};
 ///
 /// This function uniformly uses /proc, even though in some cases there are system calls that
 /// provide the same information.
+///
+/// The underlying computing system -- /proc, system tables, and clock -- is virtualized through the
+/// ProcfsAPI instance.
 
-pub fn get_process_information() -> Result<Vec<process::Process>, String> {
+pub fn get_process_information(fs: &dyn procfsapi::ProcfsAPI) -> Result<Vec<process::Process>, String> {
     // The boot time is the `btime` field of /proc/stat.  It is measured in seconds since epoch.  We
     // need this to compute the process's real time, which we need to compute ps-compatible cpu
     // utilization.
 
     let mut boot_time = 0;
-    if let Ok(s) = fs::read_to_string(path::Path::new("/proc/stat")) {
-        for l in s.split('\n') {
-            if l.starts_with("btime ") {
-                let fields = l.split_ascii_whitespace().collect::<Vec<&str>>();
-                boot_time = parse_usize_field(&fields, 1, l, "stat", 0, "btime")? as u64;
-                break;
-            }
-        }
-        if boot_time == 0 {
-            return Err(format!("Could not find btime in /proc/stat: {s}"));
+    let stat_s = fs.read_to_string("stat")?;
+    for l in stat_s.split('\n') {
+        if l.starts_with("btime ") {
+            let fields = l.split_ascii_whitespace().collect::<Vec<&str>>();
+            boot_time = parse_usize_field(&fields, 1, l, "stat", 0, "btime")? as u64;
+            break;
         }
-    } else {
-        return Err("Could not open or read /proc/stat".to_string());
-    };
+    }
+    if boot_time == 0 {
+        return Err(format!("Could not find btime in /proc/stat: {stat_s}"));
+    }
 
     // The total RAM installed is in the `MemTotal` field of /proc/meminfo.  We need this to compute
     // ps-compatible relative memory use.
 
     let mut memtotal_kib = 0;
-    if let Ok(s) = fs::read_to_string(path::Path::new("/proc/meminfo")) {
-        for l in s.split('\n') {
-            if l.starts_with("MemTotal: ") {
-                // We expect "MemTotal:\s+(\d+)\s+kB", roughly
-                let fields = l.split_ascii_whitespace().collect::<Vec<&str>>();
-                if fields.len() != 3 || fields[2] != "kB" {
-                    return Err(format!("Unexpected MemTotal in /proc/meminfo: {l}"));
-                }
-                memtotal_kib = parse_usize_field(&fields, 1, l, "meminfo", 0, "MemTotal")?;
-                break;
+    let meminfo_s = fs.read_to_string("meminfo")?;
+    for l in meminfo_s.split('\n') {
+        if l.starts_with("MemTotal: ") {
+            // We expect "MemTotal:\s+(\d+)\s+kB", roughly
+            let fields = l.split_ascii_whitespace().collect::<Vec<&str>>();
+            if fields.len() != 3 || fields[2] != "kB" {
+                return Err(format!("Unexpected MemTotal in /proc/meminfo: {l}"));
             }
+            memtotal_kib = parse_usize_field(&fields, 1, l, "meminfo", 0, "MemTotal")?;
+            break;
         }
-        if memtotal_kib == 0 {
-            return Err(format!("Could not find MemTotal in /proc/meminfo: {s}"));
-        }
-    } else {
-        return Err("Could not open or read /proc/meminfo".to_string());
-    };
+    }
+    if memtotal_kib == 0 {
+        return Err(format!(
+            "Could not find MemTotal in /proc/meminfo: {meminfo_s}"
+        ));
+    }
 
     // Enumerate all pids, and collect the uids while we're here.
     //
@@ -74,35 +66,14 @@ pub fn get_process_information() -> Result<Vec<process::Process>, String> {
     // Note that a pid may disappear between the time we see it here and the time we get around to
     // reading it, later, and that new pids may appear meanwhile.  We should ignore both issues.
 
-    let mut pids = vec![];
-    if let Ok(dir) = fs::read_dir("/proc") {
-        for dirent in dir {
-            if let Ok(dirent) = dirent {
-                if let Ok(meta) = dirent.metadata() {
-                    let uid = meta.st_uid();
-                    if let Some(name) = dirent.path().file_name() {
-                        if let Ok(pid) = name.to_string_lossy().parse::<usize>() {
-                            pids.push((pid, uid));
-                        }
-                    }
-                }
-            }
-        }
-    } else {
-        return Err("Could not open /proc".to_string());
-    };
+    let pids = fs.read_proc_pids()?;
 
     // Collect remaining system data from /proc/{pid}/stat for the enumerated pids.
 
-    // Values in "ticks" are represented as f64 here.  A typical value for CLK_TCK in 2023 is 100
-    // (checked on several different systems).  There are about 2^23 ticks per day.  2^52/2^23=29,
-    // ie 2^29 days, which is about 1.47 million years, without losing any precision.  Since we're
-    // only ever running sonar on a single node, we will not exceed that range.
-
-    let kib_per_page = page_size::get() / 1024;
+    let kib_per_page = fs.page_size_in_kib();
     let mut result = vec![];
     let mut user_table = UserTable::new();
-    let clock_ticks_per_sec = unsafe { libc::sysconf(libc::_SC_CLK_TCK) as f64 };
+    let clock_ticks_per_sec = fs.clock_ticks_per_sec() as f64;
     if clock_ticks_per_sec == 0.0 {
         return Err("Could not get a sensible CLK_TCK".to_string());
     }
@@ -118,7 +89,7 @@ pub fn get_process_information() -> Result<Vec<process::Process>, String> {
         let mut comm;
         let utime_ticks;
         let stime_ticks;
-        if let Ok(line) = fs::read_to_string(path::Path::new(&format!("/proc/{pid}/stat"))) {
+        if let Ok(line) = fs.read_to_string(&format!("{pid}/stat")) {
             // The comm field is a little tricky, it must be extracted first as the contents between
             // the first '(' and the last ')' in the line.
             let commstart = line.find('(');
@@ -131,8 +102,10 @@ pub fn get_process_information() -> Result<Vec<process::Process>, String> {
             comm = line[commstart.unwrap() + 1..commend.unwrap()].to_string();
             let s = line[commend.unwrap() + 1..].trim().to_string();
             let fields = s.split_ascii_whitespace().collect::<Vec<&str>>();
-            // NOTE relative to the `proc` documentation: All field offsets here are relative to the
-            // command, so ppid is 2, not 4, and then they are zero-based, not 1-based.
+
+            // NOTE relative to the `proc` documentation: All field offsets in the following are
+            // relative to the command (so ppid is 2, not 4), and then they are zero-based, not
+            // 1-based (so ppid is actually 1).
 
             // Fields[0] is the state.  These characters are relevant for modern kernels:
             //  R running
@@ -148,7 +121,8 @@ pub fn get_process_information() -> Result<Vec<process::Process>, String> {
             // as unsigned will fail.  For Z it also looks like some of the fields could have
             // surprising zero values; one has to be careful when dividing.
             //
-            // In particular for Z, field 5 "tpgid" has been observed to be -1.
+            // In particular for Z, field 5 "tpgid" has been observed to be -1.  For X, many of the
+            // fields are -1.
 
             // Zombie jobs cannot be ignored, because they are indicative of system health and the
             // information about their presence is used in consumers.
@@ -179,11 +153,7 @@ pub fn get_process_information() -> Result<Vec<process::Process>, String> {
             // boot_time and the current time are both time_t, ie, a 31-bit quantity in 2023 and a
             // 32-bit quantity before 2038.  clock_ticks_per_sec is on the order of 100.  Ergo
             // boot_ticks and now_ticks can be represented in about 32+7=39 bits, fine for an f64.
-            let now_ticks = SystemTime::now()
-                .duration_since(UNIX_EPOCH)
-                .unwrap()
-                .as_secs() as f64
-                * clock_ticks_per_sec;
+            let now_ticks = fs.now_in_secs_since_epoch() as f64 * clock_ticks_per_sec;
             let boot_ticks = boot_time as f64 * clock_ticks_per_sec;
 
             // start_time_ticks should be on the order of a few years, there is no risk of overflow
@@ -213,7 +183,7 @@ pub fn get_process_information() -> Result<Vec<process::Process>, String> {
 
         let size_kib;
         let rss_kib;
-        if let Ok(s) = fs::read_to_string(path::Path::new(&format!("/proc/{pid}/statm"))) {
+        if let Ok(s) = fs.read_to_string(&format!("{pid}/statm")) {
             let fields = s.split_ascii_whitespace().collect::<Vec<&str>>();
             rss_kib = parse_usize_field(&fields, 1, &s, "statm", pid, "resident set size")?
                 * kib_per_page;
@@ -234,22 +204,21 @@ pub fn get_process_information() -> Result<Vec<process::Process>, String> {
         let pcpu_value = (utime_ticks + stime_ticks) / realtime_ticks;
         let pcpu_formatted = (pcpu_value * 1000.0).round() / 10.0;
 
-        // clock_ticks_per_sec is nonzero, so these divisions will not produce NaN or Infinity
+        // clock_ticks_per_sec is nonzero, so this division will not produce NaN or Infinity
         let cputime_sec = (bsdtime_ticks / clock_ticks_per_sec).round() as usize;
 
         // Note ps uses rss not size here.  Also, ps doesn't trust rss to be <= 100% of memory, so
-        // let's not trust it either.
+        // let's not trust it either.  memtotal_kib is nonzero, so this division will not produce
+        // NaN or Infinity.
         let pmem = f64::min(
             ((rss_kib as f64) * 1000.0 / (memtotal_kib as f64)).round() / 10.0,
             99.9,
         );
 
-        let user = user_table.lookup(uid);
-
         result.push(process::Process {
             pid,
             uid: uid as usize,
-            user,
+            user: user_table.lookup(fs, uid),
             cpu_pct: pcpu_formatted,
             mem_pct: pmem,
             cputime_sec,
@@ -297,7 +266,7 @@ fn parse_usize_field(
 // The UserTable optimizes uid -> name lookup.
 
 struct UserTable {
-    ht: HashMap<uid_t, String>,
+    ht: HashMap<u32, String>,
 }
 
 impl UserTable {
@@ -305,11 +274,10 @@ impl UserTable {
         UserTable { ht: HashMap::new() }
     }
 
-    fn lookup(&mut self, uid: uid_t) -> String {
+    fn lookup(&mut self, fs: &dyn procfsapi::ProcfsAPI, uid: u32) -> String {
         if let Some(name) = self.ht.get(&uid) {
             name.clone()
-        } else if let Some(u) = get_user_by_uid(uid) {
-            let name = u.name().to_string_lossy().to_string();
+        } else if let Some(name) = fs.user_by_uid(uid) {
             self.ht.insert(uid, name.clone());
             name
         } else {
@@ -317,3 +285,182 @@ impl UserTable {
         }
     }
 }
+
+// For the parse test we use the full text of stat and meminfo, but for stat we only want the
+// 'btime' line and for meminfo we only want the 'MemTotal:' line.  Other tests can economize on the
+// input.
+
+#[test]
+pub fn procfs_parse_test() {
+    let pids = vec![(4018, 1000)];
+
+    let mut users = HashMap::new();
+    users.insert(1000, "zappa".to_string());
+
+    let mut files = HashMap::new();
+    files.insert(
+        "stat".to_string(),
+        "cpu  241155 582 127006 12838870 12445 0 3816 0 0 0
+cpu0 32528 189 19573 1597325 1493 0 1149 0 0 0
+cpu1 32258 98 17128 1597900 1618 0 550 0 0 0
+cpu2 30018 18 13638 1607769 1565 0 340 0 0 0
+cpu3 31888 23 16103 1603771 1663 0 217 0 0 0
+cpu4 32830 54 27843 1581301 1506 0 295 0 0 0
+cpu5 27206 111 10254 1618633 1509 0 325 0 0 0
+cpu6 26842 26 9906 1619446 1514 0 511 0 0 0
+cpu7 27582 61 12558 1612723 1575 0 426 0 0 0
+intr 24686011 0 9 0 0 0 0 0 0 0 46121 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 660271 642 0 0 0 0 0 0 0 0 0 0 0 0 1016 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 120 122 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 155340 23 18728 14408 20861 13683 16444 17251 14218 17364 1 1 107 159457 6997 9903 12495 7135 5125 5225 7316 7414 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1414903 2183 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
+ctxt 51751779
+btime 1698303295
+processes 30162
+procs_running 1
+procs_blocked 0
+softirq 14448542 1561885 1201818 5 226550 10931 0 58705 8120272 39 3268337".to_string());
+    files.insert(
+        "meminfo".to_string(),
+        "MemTotal:       16093776 kB
+MemFree:         5247088 kB
+MemAvailable:    8162068 kB
+Buffers:          203244 kB
+Cached:          3999448 kB
+SwapCached:            0 kB
+Active:          1405072 kB
+Inactive:        7805220 kB
+Active(anon):       6808 kB
+Inactive(anon):  6112636 kB
+Active(file):    1398264 kB
+Inactive(file):  1692584 kB
+Unevictable:      982716 kB
+Mlocked:              16 kB
+SwapTotal:       2097148 kB
+SwapFree:        2097148 kB
+Zswap:                 0 kB
+Zswapped:              0 kB
+Dirty:              2872 kB
+Writeback:             0 kB
+AnonPages:       5990404 kB
+Mapped:           672068 kB
+Shmem:           1111828 kB
+KReclaimable:     168520 kB
+Slab:             385396 kB
+SReclaimable:     168520 kB
+SUnreclaim:       216876 kB
+KernelStack:       29632 kB
+PageTables:        66172 kB
+SecPageTables:         0 kB
+NFS_Unstable:          0 kB
+Bounce:                0 kB
+WritebackTmp:          0 kB
+CommitLimit:    10144036 kB
+Committed_AS:   16010888 kB
+VmallocTotal:   34359738367 kB
+VmallocUsed:       68332 kB
+VmallocChunk:          0 kB
+Percpu:             8160 kB
+HardwareCorrupted:     0 kB
+AnonHugePages:         0 kB
+ShmemHugePages:   890880 kB
+ShmemPmdMapped:        0 kB
+FileHugePages:         0 kB
+FilePmdMapped:         0 kB
+HugePages_Total:       0
+HugePages_Free:        0
+HugePages_Rsvd:        0
+HugePages_Surp:        0
+Hugepagesize:       2048 kB
+Hugetlb:               0 kB
+DirectMap4k:      254828 kB
+DirectMap2M:     4710400 kB
+DirectMap1G:    11534336 kB
+"
+        .to_string(),
+    );
+    files.insert(
+        "4018/stat".to_string(),
+        "4018 (firefox) S 2190 2189 2189 0 -1 4194560 19293188 3117638 1823 557 51361 15728 5390 2925 20 0 187 0 16400 5144358912 184775 18446744073709551615 94466859782144 94466860597976 140720852341888 0 0 0 0 4096 17663 0 0 0 17 4 0 0 0 0 0 94466860605280 94466860610840 94466863497216 140720852350777 140720852350820 140720852350820 140720852357069 0".to_string());
+    files.insert(
+        "4018/statm".to_string(),
+        "1255967 185959 54972 200 0 316078 0".to_string(),
+    );
+
+    let ticks_per_sec = 100.0;     // We define this
+    let utime_ticks = 51361.0;     // field(/proc/4018/stat, 14)
+    let stime_ticks = 15728.0;     // field(/proc/4018/stat, 15)
+    let boot_time = 1698303295.0;  // field(/proc/stat, "btime")
+    let start_ticks = 16400.0;     // field(/proc/4018/stat, 22)
+    let rss: f64 = 185959.0 * 4.0; // pages_to_kib(field(/proc/4018/statm, 1))
+    let memtotal = 16093776.0;     // field(/proc/meminfo, "MemTotal:")
+    let size = 316078 * 4;         // pages_to_kib(field(/proc/4018/statm, 5))
+
+    // now = boot_time + start_time + utime_ticks + stime_ticks + arbitrary idle time
+    let now = (boot_time + (start_ticks / ticks_per_sec) + (utime_ticks / ticks_per_sec) + (stime_ticks / ticks_per_sec) + 2000.0) as u64;
+
+    let fs = procfsapi::MockFS::new(files, pids, users, now);
+    let info = get_process_information(&fs).unwrap();
+    assert!(info.len() == 1);
+    let p = &info[0];
+    assert!(p.pid == 4018);     // from enumeration of /proc
+    assert!(p.uid == 1000);     // ditto
+    assert!(p.user == "zappa"); // from getent
+    assert!(p.command == "firefox"); // field(/proc/4018/stat, 2)
+    assert!(p.ppid == 2190);    // field(/proc/4018/stat, 4)
+    assert!(p.session == 2189); // field(/proc/4018/stat, 6)
+
+    let now_time = now as f64;
+    let now_ticks = now_time * ticks_per_sec;
+    let boot_ticks = boot_time * ticks_per_sec;
+    let realtime_ticks = now_ticks - (boot_ticks + start_ticks);
+    let cpu_pct_value = (utime_ticks + stime_ticks) / realtime_ticks;
+    let cpu_pct = (cpu_pct_value * 1000.0).round() / 10.0;
+    assert!(p.cpu_pct == cpu_pct);
+
+    let mem_pct = (rss * 1000.0 / memtotal).round() / 10.0;
+    assert!(p.mem_pct == mem_pct);
+
+    assert!(p.mem_size_kib == size);
+}
+
+#[test]
+pub fn procfs_dead_and_undead_test() {
+    let pids = vec![(4018, 1000), (4019, 1000), (4020, 1000)];
+
+    let mut users = HashMap::new();
+    users.insert(1000, "zappa".to_string());
+
+    let mut files = HashMap::new();
+    files.insert("stat".to_string(), "btime 1698303295".to_string());
+    files.insert("meminfo".to_string(), "MemTotal:       16093776 kB".to_string());
+    files.insert(
+        "4018/stat".to_string(),
+        "4018 (firefox) S 2190 2189 2189 0 -1 4194560 19293188 3117638 1823 557 51361 15728 5390 2925 20 0 187 0 16400 5144358912 184775 18446744073709551615 94466859782144 94466860597976 140720852341888 0 0 0 0 4096 17663 0 0 0 17 4 0 0 0 0 0 94466860605280 94466860610840 94466863497216 140720852350777 140720852350820 140720852350820 140720852357069 0".to_string());
+    files.insert(
+        "4019/stat".to_string(),
+        "4019 (firefox) Z 2190 2189 2189 0 -1 4194560 19293188 3117638 1823 557 51361 15728 5390 2925 20 0 187 0 16400 5144358912 184775 18446744073709551615 94466859782144 94466860597976 140720852341888 0 0 0 0 4096 17663 0 0 0 17 4 0 0 0 0 0 94466860605280 94466860610840 94466863497216 140720852350777 140720852350820 140720852350820 140720852357069 0".to_string());
+    files.insert(
+        "4020/stat".to_string(),
+        "4020 (python3) X 0 -1 -1 0 -1 4243524 0 0 0 0 0 0 0 0 20 0 0 0 10643829 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 17 3 0 0 0 0 0 0 0 0 0 0 0 0 0".to_string());
+
+    files.insert(
+        "4018/statm".to_string(),
+        "1255967 185959 54972 200 0 316078 0".to_string(),
+    );
+    files.insert(
+        "4019/statm".to_string(),
+        "1255967 185959 54972 200 0 316078 0".to_string(),
+    );
+    files.insert(
+        "4020/statm".to_string(),
+        "1255967 185959 54972 200 0 316078 0".to_string(),
+    );
+
+    let fs = procfsapi::MockFS::new(files, pids, users, procfsapi::unix_now());
+    let info = get_process_information(&fs).unwrap();
+
+    // 4020 should be dropped - it's dead
+    assert!(info.len() == 2);
+
+    assert!(info[0].pid == 4018);
+    assert!(info[0].command == "firefox");
+    assert!(info[1].pid == 4019);
+    assert!(info[1].command == "firefox <defunct>");
+}
diff --git a/src/procfsapi.rs b/src/procfsapi.rs
new file mode 100644
index 0000000..f2dd1f3
--- /dev/null
+++ b/src/procfsapi.rs
@@ -0,0 +1,167 @@
+// This creates a API by which procfs can access the underlying computing system, allowing the
+// system to be virtualized.  In turn, that allows sensible test cases to be written.
+
+extern crate libc;
+extern crate page_size;
+extern crate users;
+
+use users::get_user_by_uid;
+
+use std::fs;
+use std::os::linux::fs::MetadataExt;
+use std::path;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+#[cfg(test)]
+use std::collections::HashMap;
+
+pub trait ProcfsAPI {
+    // Open /proc/<path> (which can have multiple path elements, eg, {PID}/filename), read it, and
+    // return its entire contents as a string.  Return a sensible error message if the file can't
+    // be opened or read.
+    fn read_to_string(&self, path: &str) -> Result<String, String>;
+
+    // Return (pid,uid) for every file /proc/{PID}.  Return a sensible error message in case
+    // something goes really, really wrong, but otherwise try to make the best of it.
+    fn read_proc_pids(&self) -> Result<Vec<(usize, u32)>, String>;
+
+    // Try to figure out the user's name from system tables, this may be an expensive operation.
+    fn user_by_uid(&self, uid: u32) -> Option<String>;
+
+    // Return the value of CLK_TCK, or 0 on error.
+    fn clock_ticks_per_sec(&self) -> usize;
+
+    // Return the page size measured in KB
+    fn page_size_in_kib(&self) -> usize;
+
+    // Return the current time in seconds since Unix epoch.
+    fn now_in_secs_since_epoch(&self) -> u64;
+}
+
+// RealFS is used to actually access /proc, system tables, and system clock.
+
+pub struct RealFS {}
+
+impl RealFS {
+    pub fn new() -> RealFS {
+        RealFS {}
+    }
+}
+
+impl ProcfsAPI for RealFS {
+    fn read_to_string(&self, path: &str) -> Result<String, String> {
+        let filename = format!("/proc/{path}");
+        match fs::read_to_string(path::Path::new(&filename)) {
+            Ok(s) => Ok(s),
+            Err(_) => Err(format!("Unable to read {filename}")),
+        }
+    }
+
+    fn read_proc_pids(&self) -> Result<Vec<(usize, u32)>, String> {
+        let mut pids = vec![];
+        if let Ok(dir) = fs::read_dir("/proc") {
+            for dirent in dir {
+                if let Ok(dirent) = dirent {
+                    if let Ok(meta) = dirent.metadata() {
+                        let uid = meta.st_uid();
+                        if let Some(name) = dirent.path().file_name() {
+                            if let Ok(pid) = name.to_string_lossy().parse::<usize>() {
+                                pids.push((pid, uid));
+                            }
+                        }
+                    }
+                }
+            }
+        } else {
+            return Err("Could not open /proc".to_string());
+        };
+        Ok(pids)
+    }
+
+    fn user_by_uid(&self, uid: u32) -> Option<String> {
+        get_user_by_uid(uid).map(|u| u.name().to_string_lossy().to_string())
+    }
+
+    fn clock_ticks_per_sec(&self) -> usize {
+        unsafe { libc::sysconf(libc::_SC_CLK_TCK) as usize }
+    }
+
+    fn page_size_in_kib(&self) -> usize {
+        page_size::get() / 1024
+    }
+
+    fn now_in_secs_since_epoch(&self) -> u64 {
+        unix_now()
+    }
+}
+
+pub fn unix_now() -> u64 {
+    SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .unwrap()
+        .as_secs()
+}
+
+// MockFS is used for testing, it is instantiated with the values we want it to return.
+
+#[cfg(test)]
+pub struct MockFS {
+    files: HashMap<String, String>,
+    pids: Vec<(usize, u32)>,
+    users: HashMap<u32, String>,
+    ticks_per_sec: usize,
+    pagesz: usize,
+    now: u64,
+}
+
+#[cfg(test)]
+impl MockFS {
+    pub fn new(
+        files: HashMap<String, String>,
+        pids: Vec<(usize, u32)>,
+        users: HashMap<u32, String>,
+        now: u64,
+    ) -> MockFS {
+        MockFS {
+            files,
+            pids,
+            users,
+            ticks_per_sec: 100,
+            pagesz: 4,
+            now,
+        }
+    }
+}
+
+#[cfg(test)]
+impl ProcfsAPI for MockFS {
+    fn read_to_string(&self, path: &str) -> Result<String, String> {
+        match self.files.get(path) {
+            Some(s) => Ok(s.clone()),
+            None => Err(format!("Unable to read /proc/{path}")),
+        }
+    }
+
+    fn read_proc_pids(&self) -> Result<Vec<(usize, u32)>, String> {
+        Ok(self.pids.clone())
+    }
+
+    fn user_by_uid(&self, uid: u32) -> Option<String> {
+        match self.users.get(&uid) {
+            Some(s) => Some(s.clone()),
+            None => None,
+        }
+    }
+
+    fn clock_ticks_per_sec(&self) -> usize {
+        self.ticks_per_sec
+    }
+
+    fn page_size_in_kib(&self) -> usize {
+        self.pagesz
+    }
+
+    fn now_in_secs_since_epoch(&self) -> u64 {
+        self.now
+    }
+}
diff --git a/src/ps.rs b/src/ps.rs
index a02deae..7e63087 100644
--- a/src/ps.rs
+++ b/src/ps.rs
@@ -9,6 +9,7 @@ use crate::jobs;
 use crate::nvidia;
 use crate::process;
 use crate::procfs;
+use crate::procfsapi;
 use crate::util::{three_places, time_iso8601};
 
 use csv::{Writer, WriterBuilder};
@@ -189,11 +190,14 @@ pub fn create_snapshot(jobs: &mut dyn jobs::JobManager, opts: &PsOptions) {
     }
     */
 
-    let procinfo_probe = match procfs::get_process_information() {
-        Ok(result) => Ok(result),
-        Err(msg) => {
-            eprintln!("INFO: procfs failed: {}", msg);
-            process::get_process_information()
+    let procinfo_probe = {
+        let fs = procfsapi::RealFS::new();
+        match procfs::get_process_information(&fs) {
+            Ok(result) => Ok(result),
+            Err(msg) => {
+                eprintln!("INFO: procfs failed: {}", msg);
+                process::get_process_information()
+            }
         }
     };
     if let Err(e) = procinfo_probe {