diff --git a/src/app/main/main.c b/src/app/main/main.c
index 03b3a95d03..15b6b48ff2 100644
--- a/src/app/main/main.c
+++ b/src/app/main/main.c
@@ -73,6 +73,7 @@
 #include "lib/geoip/geoip.h"
 
 #include "lib/process/waitpid.h"
+#include "lib/process/process.h"
 
 #include "lib/meminfo/meminfo.h"
 #include "lib/osinfo/uname.h"
@@ -557,6 +558,10 @@ tor_init(int argc, char *argv[])
   rend_cache_init();
   addressmap_init(); /* Init the client dns cache. Do it always, since it's
                       * cheap. */
+
+  /* Initialize Process subsystem. */
+  process_init();
+
   /* Initialize the HS subsystem. */
   hs_init();
 
@@ -784,6 +789,7 @@ tor_free_all(int postfork)
   circuitmux_ewma_free_all();
   accounting_free_all();
   protover_summary_cache_free_all();
+  process_free_all();
 
   if (!postfork) {
     config_free_all();
diff --git a/src/lib/process/.may_include b/src/lib/process/.may_include
index a2d57a52f3..b1c50c24a6 100644
--- a/src/lib/process/.may_include
+++ b/src/lib/process/.may_include
@@ -4,8 +4,9 @@ lib/cc/*.h
 lib/container/*.h
 lib/ctime/*.h
 lib/err/*.h
-lib/intmath/*.h
+lib/evloop/*.h
 lib/fs/*.h
+lib/intmath/*.h
 lib/log/*.h
 lib/malloc/*.h
 lib/net/*.h
@@ -15,4 +16,4 @@ lib/subsys/*.h
 lib/testsupport/*.h
 lib/thread/*.h
 
-ht.h
\ No newline at end of file
+ht.h
diff --git a/src/lib/process/include.am b/src/lib/process/include.am
index 2aa30cc3c1..1d213d1c00 100644
--- a/src/lib/process/include.am
+++ b/src/lib/process/include.am
@@ -9,6 +9,7 @@ src_lib_libtor_process_a_SOURCES =		\
 	src/lib/process/daemon.c		\
 	src/lib/process/env.c			\
 	src/lib/process/pidfile.c		\
+	src/lib/process/process.c		\
 	src/lib/process/restrict.c		\
 	src/lib/process/setuid.c		\
 	src/lib/process/subprocess.c		\
@@ -24,6 +25,7 @@ noinst_HEADERS +=				\
 	src/lib/process/daemon.h		\
 	src/lib/process/env.h			\
 	src/lib/process/pidfile.h		\
+	src/lib/process/process.h		\
 	src/lib/process/restrict.h		\
 	src/lib/process/setuid.h		\
 	src/lib/process/subprocess.h		\
diff --git a/src/lib/process/process.c b/src/lib/process/process.c
new file mode 100644
index 0000000000..d3967a52da
--- /dev/null
+++ b/src/lib/process/process.c
@@ -0,0 +1,644 @@
+/* Copyright (c) 2003, Roger Dingledine
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2018, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file process.c
+ * \brief Module for working with other processes.
+ **/
+
+#define PROCESS_PRIVATE
+#include "lib/container/buffers.h"
+#include "lib/net/buffers_net.h"
+#include "lib/container/smartlist.h"
+#include "lib/log/log.h"
+#include "lib/log/util_bug.h"
+#include "lib/process/process.h"
+#include "lib/process/env.h"
+
+#ifdef HAVE_STDDEF_H
+#include <stddef.h>
+#endif
+
+/** A list of all <b>process_t</b> instances currently allocated. */
+static smartlist_t *processes;
+
+/** Structure to represent a child process. */
+struct process_t {
+  /** Process status. */
+  process_status_t status;
+
+  /** Which protocol is the process using? */
+  process_protocol_t protocol;
+
+  /** Which function to call when we have data ready from stdout? */
+  process_read_callback_t stdout_read_callback;
+
+  /** Which function to call when we have data ready from stderr? */
+  process_read_callback_t stderr_read_callback;
+
+  /** Which function call when our process terminated? */
+  process_exit_callback_t exit_callback;
+
+  /** Our exit code when the process have terminated. */
+  process_exit_code_t exit_code;
+
+  /** Name of the command we want to execute (for example: /bin/ls). */
+  char *command;
+
+  /** The arguments used for the new process. */
+  smartlist_t *arguments;
+
+  /** The environment used for the new process. */
+  smartlist_t *environment;
+
+  /** Buffer to store data from stdout when it is read. */
+  buf_t *stdout_buffer;
+
+  /** Buffer to store data from stderr when it is read. */
+  buf_t *stderr_buffer;
+
+  /** Buffer to store data to stdin before it is written. */
+  buf_t *stdin_buffer;
+
+  /** Do we need to store some custom data with the process? */
+  void *data;
+};
+
+/** Convert a given process status in <b>status</b> to its string
+ * representation. */
+const char *
+process_status_to_string(process_status_t status)
+{
+  switch (status) {
+  case PROCESS_STATUS_NOT_RUNNING:
+    return "not running";
+  case PROCESS_STATUS_RUNNING:
+    return "running";
+  case PROCESS_STATUS_ERROR:
+    return "error";
+  }
+
+  /* LCOV_EXCL_START */
+  tor_assert_unreached();
+  return NULL;
+  /* LCOV_EXCL_STOP */
+}
+
+/** Convert a given process protocol in <b>protocol</b> to its string
+ * representation. */
+const char *
+process_protocol_to_string(process_protocol_t protocol)
+{
+  switch (protocol) {
+  case PROCESS_PROTOCOL_LINE:
+    return "Line";
+  case PROCESS_PROTOCOL_RAW:
+    return "Raw";
+  }
+
+  /* LCOV_EXCL_START */
+  tor_assert_unreached();
+  return NULL;
+  /* LCOV_EXCL_STOP */
+}
+
+/** Initialize the Process subsystem.  This function initializes the Process
+ * subsystem's global state. For cleaning up, <b>process_free_all()</b> should
+ * be called. */
+void
+process_init(void)
+{
+  processes = smartlist_new();
+}
+
+/** Free up all resources that is handled by the Process subsystem. Note that
+ * this call does not terminate already running processes. */
+void
+process_free_all(void)
+{
+  SMARTLIST_FOREACH(processes, process_t *, x, process_free(x));
+  smartlist_free(processes);
+}
+
+/** Get a list of all processes.  This function returns a smartlist of
+ * <b>process_t</b> containing all the currently allocated processes. */
+const smartlist_t *
+process_get_all_processes(void)
+{
+  return processes;
+}
+
+/** Allocate and initialize a new process.  This function returns a newly
+ * allocated and initialized process data, which can be used to configure and
+ * later run a subprocess of Tor. Use the various <b>process_set_*()</b>
+ * methods to configure it and run the process using <b>process_exec()</b>. Use
+ * <b>command</b> to specify the path to the command to run. You can either
+ * specify an absolute path to the command or relative where Tor will use the
+ * underlying operating system's functionality for finding the command to run.
+ * */
+process_t *
+process_new(const char *command)
+{
+  tor_assert(command);
+
+  process_t *process;
+  process = tor_malloc_zero(sizeof(process_t));
+
+  /* Set our command. */
+  process->command = tor_strdup(command);
+
+  /* By default we are not running. */
+  process->status = PROCESS_STATUS_NOT_RUNNING;
+
+  /* Prepare process environment. */
+  process->arguments = smartlist_new();
+  process->environment = smartlist_new();
+
+  /* Prepare the buffers. */
+  process->stdout_buffer = buf_new();
+  process->stderr_buffer = buf_new();
+  process->stdin_buffer = buf_new();
+
+  smartlist_add(processes, process);
+
+  return process;
+}
+
+/** Deallocate the given process in <b>process</b>. */
+void
+process_free_(process_t *process)
+{
+  if (! process)
+    return;
+
+  /* Cleanup parameters. */
+  tor_free(process->command);
+
+  /* Cleanup arguments and environment. */
+  SMARTLIST_FOREACH(process->arguments, char *, x, tor_free(x));
+  smartlist_free(process->arguments);
+
+  SMARTLIST_FOREACH(process->environment, char *, x, tor_free(x));
+  smartlist_free(process->environment);
+
+  /* Cleanup the buffers. */
+  buf_free(process->stdout_buffer);
+  buf_free(process->stderr_buffer);
+  buf_free(process->stdin_buffer);
+
+  smartlist_remove(processes, process);
+
+  tor_free(process);
+}
+
+/** Execute the given process.  This function executes the given process as a
+ * subprocess of Tor. Returns <b>PROCESS_STATUS_RUNNING</b> upon success. */
+process_status_t
+process_exec(process_t *process)
+{
+  tor_assert(process);
+
+  process_status_t status = PROCESS_STATUS_NOT_RUNNING;
+
+  log_info(LD_PROCESS, "Starting new process: %s", process->command);
+
+  /* Update our state. */
+  process_set_status(process, status);
+
+  if (status != PROCESS_STATUS_RUNNING) {
+    log_warn(LD_PROCESS, "Failed to start process: %s",
+             process_get_command(process));
+  }
+
+  return status;
+}
+
+/** Set the callback function for output from the child process's standard out
+ * handle.  This function sets the callback function which is called every time
+ * the child process have written output to its standard out file handle.
+ *
+ * Use <b>process_set_protocol(process, PROCESS_PROTOCOL_LINE)</b> if you want
+ * the callback to only contain complete "\n" or "\r\n" terminated lines. */
+void
+process_set_stdout_read_callback(process_t *process,
+                                 process_read_callback_t callback)
+{
+  tor_assert(process);
+  process->stdout_read_callback = callback;
+}
+
+/** Set the callback function for output from the child process's standard
+ * error handle.  This function sets the callback function which is called
+ * every time the child process have written output to its standard error file
+ * handle.
+ *
+ * Use <b>process_set_protocol(process, PROCESS_PROTOCOL_LINE)</b> if you want
+ * the callback to only contain complete "\n" or "\r\n" terminated lines. */
+void
+process_set_stderr_read_callback(process_t *process,
+                                 process_read_callback_t callback)
+{
+  tor_assert(process);
+  process->stderr_read_callback = callback;
+}
+
+/** Set the callback function for process exit notification.  The
+ * <b>callback</b> function will be called every time your child process have
+ * terminated. */
+void
+process_set_exit_callback(process_t *process,
+                          process_exit_callback_t callback)
+{
+  tor_assert(process);
+  process->exit_callback = callback;
+}
+
+/** Get the current command of the given process. */
+const char *
+process_get_command(const process_t *process)
+{
+  tor_assert(process);
+  return process->command;
+}
+
+void
+process_set_protocol(process_t *process, process_protocol_t protocol)
+{
+  tor_assert(process);
+  process->protocol = protocol;
+}
+
+/** Get the currently used protocol of the given process. */
+process_protocol_t
+process_get_protocol(const process_t *process)
+{
+  tor_assert(process);
+  return process->protocol;
+}
+
+/** Set opague pointer to data.  This function allows you to store a pointer to
+ * your own data in the given process. Use <b>process_get_data()</b> in the
+ * various callback functions to retrieve the data again.
+ *
+ * Note that the given process does NOT take ownership of the data and you are
+ * responsible for freeing up any resources allocated by the given data.
+ * */
+void
+process_set_data(process_t *process, void *data)
+{
+  tor_assert(process);
+  process->data = data;
+}
+
+/** Get the opaque pointer to callback data from the given process.  This
+ * function allows you get the data you stored with <b>process_set_data()</b>
+ * in the different callback functions. */
+void *
+process_get_data(const process_t *process)
+{
+  tor_assert(process);
+  return process->data;
+}
+
+/** Set the status of a given process. */
+void
+process_set_status(process_t *process, process_status_t status)
+{
+  tor_assert(process);
+  process->status = status;
+}
+
+/** Get the status of the given process. */
+process_status_t
+process_get_status(const process_t *process)
+{
+  tor_assert(process);
+  return process->status;
+}
+
+/** Append an argument to the list of arguments in the given process. */
+void
+process_append_argument(process_t *process, const char *argument)
+{
+  tor_assert(process);
+  tor_assert(argument);
+
+  smartlist_add(process->arguments, tor_strdup(argument));
+}
+
+/** Returns a list of arguments (excluding the command itself) from the
+ * given process. */
+const smartlist_t *
+process_get_arguments(const process_t *process)
+{
+  tor_assert(process);
+  return process->arguments;
+}
+
+/** Returns a newly allocated Unix style argument vector. Use <b>tor_free()</b>
+ * to deallocate it after use. */
+char **
+process_get_argv(const process_t *process)
+{
+  tor_assert(process);
+
+  /** Generate a Unix style process argument vector from our process's
+   * arguments smartlist_t. */
+  char **argv = NULL;
+
+  char *filename = process->command;
+  const smartlist_t *arguments = process->arguments;
+  const size_t size = smartlist_len(arguments);
+
+  /* Make space for the process filename as argv[0] and a trailing NULL. */
+  argv = tor_malloc_zero(sizeof(char *) * (size + 2));
+
+  /* Set our filename as first argument. */
+  argv[0] = filename;
+
+  /* Put in the rest of the values from arguments. */
+  SMARTLIST_FOREACH_BEGIN(arguments, char *, arg_val) {
+    tor_assert(arg_val != NULL);
+
+    argv[arg_val_sl_idx + 1] = arg_val;
+  } SMARTLIST_FOREACH_END(arg_val);
+
+  return argv;
+}
+
+/** Set the given <b>key</b>/<b>value</b> pair as environment variable in the
+ * given process. */
+void
+process_set_environment(process_t *process,
+                        const char *key,
+                        const char *value)
+{
+  tor_assert(process);
+  tor_assert(key);
+  tor_assert(value);
+
+  smartlist_add_asprintf(process->environment, "%s=%s", key, value);
+}
+
+/** Returns a newly allocated <b>process_environment_t</b> containing the
+ * environment variables for the given process. */
+process_environment_t *
+process_get_environment(const process_t *process)
+{
+  tor_assert(process);
+  return process_environment_make(process->environment);
+}
+
+/** Write <b>size</b> bytes of <b>data</b> to the given process's standard
+ * input. */
+void
+process_write(process_t *process,
+              const uint8_t *data, size_t size)
+{
+  tor_assert(process);
+  tor_assert(data);
+
+  buf_add(process->stdin_buffer, (char *)data, size);
+  process_write_stdin(process, process->stdin_buffer);
+}
+
+/** As tor_vsnprintf(), but write the data to the given process's standard
+ * input. */
+void
+process_vprintf(process_t *process,
+                const char *format, va_list args)
+{
+  tor_assert(process);
+  tor_assert(format);
+
+  int size;
+  char *data;
+
+  size = tor_vasprintf(&data, format, args);
+  process_write(process, (uint8_t *)data, size);
+  tor_free(data);
+}
+
+/** As tor_snprintf(), but write the data to the given process's standard
+ * input. */
+void
+process_printf(process_t *process,
+               const char *format, ...)
+{
+  tor_assert(process);
+  tor_assert(format);
+
+  va_list ap;
+  va_start(ap, format);
+  process_vprintf(process, format, ap);
+  va_end(ap);
+}
+
+/** This function is called by the Process backend when a given process have
+ * data that is ready to be read from the child process's standard output
+ * handle. */
+void
+process_notify_event_stdout(process_t *process)
+{
+  tor_assert(process);
+
+  int ret;
+  ret = process_read_stdout(process, process->stdout_buffer);
+
+  if (ret > 0)
+    process_read_data(process,
+                      process->stdout_buffer,
+                      process->stdout_read_callback);
+}
+
+/** This function is called by the Process backend when a given process have
+ * data that is ready to be read from the child process's standard error
+ * handle. */
+void
+process_notify_event_stderr(process_t *process)
+{
+  tor_assert(process);
+
+  int ret;
+  ret = process_read_stderr(process, process->stderr_buffer);
+
+  if (ret > 0)
+    process_read_data(process,
+                      process->stderr_buffer,
+                      process->stderr_read_callback);
+}
+
+/** This function is called by the Process backend when a given process is
+ * allowed to begin writing data to the standard input of the child process. */
+void
+process_notify_event_stdin(process_t *process)
+{
+  tor_assert(process);
+
+  process_write_stdin(process, process->stdin_buffer);
+}
+
+/** This function is called by the Process backend when a given process have
+ * terminated. The exit status code is passed in <b>exit_code</b>. We mark the
+ * process as no longer running and calls the <b>exit_callback</b> with
+ * information about the process termination. */
+void
+process_notify_event_exit(process_t *process, process_exit_code_t exit_code)
+{
+  tor_assert(process);
+
+  log_debug(LD_PROCESS,
+            "Process terminated with exit code: %"PRIu64, exit_code);
+
+  /* Update our state. */
+  process_set_status(process, PROCESS_STATUS_NOT_RUNNING);
+  process->exit_code = exit_code;
+
+  /* Call our exit callback, if it exists. */
+  if (process->exit_callback) {
+    process->exit_callback(process, exit_code);
+  }
+}
+
+/** This function is called whenever the Process backend have notified us that
+ * there is data to be read from its standard out handle. Returns the number of
+ * bytes that have been put into the given buffer. */
+MOCK_IMPL(STATIC int, process_read_stdout, (process_t *process, buf_t *buffer))
+{
+  tor_assert(process);
+  tor_assert(buffer);
+
+  return 0;
+}
+
+/** This function is called whenever the Process backend have notified us that
+ * there is data to be read from its standard error handle. Returns the number
+ * of bytes that have been put into the given buffer. */
+MOCK_IMPL(STATIC int, process_read_stderr, (process_t *process, buf_t *buffer))
+{
+  tor_assert(process);
+  tor_assert(buffer);
+
+  return 0;
+}
+
+/** This function calls the backend function for the given process whenever
+ * there is data to be written to the backends' file handles. */
+MOCK_IMPL(STATIC void, process_write_stdin,
+          (process_t *process, buf_t *buffer))
+{
+  tor_assert(process);
+  tor_assert(buffer);
+}
+
+/** This function calls the protocol handlers based on the value of
+ * <b>process_get_protocol(process)</b>. Currently we call
+ * <b>process_read_buffer()</b> for <b>PROCESS_PROTOCOL_RAW</b> and
+ * <b>process_read_lines()</b> for <b>PROCESS_PROTOCOL_LINE</b>. */
+STATIC void
+process_read_data(process_t *process,
+                  buf_t *buffer,
+                  process_read_callback_t callback)
+{
+  tor_assert(process);
+  tor_assert(buffer);
+
+  switch (process_get_protocol(process)) {
+  case PROCESS_PROTOCOL_RAW:
+    process_read_buffer(process, buffer, callback);
+    break;
+  case PROCESS_PROTOCOL_LINE:
+    process_read_lines(process, buffer, callback);
+    break;
+  default:
+    /* LCOV_EXCL_START */
+    tor_assert_unreached();
+    return;
+    /* LCOV_EXCL_STOP */
+  }
+}
+
+/** This function takes the content of the given <b>buffer</b> and passes it to
+ * the given <b>callback</b> function, but ensures that an additional zero byte
+ * is added to the end of the data such that the given callback implementation
+ * can threat the content as a ASCIIZ string. */
+STATIC void
+process_read_buffer(process_t *process,
+                    buf_t *buffer,
+                    process_read_callback_t callback)
+{
+  tor_assert(process);
+  tor_assert(buffer);
+
+  const size_t size = buf_datalen(buffer);
+
+  /* We allocate an extra byte for the zero byte in the end. */
+  char *data = tor_malloc_zero(size + 1);
+
+  buf_get_bytes(buffer, data, size);
+  log_debug(LD_PROCESS, "Read data from process");
+
+  if (callback)
+    callback(process, data, size);
+
+  tor_free(data);
+}
+
+/** This function tries to extract complete lines from the given <b>buffer</b>
+ * and calls the given <b>callback</b> function whenever it has a complete
+ * line. Before calling <b>callback</b> we remove the trailing "\n" or "\r\n"
+ * from the line. If we are unable to extract a complete line we leave the data
+ * in the buffer for next call. */
+STATIC void
+process_read_lines(process_t *process,
+                   buf_t *buffer,
+                   process_read_callback_t callback)
+{
+  tor_assert(process);
+  tor_assert(buffer);
+
+  const size_t size = buf_datalen(buffer) + 1;
+  size_t line_size = 0;
+  char *data = tor_malloc_zero(size);
+  int ret;
+
+  while (true) {
+    line_size = size;
+    ret = buf_get_line(buffer, data, &line_size);
+
+    /* A complete line should always be smaller than the size of our
+     * buffer. */
+    tor_assert(ret != -1);
+
+    /* Remove \n from the end of the line. */
+    if (data[line_size - 1] == '\n') {
+      data[line_size - 1] = '\0';
+      --line_size;
+    }
+
+    /* Remove \r from the end of the line. */
+    if (data[line_size - 1] == '\r') {
+      data[line_size - 1] = '\0';
+      --line_size;
+    }
+
+    if (ret == 1) {
+      log_debug(LD_PROCESS, "Read line from process: \"%s\"", data);
+
+      if (callback)
+        callback(process, data, line_size);
+
+      /* We have read a whole line, let's see if there is more lines to read.
+       * */
+      continue;
+    }
+
+    /* No complete line for us to read. We are done for now. */
+    tor_assert_nonfatal(ret == 0);
+    break;
+  }
+
+  tor_free(data);
+}
diff --git a/src/lib/process/process.h b/src/lib/process/process.h
new file mode 100644
index 0000000000..cf20a5d80b
--- /dev/null
+++ b/src/lib/process/process.h
@@ -0,0 +1,127 @@
+/* Copyright (c) 2003-2004, Roger Dingledine
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2018, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file process.h
+ * \brief Header for process.c
+ **/
+
+#ifndef TOR_PROCESS_H
+#define TOR_PROCESS_H
+
+#include "orconfig.h"
+#include "lib/malloc/malloc.h"
+#include "lib/string/printf.h"
+
+/** Maximum number of bytes to write to a process' stdin. */
+#define PROCESS_MAX_WRITE (1024)
+
+/** Maximum number of bytes to read from a process' stdout/stderr. */
+#define PROCESS_MAX_READ  (1024)
+
+typedef enum {
+  /** The process is not running. */
+  PROCESS_STATUS_NOT_RUNNING,
+
+  /** The process is running. */
+  PROCESS_STATUS_RUNNING,
+
+  /** The process is in an erroneous state. */
+  PROCESS_STATUS_ERROR
+} process_status_t;
+
+const char *process_status_to_string(process_status_t status);
+
+typedef enum {
+  /** Pass complete \n-terminated lines to the
+   *  callback (with the \n or \r\n removed). */
+  PROCESS_PROTOCOL_LINE,
+
+  /** Pass the raw response from read() to the callback. */
+  PROCESS_PROTOCOL_RAW
+} process_protocol_t;
+
+const char *process_protocol_to_string(process_protocol_t protocol);
+
+struct process_t;
+typedef struct process_t process_t;
+
+typedef uint64_t process_exit_code_t;
+
+typedef void (*process_read_callback_t)(process_t *,
+                                        char *,
+                                        size_t);
+typedef void (*process_exit_callback_t)(process_t *,
+                                        process_exit_code_t);
+
+void process_init(void);
+void process_free_all(void);
+const smartlist_t *process_get_all_processes(void);
+
+process_t *process_new(const char *command);
+void process_free_(process_t *process);
+#define process_free(s) FREE_AND_NULL(process_t, process_free_, (s))
+
+process_status_t process_exec(process_t *process);
+
+void process_set_stdout_read_callback(process_t *,
+                                      process_read_callback_t);
+void process_set_stderr_read_callback(process_t *,
+                                      process_read_callback_t);
+void process_set_exit_callback(process_t *,
+                               process_exit_callback_t);
+
+const char *process_get_command(const process_t *process);
+
+void process_append_argument(process_t *process, const char *argument);
+const smartlist_t *process_get_arguments(const process_t *process);
+char **process_get_argv(const process_t *process);
+
+void process_set_environment(process_t *process,
+                             const char *key,
+                             const char *value);
+
+struct process_environment_t;
+struct process_environment_t *process_get_environment(const process_t *);
+
+void process_set_protocol(process_t *process, process_protocol_t protocol);
+process_protocol_t process_get_protocol(const process_t *process);
+
+void process_set_data(process_t *process, void *data);
+void *process_get_data(const process_t *process);
+
+void process_set_status(process_t *process, process_status_t status);
+process_status_t process_get_status(const process_t *process);
+
+void process_write(process_t *process,
+                   const uint8_t *data, size_t size);
+void process_vprintf(process_t *process,
+                     const char *format, va_list args) CHECK_PRINTF(2, 0);
+void process_printf(process_t *process,
+                    const char *format, ...) CHECK_PRINTF(2, 3);
+
+void process_notify_event_stdout(process_t *process);
+void process_notify_event_stderr(process_t *process);
+void process_notify_event_stdin(process_t *process);
+void process_notify_event_exit(process_t *process,
+                               process_exit_code_t);
+
+#ifdef PROCESS_PRIVATE
+MOCK_DECL(STATIC int, process_read_stdout, (process_t *, buf_t *));
+MOCK_DECL(STATIC int, process_read_stderr, (process_t *, buf_t *));
+MOCK_DECL(STATIC void, process_write_stdin, (process_t *, buf_t *));
+
+STATIC void process_read_data(process_t *process,
+                              buf_t *buffer,
+                              process_read_callback_t callback);
+STATIC void process_read_buffer(process_t *process,
+                                buf_t *buffer,
+                                process_read_callback_t callback);
+STATIC void process_read_lines(process_t *process,
+                               buf_t *buffer,
+                               process_read_callback_t callback);
+#endif /* defined(PROCESS_PRIVATE). */
+
+#endif /* defined(TOR_PROCESS_H). */
diff --git a/src/test/include.am b/src/test/include.am
index e5eae56e25..482897c7a5 100644
--- a/src/test/include.am
+++ b/src/test/include.am
@@ -153,6 +153,7 @@ src_test_test_SOURCES += \
 	src/test/test_pem.c \
 	src/test/test_periodic_event.c \
 	src/test/test_policy.c \
+	src/test/test_process.c \
 	src/test/test_procmon.c \
 	src/test/test_proto_http.c \
 	src/test/test_proto_misc.c \
diff --git a/src/test/test.c b/src/test/test.c
index 17b736d305..b13c3ffbe2 100644
--- a/src/test/test.c
+++ b/src/test/test.c
@@ -898,6 +898,7 @@ struct testgroup_t testgroups[] = {
   { "periodic-event/" , periodic_event_tests },
   { "policy/" , policy_tests },
   { "procmon/", procmon_tests },
+  { "process/", process_tests },
   { "proto/http/", proto_http_tests },
   { "proto/misc/", proto_misc_tests },
   { "protover/", protover_tests },
diff --git a/src/test/test.h b/src/test/test.h
index 092356f0fb..7c17389776 100644
--- a/src/test/test.h
+++ b/src/test/test.h
@@ -241,6 +241,7 @@ extern struct testcase_t pem_tests[];
 extern struct testcase_t periodic_event_tests[];
 extern struct testcase_t policy_tests[];
 extern struct testcase_t procmon_tests[];
+extern struct testcase_t process_tests[];
 extern struct testcase_t proto_http_tests[];
 extern struct testcase_t proto_misc_tests[];
 extern struct testcase_t protover_tests[];
diff --git a/src/test/test_process.c b/src/test/test_process.c
new file mode 100644
index 0000000000..2adbde7ad2
--- /dev/null
+++ b/src/test/test_process.c
@@ -0,0 +1,558 @@
+/* Copyright (c) 2018, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file test_process.c
+ * \brief Test cases for the Process API.
+ */
+
+#include "orconfig.h"
+#include "core/or/or.h"
+#include "test/test.h"
+
+#define PROCESS_PRIVATE
+#include "lib/process/process.h"
+
+static const char *stdout_read_buffer;
+static const char *stderr_read_buffer;
+
+struct process_data_t {
+  smartlist_t *stdout_data;
+  smartlist_t *stderr_data;
+  smartlist_t *stdin_data;
+  process_exit_code_t exit_code;
+};
+
+typedef struct process_data_t process_data_t;
+
+static process_data_t *
+process_data_new(void)
+{
+  process_data_t *process_data = tor_malloc_zero(sizeof(process_data_t));
+  process_data->stdout_data = smartlist_new();
+  process_data->stderr_data = smartlist_new();
+  process_data->stdin_data = smartlist_new();
+  return process_data;
+}
+
+static void
+process_data_free(process_data_t *process_data)
+{
+  if (process_data == NULL)
+    return;
+
+  SMARTLIST_FOREACH(process_data->stdout_data, char *, x, tor_free(x));
+  SMARTLIST_FOREACH(process_data->stderr_data, char *, x, tor_free(x));
+  SMARTLIST_FOREACH(process_data->stdin_data, char *, x, tor_free(x));
+
+  smartlist_free(process_data->stdout_data);
+  smartlist_free(process_data->stderr_data);
+  smartlist_free(process_data->stdin_data);
+  tor_free(process_data);
+}
+
+static int
+process_mocked_read_stdout(process_t *process, buf_t *buffer)
+{
+  (void)process;
+
+  if (stdout_read_buffer != NULL) {
+    buf_add_string(buffer, stdout_read_buffer);
+    stdout_read_buffer = NULL;
+  }
+
+  return (int)buf_datalen(buffer);
+}
+
+static int
+process_mocked_read_stderr(process_t *process, buf_t *buffer)
+{
+  (void)process;
+
+  if (stderr_read_buffer != NULL) {
+    buf_add_string(buffer, stderr_read_buffer);
+    stderr_read_buffer = NULL;
+  }
+
+  return (int)buf_datalen(buffer);
+}
+
+static void
+process_mocked_write_stdin(process_t *process, buf_t *buffer)
+{
+  const size_t size = buf_datalen(buffer);
+
+  if (size == 0)
+    return;
+
+  char *data = tor_malloc_zero(size + 1);
+  process_data_t *process_data = process_get_data(process);
+
+  buf_get_bytes(buffer, data, size);
+  smartlist_add(process_data->stdin_data, data);
+}
+
+static void
+process_stdout_callback(process_t *process, char *data, size_t size)
+{
+  tt_ptr_op(process, OP_NE, NULL);
+  tt_ptr_op(data, OP_NE, NULL);
+  tt_int_op(strlen(data), OP_EQ, size);
+
+  process_data_t *process_data = process_get_data(process);
+  smartlist_add(process_data->stdout_data, tor_strdup(data));
+
+ done:
+  return;
+}
+
+static void
+process_stderr_callback(process_t *process, char *data, size_t size)
+{
+  tt_ptr_op(process, OP_NE, NULL);
+  tt_ptr_op(data, OP_NE, NULL);
+  tt_int_op(strlen(data), OP_EQ, size);
+
+  process_data_t *process_data = process_get_data(process);
+  smartlist_add(process_data->stderr_data, tor_strdup(data));
+
+ done:
+  return;
+}
+
+static void
+process_exit_callback(process_t *process, process_exit_code_t exit_code)
+{
+  tt_ptr_op(process, OP_NE, NULL);
+
+  process_data_t *process_data = process_get_data(process);
+  process_data->exit_code = exit_code;
+
+ done:
+  return;
+}
+
+static void
+test_default_values(void *arg)
+{
+  (void)arg;
+  process_init();
+
+  process_t *process = process_new("/path/to/nothing");
+
+  /* We are not running by default. */
+  tt_int_op(PROCESS_STATUS_NOT_RUNNING, OP_EQ, process_get_status(process));
+
+  /* We use the line protocol by default. */
+  tt_int_op(PROCESS_PROTOCOL_LINE, OP_EQ, process_get_protocol(process));
+
+  /* We don't set any custom data by default. */
+  tt_ptr_op(NULL, OP_EQ, process_get_data(process));
+
+  /* Our command was given to the process_t's constructor in process_new(). */
+  tt_str_op("/path/to/nothing", OP_EQ, process_get_command(process));
+
+  /* Make sure we are listed in the list of proccesses. */
+  tt_assert(smartlist_contains(process_get_all_processes(),
+                               process));
+
+  /* Our arguments should be empty. */
+  tt_int_op(0, OP_EQ,
+            smartlist_len(process_get_arguments(process)));
+
+ done:
+  process_free(process);
+  process_free_all();
+}
+
+static void
+test_stringified_types(void *arg)
+{
+  (void)arg;
+
+  /* process_protocol_t values. */
+  tt_str_op("Raw", OP_EQ, process_protocol_to_string(PROCESS_PROTOCOL_RAW));
+  tt_str_op("Line", OP_EQ, process_protocol_to_string(PROCESS_PROTOCOL_LINE));
+
+  /* process_status_t values. */
+  tt_str_op("not running", OP_EQ,
+            process_status_to_string(PROCESS_STATUS_NOT_RUNNING));
+  tt_str_op("running", OP_EQ,
+            process_status_to_string(PROCESS_STATUS_RUNNING));
+  tt_str_op("error", OP_EQ,
+            process_status_to_string(PROCESS_STATUS_ERROR));
+
+ done:
+  return;
+}
+
+static void
+test_line_protocol_simple(void *arg)
+{
+  (void)arg;
+  process_init();
+
+  process_data_t *process_data = process_data_new();
+
+  process_t *process = process_new("");
+  process_set_data(process, process_data);
+
+  process_set_stdout_read_callback(process, process_stdout_callback);
+  process_set_stderr_read_callback(process, process_stderr_callback);
+
+  MOCK(process_read_stdout, process_mocked_read_stdout);
+  MOCK(process_read_stderr, process_mocked_read_stderr);
+
+  /* Make sure we are running with the line protocol. */
+  tt_int_op(PROCESS_PROTOCOL_LINE, OP_EQ, process_get_protocol(process));
+
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  stdout_read_buffer = "Hello stdout\n";
+  process_notify_event_stdout(process);
+  tt_ptr_op(NULL, OP_EQ, stdout_read_buffer);
+
+  stderr_read_buffer = "Hello stderr\r\n";
+  process_notify_event_stderr(process);
+  tt_ptr_op(NULL, OP_EQ, stderr_read_buffer);
+
+  /* Data should be ready. */
+  tt_int_op(1, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(1, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  /* Check if the data is correct. */
+  tt_str_op(smartlist_get(process_data->stdout_data, 0), OP_EQ,
+            "Hello stdout");
+  tt_str_op(smartlist_get(process_data->stderr_data, 0), OP_EQ,
+            "Hello stderr");
+
+ done:
+  process_data_free(process_data);
+  process_free(process);
+  process_free_all();
+
+  UNMOCK(process_read_stdout);
+  UNMOCK(process_read_stderr);
+}
+
+static void
+test_line_protocol_multi(void *arg)
+{
+  (void)arg;
+  process_init();
+
+  process_data_t *process_data = process_data_new();
+
+  process_t *process = process_new("");
+  process_set_data(process, process_data);
+  process_set_stdout_read_callback(process, process_stdout_callback);
+  process_set_stderr_read_callback(process, process_stderr_callback);
+
+  MOCK(process_read_stdout, process_mocked_read_stdout);
+  MOCK(process_read_stderr, process_mocked_read_stderr);
+
+  /* Make sure we are running with the line protocol. */
+  tt_int_op(PROCESS_PROTOCOL_LINE, OP_EQ, process_get_protocol(process));
+
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  stdout_read_buffer = "Hello stdout\r\nOnion Onion Onion\nA B C D\r\n\r\n";
+  process_notify_event_stdout(process);
+  tt_ptr_op(NULL, OP_EQ, stdout_read_buffer);
+
+  stderr_read_buffer = "Hello stderr\nFoo bar baz\nOnion Onion Onion\n";
+  process_notify_event_stderr(process);
+  tt_ptr_op(NULL, OP_EQ, stderr_read_buffer);
+
+  /* Data should be ready. */
+  tt_int_op(4, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(3, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  /* Check if the data is correct. */
+  tt_str_op(smartlist_get(process_data->stdout_data, 0), OP_EQ,
+            "Hello stdout");
+  tt_str_op(smartlist_get(process_data->stdout_data, 1), OP_EQ,
+            "Onion Onion Onion");
+  tt_str_op(smartlist_get(process_data->stdout_data, 2), OP_EQ,
+            "A B C D");
+  tt_str_op(smartlist_get(process_data->stdout_data, 3), OP_EQ,
+            "");
+
+  tt_str_op(smartlist_get(process_data->stderr_data, 0), OP_EQ,
+            "Hello stderr");
+  tt_str_op(smartlist_get(process_data->stderr_data, 1), OP_EQ,
+            "Foo bar baz");
+  tt_str_op(smartlist_get(process_data->stderr_data, 2), OP_EQ,
+            "Onion Onion Onion");
+
+ done:
+  process_data_free(process_data);
+  process_free(process);
+  process_free_all();
+
+  UNMOCK(process_read_stdout);
+  UNMOCK(process_read_stderr);
+}
+
+static void
+test_line_protocol_partial(void *arg)
+{
+  (void)arg;
+  process_init();
+
+  process_data_t *process_data = process_data_new();
+
+  process_t *process = process_new("");
+  process_set_data(process, process_data);
+  process_set_stdout_read_callback(process, process_stdout_callback);
+  process_set_stderr_read_callback(process, process_stderr_callback);
+
+  MOCK(process_read_stdout, process_mocked_read_stdout);
+  MOCK(process_read_stderr, process_mocked_read_stderr);
+
+  /* Make sure we are running with the line protocol. */
+  tt_int_op(PROCESS_PROTOCOL_LINE, OP_EQ, process_get_protocol(process));
+
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  stdout_read_buffer = "Hello stdout this is a partial line ...";
+  process_notify_event_stdout(process);
+  tt_ptr_op(NULL, OP_EQ, stdout_read_buffer);
+
+  stderr_read_buffer = "Hello stderr this is a partial line ...";
+  process_notify_event_stderr(process);
+  tt_ptr_op(NULL, OP_EQ, stderr_read_buffer);
+
+  /* Data should NOT be ready. */
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  stdout_read_buffer = " the end\nAnother partial string goes here ...";
+  process_notify_event_stdout(process);
+  tt_ptr_op(NULL, OP_EQ, stdout_read_buffer);
+
+  stderr_read_buffer = " the end\nAnother partial string goes here ...";
+  process_notify_event_stderr(process);
+  tt_ptr_op(NULL, OP_EQ, stderr_read_buffer);
+
+  /* Some data should be ready. */
+  tt_int_op(1, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(1, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  stdout_read_buffer = " the end\nFoo bar baz\n";
+  process_notify_event_stdout(process);
+  tt_ptr_op(NULL, OP_EQ, stdout_read_buffer);
+
+  stderr_read_buffer = " the end\nFoo bar baz\n";
+  process_notify_event_stderr(process);
+  tt_ptr_op(NULL, OP_EQ, stderr_read_buffer);
+
+  /* Some data should be ready. */
+  tt_int_op(3, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(3, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  /* Check if the data is correct. */
+  tt_str_op(smartlist_get(process_data->stdout_data, 0), OP_EQ,
+                          "Hello stdout this is a partial line ... the end");
+  tt_str_op(smartlist_get(process_data->stdout_data, 1), OP_EQ,
+                          "Another partial string goes here ... the end");
+  tt_str_op(smartlist_get(process_data->stdout_data, 2), OP_EQ,
+                          "Foo bar baz");
+
+  tt_str_op(smartlist_get(process_data->stderr_data, 0), OP_EQ,
+                          "Hello stderr this is a partial line ... the end");
+  tt_str_op(smartlist_get(process_data->stderr_data, 1), OP_EQ,
+                          "Another partial string goes here ... the end");
+  tt_str_op(smartlist_get(process_data->stderr_data, 2), OP_EQ,
+                          "Foo bar baz");
+
+ done:
+  process_data_free(process_data);
+  process_free(process);
+  process_free_all();
+
+  UNMOCK(process_read_stdout);
+  UNMOCK(process_read_stderr);
+}
+
+static void
+test_raw_protocol_simple(void *arg)
+{
+  (void)arg;
+  process_init();
+
+  process_data_t *process_data = process_data_new();
+
+  process_t *process = process_new("");
+  process_set_data(process, process_data);
+  process_set_protocol(process, PROCESS_PROTOCOL_RAW);
+
+  process_set_stdout_read_callback(process, process_stdout_callback);
+  process_set_stderr_read_callback(process, process_stderr_callback);
+
+  MOCK(process_read_stdout, process_mocked_read_stdout);
+  MOCK(process_read_stderr, process_mocked_read_stderr);
+
+  /* Make sure we are running with the raw protocol. */
+  tt_int_op(PROCESS_PROTOCOL_RAW, OP_EQ, process_get_protocol(process));
+
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(0, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  stdout_read_buffer = "Hello stdout\n";
+  process_notify_event_stdout(process);
+  tt_ptr_op(NULL, OP_EQ, stdout_read_buffer);
+
+  stderr_read_buffer = "Hello stderr\n";
+  process_notify_event_stderr(process);
+  tt_ptr_op(NULL, OP_EQ, stderr_read_buffer);
+
+  /* Data should be ready. */
+  tt_int_op(1, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(1, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  stdout_read_buffer = "Hello, again, stdout\nThis contains multiple lines";
+  process_notify_event_stdout(process);
+  tt_ptr_op(NULL, OP_EQ, stdout_read_buffer);
+
+  stderr_read_buffer = "Hello, again, stderr\nThis contains multiple lines";
+  process_notify_event_stderr(process);
+  tt_ptr_op(NULL, OP_EQ, stderr_read_buffer);
+
+  /* Data should be ready. */
+  tt_int_op(2, OP_EQ, smartlist_len(process_data->stdout_data));
+  tt_int_op(2, OP_EQ, smartlist_len(process_data->stderr_data));
+
+  /* Check if the data is correct. */
+  tt_str_op(smartlist_get(process_data->stdout_data, 0), OP_EQ,
+            "Hello stdout\n");
+  tt_str_op(smartlist_get(process_data->stdout_data, 1), OP_EQ,
+            "Hello, again, stdout\nThis contains multiple lines");
+
+  tt_str_op(smartlist_get(process_data->stderr_data, 0), OP_EQ,
+            "Hello stderr\n");
+  tt_str_op(smartlist_get(process_data->stderr_data, 1), OP_EQ,
+            "Hello, again, stderr\nThis contains multiple lines");
+
+ done:
+  process_data_free(process_data);
+  process_free(process);
+  process_free_all();
+
+  UNMOCK(process_read_stdout);
+  UNMOCK(process_read_stderr);
+}
+
+static void
+test_write_simple(void *arg)
+{
+  (void)arg;
+
+  process_init();
+
+  process_data_t *process_data = process_data_new();
+
+  process_t *process = process_new("");
+  process_set_data(process, process_data);
+
+  MOCK(process_write_stdin, process_mocked_write_stdin);
+
+  process_write(process, (uint8_t *)"Hello world\n", 12);
+  process_notify_event_stdin(process);
+  tt_int_op(1, OP_EQ, smartlist_len(process_data->stdin_data));
+
+  process_printf(process, "Hello %s !\n", "moon");
+  process_notify_event_stdin(process);
+  tt_int_op(2, OP_EQ, smartlist_len(process_data->stdin_data));
+
+ done:
+  process_data_free(process_data);
+  process_free(process);
+  process_free_all();
+
+  UNMOCK(process_write_stdin);
+}
+
+static void
+test_exit_simple(void *arg)
+{
+  (void)arg;
+
+  process_init();
+
+  process_data_t *process_data = process_data_new();
+
+  process_t *process = process_new("");
+  process_set_data(process, process_data);
+  process_set_exit_callback(process, process_exit_callback);
+
+  /* Our default is 0. */
+  tt_int_op(0, OP_EQ, process_data->exit_code);
+
+  /* Fake that we are a running process. */
+  process_set_status(process, PROCESS_STATUS_RUNNING);
+  tt_int_op(process_get_status(process), OP_EQ, PROCESS_STATUS_RUNNING);
+
+  /* Fake an exit. */
+  process_notify_event_exit(process, 1337);
+
+  /* Check if our state changed and if our callback fired. */
+  tt_int_op(process_get_status(process), OP_EQ, PROCESS_STATUS_NOT_RUNNING);
+  tt_int_op(1337, OP_EQ, process_data->exit_code);
+
+ done:
+  process_set_data(process, process_data);
+  process_data_free(process_data);
+  process_free(process);
+  process_free_all();
+}
+
+static void
+test_argv_simple(void *arg)
+{
+  (void)arg;
+  process_init();
+
+  process_t *process = process_new("/bin/cat");
+  char **argv = NULL;
+
+  /* Setup some arguments. */
+  process_append_argument(process, "foo");
+  process_append_argument(process, "bar");
+  process_append_argument(process, "baz");
+
+  /* Check the number of elements. */
+  tt_int_op(3, OP_EQ,
+            smartlist_len(process_get_arguments(process)));
+
+  /* Let's try to convert it into a Unix style char **argv. */
+  argv = process_get_argv(process);
+
+  /* Check our values. */
+  tt_str_op(argv[0], OP_EQ, "/bin/cat");
+  tt_str_op(argv[1], OP_EQ, "foo");
+  tt_str_op(argv[2], OP_EQ, "bar");
+  tt_str_op(argv[3], OP_EQ, "baz");
+  tt_ptr_op(argv[4], OP_EQ, NULL);
+
+ done:
+  tor_free(argv);
+  process_free(process);
+  process_free_all();
+}
+
+struct testcase_t process_tests[] = {
+  { "default_values", test_default_values, TT_FORK, NULL, NULL },
+  { "stringified_types", test_stringified_types, TT_FORK, NULL, NULL },
+  { "line_protocol_simple", test_line_protocol_simple, TT_FORK, NULL, NULL },
+  { "line_protocol_multi", test_line_protocol_multi, TT_FORK, NULL, NULL },
+  { "line_protocol_partial", test_line_protocol_partial, TT_FORK, NULL, NULL },
+  { "raw_protocol_simple", test_raw_protocol_simple, TT_FORK, NULL, NULL },
+  { "write_simple", test_write_simple, TT_FORK, NULL, NULL },
+  { "exit_simple", test_exit_simple, TT_FORK, NULL, NULL },
+  { "argv_simple", test_argv_simple, TT_FORK, NULL, NULL },
+  END_OF_TESTCASES
+};