Add log-file option to automatically redirect stderr to a file

From bfgminer b0240dd004

compat.h

@@ -4,6 +4,7 @@
 #ifdef WIN32
 #include "config.h"
 #include <errno.h>
+#include <fcntl.h>
 #include <time.h>
 #include <pthread.h>
 #include <sys/time.h>

doc/configuration.md

@@ -465,6 +465,7 @@ sgminer 4.2.1-116-g2e8b-dirty
   * [incognito](#incognito)
   * [kernel-path](#kernel-path)
   * [log](#log)
+  * [log-file](#log-file)
   * [log-show-date](#log-show-date)
   * [lowmem](#lowmem)
   * [monitor](#monitor)
@@ -1973,6 +1974,22 @@ Set the interval in seconds between log outputs.
 
 [Top](#configuration-and-command-line-options) :: [Config-file and CLI options](#config-file-and-cli-options) :: [Miscellaneous Options](#miscellaneous-options)
 
+### log-file
+
+Log stderr to file.
+
+*Available*: Global
+
+*Config File Syntax:* `"log-file":"<path>"`
+
+*Command Line Syntax:* `--log-file <path>`
+
+*Argument:* `path` Path to log file, or FD number, or `-` to redirect to stdout.
+
+*Default:* will log to stderr
+
+[Top](#configuration-and-command-line-options) :: [Config-file and CLI options](#config-file-and-cli-options) :: [Miscellaneous Options](#miscellaneous-options)
+
 ### log-show-date
 
 Show a timestamp on every log line.

sgminer.c

@@ -1116,6 +1116,36 @@ static char *set_schedtime(const char *arg, struct schedtime *st)
   return NULL;
 }
 
+static char *set_log_file(char *arg)
+{
+  char *r = "";
+  long int i = strtol(arg, &r, 10);
+  int fd, stderr_fd = fileno(stderr);
+
+  if ((!*r) && i >= 0 && i <= INT_MAX)
+    fd = i;
+  else
+  if (!strcmp(arg, "-"))
+  {
+    fd = fileno(stdout);
+    if (unlikely(fd == -1))
+      return "Standard output missing for log-file";
+  }
+  else
+  {
+    fd = open(arg, O_WRONLY | O_APPEND | O_CREAT, S_IRUSR | S_IWUSR);
+    if (unlikely(fd == -1))
+      return "Failed to open %s for log-file";
+  }
+
+  close(stderr_fd);
+  if (unlikely(-1 == dup2(fd, stderr_fd)))
+    return "Failed to dup2 for log-file";
+  close(fd);
+
+  return NULL;
+}
+
 static char* set_sharelog(char *arg)
 {
   char *r = "";
@@ -1415,6 +1445,9 @@ struct opt_table opt_config_table[] = {
   OPT_WITH_ARG("--log|-l",
       set_int_0_to_9999, opt_show_intval, &opt_log_interval,
       "Interval in seconds between log output"),
+ OPT_WITH_ARG("--log-file|-L",
+      set_log_file, NULL, NULL,
+      "Log stderr to file"),
   OPT_WITHOUT_ARG("--log-show-date|-L",
       opt_set_bool, &opt_log_show_date,
       "Show date on every log line"),