Implement xrdb reading without popen

Avoid using the shell, awk and cut command

src/main.c

@@ -7,7 +7,7 @@
   #include <windows.h>
 #elif __linux__
   #include <unistd.h>
-  #include <stdlib.h>
+  #include "xrdb_parse.h"
 #elif __APPLE__
   #include <mach-o/dyld.h>
 #endif
@@ -15,40 +15,17 @@
 
 SDL_Window *window;
 
-
 static double get_scale(void) {
+#if _WIN32
   float dpi;
   SDL_GetDisplayDPI(0, NULL, &dpi, NULL);
-#if _WIN32
   return dpi / 96.0;
 #elif __linux__
-  FILE *stream;
-  char *xrdb = "xrdb -query | grep dpi | cut -f 2";
-
-  // In case something goes wrong in popen (e.g., fork or exec calls) or any
-  // other calls listed below we return the dpi equal to 1.
-  if ((stream = popen(xrdb, "r")) == NULL)
-    return 1.0;
-
-  char *line = NULL;
-  size_t buffer = 0;
-  ssize_t length = getline(&line, &buffer, stream);
-
-  if (length == -1) {
-    free(line);
-    pclose(stream);
-    return 1.0;
+  int xft_dpi = xrdb_find_dpi();
+  if (xft_dpi > 0) {
+    return xft_dpi / 96.0;
   }
-
-  if (pclose(stream)) {
-    free(line);
   return 1.0;
-  }
-
-  dpi = strtol(line, NULL, 10) / 96.0;
-  free(line);
-
-  return dpi;
 #else
   return 1.0;
 #endif

src/meson.build

@@ -5,6 +5,7 @@ lite_sources = [
     'api/system.c',
     'renderer.c',
     'rencache.c',
+    'xrdb_parse.c',
     'main.c',
 ]
 

src/xrdb_parse.c

@@ -0,0 +1,139 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "xrdb_parse.h"
+
+extern char **environ;
+
+static int join_path(char buf[], int buf_size, const char *path, const char *name) {
+  const char *path_delim = strchr(path, ':');
+  if (!path_delim) {
+    path_delim = path + strlen(path);
+  }
+  const int path_len = path_delim - path, name_len = strlen(name);
+  if (buf_size < path_len + 1 + name_len + 1) return -1;
+  memcpy(buf, path, path_len);
+  buf[path_len] = '/';
+  memcpy(buf + path_len + 1, name, name_len + 1);
+  return 0;
+}
+
+
+static int xrdb_popen (int *pid_ptr) {
+  int fd[2];
+
+  pipe(fd);
+  int read_fd = fd[STDIN_FILENO];
+  int write_fd = fd[STDOUT_FILENO];
+
+  char *path = getenv("PATH");
+
+  int pid = fork();
+  if (pid == 0) {
+    close(read_fd);
+    dup2(write_fd, STDOUT_FILENO);
+    close(write_fd);
+    char xrdb_filename[256];
+    while (path) {
+      char *xrgb_argv[3] = {"xrdb", "-query", NULL};
+      if (join_path(xrdb_filename, 256, path, "xrdb") == 0) {
+        execve(xrdb_filename, xrgb_argv, environ);
+      }
+      path = strchr(path, ':');
+      if (path) {
+        path++;
+      }
+    }
+  } else {
+    *pid_ptr = pid;
+    close(write_fd);
+    return read_fd;
+  }
+  return -1;
+}
+
+
+static int xrdb_try_dpi_match(const char *line, int len) {
+  if (len >= 8 && strncmp(line, "Xft.dpi:", 8) == 0 && len - 8 + 1 <= 64) {
+    char buf[64];
+    memcpy(buf, line + 8, len - 8);
+    buf[len - 8] = 0;
+    return strtol(buf, NULL, 10);
+  }
+  return -1;
+}
+
+#define XRDB_READ_BUF_SIZE 256
+static void xrdb_complete_reading(int read_fd) {
+  char buf[XRDB_READ_BUF_SIZE];
+  while (1) {
+    int nr = read(read_fd, buf, XRDB_READ_BUF_SIZE);
+    if (nr <= 0) return;
+  }
+}
+
+static int xrdb_parse_for_dpi(int read_fd) {
+  char buf[XRDB_READ_BUF_SIZE];
+  char read_buf_offset = 0;
+  while (1) {
+    int nr = read(read_fd, buf + read_buf_offset, XRDB_READ_BUF_SIZE - read_buf_offset);
+    if (nr > 0) {
+      const char * const buf_limit = buf + nr + read_buf_offset;
+      char *line_start = buf;
+      while (line_start < buf_limit) {
+        char *nlptr = line_start;
+        while (nlptr < buf_limit && *nlptr != '\n') {
+          nlptr++;
+        }
+        if (nlptr < buf_limit) {
+          int dpi_read = xrdb_try_dpi_match(line_start, nlptr - line_start);
+          if (dpi_read > 0) {
+            xrdb_complete_reading(read_fd);
+            return dpi_read;
+          }
+          line_start = nlptr + 1;
+        } else {
+          /* No newline found: copy the remaining part at the beginning of buf
+             and read again from file descriptor. */
+          const int rem = nr + read_buf_offset - (line_start - buf);
+          if (rem >= XRDB_READ_BUF_SIZE) {
+            /* Line is too long for the buffer: skip. */
+            read_buf_offset = 0;
+          } else {
+            int c = 0;
+            while (c < rem) {
+              buf[c] = line_start[c];
+              c++;
+            }
+            read_buf_offset = rem;
+          }
+          break;
+        }
+      }
+    } else {
+      if (nr == 0 && read_buf_offset > 0) {
+        int dpi_read = xrdb_try_dpi_match(buf, read_buf_offset);
+        if (dpi_read > 0) {
+          return dpi_read;
+        }
+      }
+      break;
+    }
+  }
+  return -1;
+}
+
+int xrdb_find_dpi() {
+  int xrdb_pid;
+  int read_fd = xrdb_popen(&xrdb_pid);
+  if (read_fd >= 0) {
+    int dpi_read = xrdb_parse_for_dpi(read_fd);
+    int child_status;
+    waitpid(xrdb_pid, &child_status, 0);
+    close(read_fd);
+    return dpi_read;
+  }
+  return -1;
+}

src/xrdb_parse.h

@@ -0,0 +1,6 @@
+#ifndef XRDB_PARSE_H
+#define XRDB_PARSE_H
+
+extern int xrdb_find_dpi();
+
+#endif