util: add os_read_file() helper

readN() taken from igt.
os_read_file() inspired by igt_sysfs_get()

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/util/Makefile.sources

@@ -29,6 +29,8 @@ MESA_UTIL_FILES := \
 	mesa-sha1.h \
 	os_time.c \
 	os_time.h \
+	os_file.c \
+	os_file.h \
 	os_misc.c \
 	os_misc.h \
 	u_process.c \

src/util/meson.build

@@ -52,6 +52,7 @@ files_mesa_util = files(
   'mesa-sha1.h',
   'os_time.c',
   'os_time.h',
+  'os_file.c',
   'os_misc.c',
   'os_misc.h',
   'u_process.c',

src/util/os_file.c

@@ -0,0 +1,129 @@
+/*
+ * Copyright 2019 Intel Corporation
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "os_file.h"
+
+#include <errno.h>
+
+#if defined(__linux__)
+
+#include <fcntl.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+
+static ssize_t
+readN(int fd, char *buf, size_t len)
+{
+   int err = -ENODATA;
+   size_t total = 0;
+   do {
+      ssize_t ret = read(fd, buf + total, len - total);
+
+      if (ret < 0)
+         ret = -errno;
+
+      if (ret == -EINTR || ret == -EAGAIN)
+         continue;
+
+      if (ret <= 0)
+         break;
+
+      total += ret;
+   } while (total != len);
+
+   return total ? total : err;
+}
+
+static char *
+read_grow(int fd)
+{
+   size_t len = 64;
+
+   char *buf = malloc(len);
+   if (!buf) {
+      close(fd);
+      errno = -ENOMEM;
+      return NULL;
+   }
+
+   ssize_t read;
+   size_t offset = 0, remaining = len - 1;
+   while ((read = readN(fd, buf + offset, remaining)) == remaining) {
+      char *newbuf = realloc(buf, 2 * len);
+      if (!newbuf) {
+         free(buf);
+         close(fd);
+         errno = -ENOMEM;
+         return NULL;
+      }
+
+      buf = newbuf;
+      len *= 2;
+      offset += read;
+      remaining = len - offset - 1;
+   }
+
+   close(fd);
+
+   if (read > 0)
+      offset += read;
+
+   buf[offset] = '\0';
+
+   return buf;
+}
+
+char *
+os_read_file(const char *filename)
+{
+   size_t len = 0;
+
+   int fd = open(filename, O_RDONLY);
+   if (fd == -1) {
+      /* errno set by open() */
+      return NULL;
+   }
+
+   struct stat stat;
+   if (fstat(fd, &stat) == 0)
+      len = stat.st_size;
+
+   if (!len)
+      return read_grow(fd);
+
+   /* add NULL terminator */
+   len++;
+
+   char *buf = malloc(len);
+   if (!buf) {
+      close(fd);
+      errno = -ENOMEM;
+      return NULL;
+   }
+
+   ssize_t read = readN(fd, buf, len - 1);
+
+   close(fd);
+
+   if (read == -1)
+      return NULL;
+
+   buf[read] = '\0';
+
+   return buf;
+}
+
+#else
+
+char *
+os_read_file(const char *filename)
+{
+   errno = -ENOSYS;
+   return NULL;
+}
+
+#endif

src/util/os_file.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2019 Intel Corporation
+ * SPDX-License-Identifier: MIT
+ *
+ * File operations helpers
+ */
+
+#ifndef _OS_FILE_H_
+#define _OS_FILE_H_
+
+#ifdef  __cplusplus
+extern "C" {
+#endif
+
+/*
+ * Read a file.
+ * Returns a char* that the caller must free(), or NULL and sets errno.
+ */
+char *
+os_read_file(const char *filename);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _OS_FILE_H_ */