sandbox: Add a function to read a line from a file

Provide a version of fgets() which works on file descriptors. This can
be used to read lines from a file, as will be needed for the ulib
test-program.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/os.c

@@ -150,6 +150,38 @@ int os_unlink(const char *pathname)
 	return unlink(pathname);
 }
 
+char *os_fgets(char *str, int size, int fd)
+{
+	char *s = str;
+	int n = size - 1;
+	int i;
+
+	if (n <= 0 || !str)
+		return NULL;
+
+	for (i = 0; i < n; i++) {
+		ssize_t ret;
+		char c;
+
+		ret  = read(fd, &c, 1);
+		if (ret <= 0) {
+			/* EOF or error */
+			if (!i)
+				return NULL;
+			break;
+		}
+
+		*s++ = c;
+
+		if (c == '\n')
+			break;
+	}
+
+	*s = '\0';
+
+	return str;
+}
+
 void os_exit(int exit_code)
 {
 	exit(exit_code);

include/os.h

@@ -106,6 +106,20 @@ int os_isatty(int fd);
  */
 int os_unlink(const char *pathname);
 
+/**
+ * os_fgets() - read a string from a file stream
+ *
+ * Reads at most @size - 1 characters from the stream and stores them in str.
+ * Reading stops after an EOF or a newline. If a newline is read, it is
+ * stored in str. A terminating nul byte is appended.
+ *
+ * @str:	Buffer to store the string
+ * @size:	Maximum number of characters to read (including null terminator)
+ * @fd:		File descriptor to read from
+ * Return:	str on success, NULL on error, or EOF with no characters read
+ */
+char *os_fgets(char *str, int size, int fd);
+
 /** os_persistent_fname() - Find the path to a test file
  *
  * @buf: Buffer to hold path