libutil: add read_data style wrapper around pread()

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

3 files changed, 41 insertions, 0 deletions

diff --git a/include/util.h b/include/util.h
index 8b79c30..0c36160 100644
--- a/include/util.h
+++ b/include/util.h
@@ -40,6 +40,12 @@ int write_data(const char *errstr, int fd, const void *data, size_t size);
 int read_data(const char *errstr, int fd, void *buffer, size_t size);
 
 /*
+  Similar to read_data but wrapps pread() instead of read().
+*/
+int read_data_at(const char *errstr, off_t location,
+		 int fd, void *buffer, size_t size);
+
+/*
   A common implementation of the '--version' command line flag.
 
   Prints out version information. The program name is extracted from the
diff --git a/lib/Makemodule.am b/lib/Makemodule.am
index ab2030c..0ea95fa 100644
--- a/lib/Makemodule.am
+++ b/lib/Makemodule.am
@@ -42,6 +42,7 @@ libutil_a_SOURCES += lib/util/read_data.c include/util.h
 libutil_a_SOURCES += lib/util/print_version.c lib/util/mkdir_p.c
 libutil_a_SOURCES += lib/util/str_table.c include/str_table.h
 libutil_a_SOURCES += lib/util/dirstack.c lib/util/padd_file.c
+libutil_a_SOURCES += lib/util/read_data_at.c
 
 if WITH_GZIP
 libcompress_a_SOURCES += lib/comp/gzip.c
diff --git a/lib/util/read_data_at.c b/lib/util/read_data_at.c
new file mode 100644
index 0000000..98432d9
--- /dev/null
+++ b/lib/util/read_data_at.c
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+#include "config.h"
+
+#include <unistd.h>
+#include <errno.h>
+#include <stdio.h>
+
+#include "util.h"
+
+int read_data_at(const char *errstr, off_t location, int fd,
+		 void *buffer, size_t size)
+{
+	ssize_t ret;
+
+	while (size > 0) {
+		ret = pread(fd, buffer, size, location);
+		if (ret < 0) {
+			if (errno == EINTR)
+				continue;
+			perror(errstr);
+			return -1;
+		}
+		if (ret == 0) {
+			fprintf(stderr, "%s: short read\n", errstr);
+			return -1;
+		}
+
+		size -= ret;
+		buffer = (char *)buffer + ret;
+		location += ret;
+	}
+
+	return 0;
+}