tar,smemcap: commonalyze checksumming code for tar header

function                                             old     new   delta
chksum_and_xwrite_tar_header                           -      99     +99
writeheader                                          280     199     -81
chksum_and_xwrite                                    102       -    -102
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/1 up/down: 99/-183)           Total: -84 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/archival/chksum_and_xwrite_tar_header.c b/archival/chksum_and_xwrite_tar_header.c
new file mode 100644
index 000000000..25934f898
--- /dev/null
+++ b/archival/chksum_and_xwrite_tar_header.c
@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2021 Denys Vlasenko <vda.linux@googlemail.com>
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+//kbuild:lib-$(CONFIG_FEATURE_TAR_CREATE) += chksum_and_xwrite_tar_header.o
+//kbuild:lib-$(CONFIG_SMEMCAP) += chksum_and_xwrite_tar_header.o
+
+#include "libbb.h"
+#include "bb_archive.h"
+
+void FAST_FUNC chksum_and_xwrite_tar_header(int fd, struct tar_header_t *hp)
+{
+	/* POSIX says that checksum is done on unsigned bytes
+	 * (Sun and HP-UX gets it wrong... more details in
+	 * GNU tar source) */
+	const unsigned char *cp;
+	int chksum, size;
+
+	strcpy(hp->magic, "ustar  ");
+
+	/* Calculate and store the checksum (the sum of all of the bytes of
+	 * the header).  The checksum field must be filled with blanks for the
+	 * calculation.  The checksum field is formatted differently from the
+	 * other fields: it has 6 digits, a NUL, then a space -- rather than
+	 * digits, followed by a NUL like the other fields... */
+	memset(hp->chksum, ' ', sizeof(hp->chksum));
+	cp = (const unsigned char *) hp;
+	chksum = 0;
+	size = sizeof(*hp);
+	do { chksum += *cp++; } while (--size);
+	sprintf(hp->chksum, "%06o", chksum);
+
+	xwrite(fd, hp, sizeof(*hp));
+}