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>

4 files changed, 39 insertions, 44 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));
+}
diff --git a/archival/tar.c b/archival/tar.c
index 94fb61a29..9de37592e 100644
--- a/archival/tar.c
+++ b/archival/tar.c
@@ -254,32 +254,6 @@ static void putOctal(char *cp, int len, off_t value)
 }
 #define PUT_OCTAL(a, b) putOctal((a), sizeof(a), (b))
 
-static void chksum_and_xwrite(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 (i.e., 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 null, then a space -- rather than
-         * digits, followed by a null 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);
-        putOctal(hp->chksum, sizeof(hp->chksum)-1, chksum);
-
-        /* Now write the header out to disk */
-        xwrite(fd, hp, sizeof(*hp));
-}
-
 # if ENABLE_FEATURE_TAR_GNU_EXTENSIONS
 static void writeLongname(int fd, int type, const char *name, int dir)
 {
@@ -310,7 +284,7 @@ static void writeLongname(int fd, int type, const char *name, int dir)
         /* + dir: account for possible '/' */
 
         PUT_OCTAL(header.size, size);
-        chksum_and_xwrite(fd, &header);
+        chksum_and_xwrite_tar_header(fd, &header);
 
         /* Write filename[/] and pad the block. */
         /* dir=0: writes 'name<NUL>', pads */
@@ -441,8 +415,7 @@ static int writeTarHeader(struct TarBallInfo *tbInfo,
                                 header_name, S_ISDIR(statbuf->st_mode));
 # endif
 
-        /* Now write the header out to disk */
-        chksum_and_xwrite(tbInfo->tarFd, &header);
+        chksum_and_xwrite_tar_header(tbInfo->tarFd, &header);
 
         /* Now do the verbose thing (or not) */
         if (tbInfo->verboseFlag) {
diff --git a/include/bb_archive.h b/include/bb_archive.h
index 9b1db5b3e..dc5e55f0a 100644
--- a/include/bb_archive.h
+++ b/include/bb_archive.h
@@ -167,6 +167,7 @@ typedef struct tar_header_t {     /* byte offset */
 struct BUG_tar_header {
         char c[sizeof(tar_header_t) == TAR_BLOCK_SIZE ? 1 : -1];
 };
+void chksum_and_xwrite_tar_header(int fd, struct tar_header_t *hp) FAST_FUNC;
 
 
 extern const char cpio_TRAILER[];
diff --git a/procps/smemcap.c b/procps/smemcap.c
index 2f8ab192e..2f1897dae 100644
--- a/procps/smemcap.c
+++ b/procps/smemcap.c
@@ -29,7 +29,6 @@ struct fileblock {
 static void writeheader(const char *path, struct stat *sb, int type)
 {
         struct tar_header_t header;
-        int i, sum;
 
         memset(&header, 0, TAR_BLOCK_SIZE);
         strcpy(header.name, path);
@@ -40,20 +39,7 @@ static void writeheader(const char *path, struct stat *sb, int type)
         sprintf(header.size, "%o", (unsigned)sb->st_size);
         sprintf(header.mtime, "%llo", sb->st_mtime & 077777777777LL);
         header.typeflag = type;
-        strcpy(header.magic, "ustar  "); /* like GNU tar */
-
-        /* 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... */
-        header.chksum[7] = ' ';
-        sum = ' ' * 7;
-        for (i = 0; i < TAR_BLOCK_SIZE; i++)
-                sum += ((unsigned char*)&header)[i];
-        sprintf(header.chksum, "%06o", sum);
-
-        xwrite(STDOUT_FILENO, &header, TAR_BLOCK_SIZE);
+        chksum_and_xwrite_tar_header(STDOUT_FILENO, &header);
 }
 
 static void archivefile(const char *path)