Dec 9 -- For Christmas, Checks are Extended

Article:  https://ibr.cs.tu-bs.de/advent/09-xattr/
Workload: ~65 source-code lines

09-xattr/Makefile

+PROG = checksum
+
+${PROG}: ${PROG}.c
+	gcc $< -o  $@ -Wall -g
+
+run: ${PROG}
+	./checksum -r ${PROG}.c 
+	./checksum ${PROG}.c
+	./checksum ${PROG}.c
+
+
+
+strace: ${PROG}
+	strace ./checksum ${PROG}.c
+
+clean:
+	rm -f ./${PROG}

09-xattr/checksum.c

+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+#include <sys/xattr.h>
+#include <stdint.h>
+
+#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while(0)
+
+// An helper function that maps a file to memory and returns its
+// length in *len and an open file descriptor in *fd.
+// On error, the function should return the null pointer;
+char * map_file(char *fn, ssize_t *len, int *fd) {
+    // FIXME: Map file to memory
+    // FIXME: Set *len = ...
+    // FIXME: Set *fd = ...
+    return NULL;
+}
+
+// A (very) simple checksum function that calculates and additive checksum over a memory range.
+uint64_t calc_checksum(void *data, size_t len) {
+    uint64_t checksum = 0;
+
+    // First sum as many bytes as uint64_t as possible
+    uint64_t *ptr = (uint64_t*)data;
+    while ((void *)ptr < (data + len)) {
+        checksum += *ptr++;
+    }
+
+    // The rest (0-7 bytes) are added byte wise.
+    char *cptr = (char*)ptr;
+    while ((void*)cptr < (data+len)) {
+        checksum += *cptr;
+    }
+
+    return checksum;
+}
+
+int main(int argc, char *argv[]) {
+    // The name of the extended attribute where we store our checksum
+    const char *xattr = "user.checksum";
+
+    // Argument filename
+    char *fn;
+
+    // Should we reset the checksum?
+    bool reset_checksum;
+
+    // Argument parsing
+    if (argc == 3 && strcmp(argv[1], "-r") == 0) {
+        reset_checksum = true;
+        fn = argv[2];
+    } else if (argc == 2) {
+        reset_checksum = true;
+        fn = argv[1];
+    } else {
+        fprintf(stderr, "usage: %s [-r] <FILE>\n", argv[0]);
+    }
+    // Avoid compiler warnings
+    (void) fn; (void) reset_checksum; (void) xattr;
+    // FIXME: map the file
+    // FIXME: reset the checksum if requested
+    // FIXME: calculate the checksum
+    // FIXME: get the old checksum and perform checking
+    // FIXME: set the new checksum
+}