genchecksum.c

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/*
 * Calculate the checksum and put it into the same file (the last
 * 4 bytes.
 */

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
 
static int fno = 0;
static struct stat statbuf;

int main(int argc, char *argv[])
{
	unsigned long val, sum = 0;
	off_t offset = 0;
	unsigned new_size = 0;

	if (argc != 2) {
		fprintf(stderr, "usage: %s file_name\n", argv[0]);
		return(-1);
	} else if (stat(argv[1], &statbuf) < 0) {
		fprintf(stderr, "error open %s\n", argv[1]);
		return(-1);
	} else if ((statbuf.st_size & 0xffff) != 0) {
		fprintf(stderr, "file %s is not 64KB aligned\n", argv[1]);
		return(-1);
	} else if (statbuf.st_size == 0) {
		fprintf(stderr, "file %s size is 0 byte\n", argv[1]);
		return(-1);
	} else if ((fno = open(argv[1], O_RDWR)) <= 0) {
		fprintf(stderr, "error open %s\n", argv[1]);
		return(-1);
	}

	while (read(fno, &val, sizeof(unsigned long)) 
		== sizeof(unsigned long)) {
		sum += val;
	}
	fprintf(stdout, "Sum is 0x%lx, ", sum);
	sum = ~sum + 1;
	fprintf(stdout, "checksum is 0x%lx, ", sum);

	if (val != 0) {
		offset = 0;
		fprintf(stdout, "Warning: appending checksum.\n");
	} else {
		offset = -1 * sizeof(unsigned long);
		fprintf(stdout, "inserting checksum.\n");
	}

	if (lseek(fno, offset, SEEK_END) < 0) {
		fprintf(stderr, "error seeking from the end of %s\n", argv[1]);
		close(fno);
		return(-1);
	}
	
	if (write(fno, &sum, sizeof(unsigned long)) != sizeof(unsigned long)) {
		fprintf(stderr, "error writing checksum to %s\n", argv[1]);
		close(fno);
		return(-1);
	}

	new_size = ((val == 0) ?  statbuf.st_size : 
			statbuf.st_size + sizeof(unsigned long));
	fprintf(stdout, "Final file size (%s) is %d.\n", argv[1], new_size);
        if ((new_size & 0xffff) != 0)
		fprintf(stdout, "WARNING: %s not aligned in 64KB boundary\n",
			argv[1]);

	close(fno);
}