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[overonion] / reverse_lib.c
1 #define _FILE_OFFSET_BITS 64
2
3 #define BUFFER_SIZE 4096
4
5 #include <err.h>
6 #include <fcntl.h>
7 #include <sys/mman.h>
8 #include <sys/stat.h>
9 #include <sys/types.h>
10 #include <sysexits.h>
11 #include <unistd.h>
12
13 static void write_all(int fd, const void *buf, size_t count) {
14 const char* cbuf = buf;
15 size_t written = 0;
16 while (written < count) {
17 int ret = write(fd, &cbuf[written], count - written);
18 if (ret == -1) err(EX_IOERR, "Could not write");
19 written += ret;
20 }
21 }
22
23 void reverse_file(const char* input_filename, int output_fd) {
24 int fd = open(input_filename, O_RDONLY);
25 if (fd == -1) err(EX_NOINPUT, "Could not open specified file");
26
27 struct stat stats;
28 if (fstat(fd, &stats) == -1) err(EX_NOINPUT, "Could not stat input");
29
30 long page_size = sysconf(_SC_PAGE_SIZE);
31 off_t pages = (stats.st_size - 1) / page_size + 1;
32 long map_size = pages * page_size;
33 char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
34 if (m == MAP_FAILED) err(EX_NOINPUT, "Could not mmap input");
35
36 char buf[BUFFER_SIZE];
37 off_t buf_offset = 0;
38 for (off_t p = stats.st_size - 1; p >= 0; p--) {
39 buf[buf_offset++] = m[p];
40 if (buf_offset >= BUFFER_SIZE) {
41 write_all(output_fd, buf, buf_offset);
42 buf_offset = 0;
43 }
44 }
45 if (buf_offset) {
46 write_all(output_fd, buf, buf_offset);
47 }
48
49 if (munmap(m, map_size) == -1) err(EX_IOERR, "Could not unmap input");
50 if (close(fd) == -1) err(EX_IOERR, "Could not close input");
51 }