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[overonion] / reverse_lib.c
1 #define _FILE_OFFSET_BITS 64
2
3 #define BUFFER_SIZE 4096
4
5 #include <sys/types.h>
6 #include <sys/stat.h>
7 #include <sys/mman.h>
8 #include <fcntl.h>
9 #include <unistd.h>
10 #include <err.h>
11 #include <sysexits.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) {
19 err(EX_IOERR, "Could not write");
20 }
21 written += ret;
22 }
23 }
24
25 void reverse_file(const char* input_filename, int output_fd) {
26 int fd = open(input_filename, O_RDONLY);
27 if (fd == -1) {
28 err(EX_NOINPUT, "Could not open specified file");
29 }
30
31 struct stat stats;
32 if (fstat(fd, &stats) == -1) {
33 err(EX_NOINPUT, "Could not stat input");
34 }
35
36 long page_size = sysconf(_SC_PAGE_SIZE);
37 off_t pages = (stats.st_size - 1) / page_size + 1;
38 long map_size = pages * page_size;
39 char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
40 if (m == MAP_FAILED) {
41 err(EX_NOINPUT, "Could not mmap input");
42 }
43
44 char buf[BUFFER_SIZE];
45 off_t buf_offset = 0;
46 for (off_t p = stats.st_size - 1; p >= 0; p--) {
47 buf[buf_offset++] = m[p];
48 if (buf_offset >= BUFFER_SIZE) {
49 write_all(output_fd, buf, buf_offset);
50 buf_offset = 0;
51 }
52 }
53 if (buf_offset) {
54 write_all(output_fd, buf, buf_offset);
55 }
56
57 if (munmap(m, map_size) == -1) {
58 err(EX_IOERR, "Could not unmap input");
59 }
60 if (close(fd) == -1) {
61 err(EX_IOERR, "Could not close input");
62 }
63 }