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[overonion] / reverse_lib.c
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1#define _FILE_OFFSET_BITS 64
2
3#define BUFFER_SIZE 4096
4
b398ee5a 5#include <err.h>
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6#include <fcntl.h>
7#include <sys/mman.h>
8#include <sys/stat.h>
9#include <sys/types.h>
b398ee5a 10#include <sysexits.h>
3adfbe9d 11#include <unistd.h>
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12
13static 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);
3a85da3a 18 if (ret == -1) err(EX_IOERR, "Could not write");
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19 written += ret;
20 }
21}
22
20615086 23void reverse_file(const char* input_filename, int output_fd) {
b398ee5a 24 int fd = open(input_filename, O_RDONLY);
3a85da3a 25 if (fd == -1) err(EX_NOINPUT, "Could not open specified file");
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26
27 struct stat stats;
3a85da3a 28 if (fstat(fd, &stats) == -1) err(EX_NOINPUT, "Could not stat input");
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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);
3a85da3a 34 if (m == MAP_FAILED) err(EX_NOINPUT, "Could not mmap input");
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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) {
20615086 41 write_all(output_fd, buf, buf_offset);
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42 buf_offset = 0;
43 }
44 }
45 if (buf_offset) {
20615086 46 write_all(output_fd, buf, buf_offset);
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47 }
48
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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");
b398ee5a 51}