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/*
* Extract a part from a file.
*
* Author: Peter Wu <lekensteyn@gmail.com>
* Date: 2012-11-01
*/
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
int main(int argc, char **argv) {
char *filename;
unsigned long seekpos, bytes_end;
if (argc < 3) {
fprintf(stderr, "Usage: %s file offset [last-exclusive-byte|+length]\n", *argv);
return 1;
}
filename = argv[1];
seekpos = strtoul(argv[2], NULL, 0);
if (argc >= 4) {
bytes_end = strtoul(argv[3], NULL, 0);
} else {
bytes_end = -1;
}
if (argc >= 4 && *argv[3] == '+') {
if (bytes_end + seekpos < bytes_end) {
fprintf(stderr, "Integer overflow on the last byte\n");
return 1;
}
bytes_end += seekpos;
}
if (bytes_end < seekpos) {
fprintf(stderr, "The last byte must be greater than the first byte.\n");
return 1;
}
FILE *fp;
if (filename[0] == '-' && filename[1] == 0)
fp = stdin;
else
fp = fopen(filename, "r");
if (!fp) {
perror(filename);
return 1;
}
unsigned long read_bytes = 0;
if (!fseek(fp, seekpos, SEEK_SET))
read_bytes += seekpos;
do {
char buf[8 * 1024];
unsigned int read_count = sizeof(buf);
unsigned long skip_bytes = 0;
if (bytes_end - read_bytes < read_count)
read_count = bytes_end - read_bytes;
/* if fseek failed, discard some bytes */
if (read_bytes < seekpos)
skip_bytes = seekpos - read_bytes;
unsigned long n = fread(buf, 1, read_count, fp);
if (!n)
break; /* EOF? */
if (n > skip_bytes)
write(STDOUT_FILENO, buf + skip_bytes, n);
read_bytes += n;
} while (read_bytes < bytes_end);
if (bytes_end != -1UL && read_bytes != bytes_end)
fprintf(stderr, "Output is truncated, missing %lu bytes.\n",
bytes_end - read_bytes);
fclose(fp);
return 0;
}
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