Write a C program that reverses the bytes of a given arbitra

Write a C program that reverses the bytes of a given arbitrary length file in-place, as in, the contents of the file are swapped without a temporary file and without producing a second file. Use mmap(). E.g. ./reverse2 infile results in the contents of infile being reversed after the call, and calling ./reverse2 infile twice should result in the same file as you started with.

Solution

#include <stdio.h>

#include <sys/types.h>

#include <stdlib.h>

#include <sys/stat.h>

#include <string.h>

#include <unistd.h>

int main(int argc, char * argv[]) {

      

File *in, *out;          

int errflg = 0;         

size_t blocksize = 0;   

size_t bufsize = 0;     

char* instr;            

char* outstr;            

char **endptr;           

char *buf;               

  char c;                  

char tmpbuf[L_tmpnam+1];

off_t inlen, ipos;        

size_t nbytes;            

static struct stat fstatus;

/* Extract options */

instr = (char *)NULL;

outstr = (char *)NULL;

/* scan for the positional arguments                              */

for (; optind < argc; optind++) {

    if (!instr) {

      instr = argv[optind];

    } else {

      if (!outstr) {

        outstr = argv[optind];

      } else {

        errflg++;

      }

    }

}

/* Report option errors                                           */

if (errflg) {

    fprintf(stderr,\"reverse: Usage: \ \");

    fprintf(stderr,\" reverse [-b blocksize] [[-i] input] [[-o] output]\ \");

    fprintf(stderr,\"\ \");

    exit(2);

}

bufsize = blocksize;

if (blocksize < 512) bufsize = 512;

if (blocksize < 1) blocksize = 1;

if(! (buf=(char *)malloc(bufsize)) ) {

    fprintf(stderr,\"reverse: Insufficient memory to allocate buffer\ \ \");

    exit(1);

}

/* Now copy from the back of the input file to the start

     of the output file, in blocks of size blocksize,

     reversing each block as it is read                               */

for (ipos = 0; ipos < inlen; ipos += blocksize ) {

    off_t postn;

    size_t kbytes=blocksize;

    int ii;

    if (ipos+blocksize > inlen) {

      kbytes += inlen-blocksize-ipos;

    }

    fseek(in, inlen-kbytes-ipos, SEEK_SET);

    if (kbytes != (nbytes = fread(buf,1,kbytes,in))) {

      fprintf(stderr,

        \"reverse: Read failed at position %ld for %ld bytes, got %ld.\ \",

        (long)(inlen-kbytes-ipos), (long)kbytes, (long)nbytes);

    }

    for(ii = 0; ii < nbytes/2; ii++) {

      char t;

      t = buf[nbytes-1-ii];

      buf[nbytes-1-ii] = buf[ii];

      buf[ii] = t;

    }

    fwrite(buf,1,nbytes,out);

}

/* close the input file, but not the output if it is stdout

     but we will do an fflush. Note the the final exit would

     have done the same closes                                        */

fclose(in);

if (out != stdout) {

    fclose(out);     

} else {

    fflush(out);      /* usually not necessary, but we\'ll be cautious */

}

exit(0);

}