A C program reverses the content of a file

The C program below reverses the content of a file.
input file with content:123456789
output file with the content reversed:987654321
To compile the program: gcc reverse.c
To run the program: ./a.out inputfile.txt
[code language=”c”]
#include <stdio.h>
#include <stdlib.h>

void reverse(FILE *fPtr)
{
long loc;
char c1,c2;

/* find end of source file
* store the location to loc, and calculate the middle location*/
fseek(fPtr, 0L, SEEK_END);
loc = ftell(fPtr);
int mid = loc/2;
//printf("Loc=%d, Size=%d\n",loc,size);

int i=0;
loc = loc-1; /* back up past end-of-file mark */
while(i<mid) {
/*set the fPtr to appropriate location for c1 and c2,
*and read one byte from that location*/
fseek(fPtr, i, SEEK_SET);
fread(&c1,1,1,fPtr);
fseek(fPtr, loc, SEEK_SET);
fread(&c2,1,1,fPtr);

/*se the fPtr to appropriate location, swich c1 with c2*/
fseek(fPtr, loc, SEEK_SET);
fwrite(&c1,1,1, fPtr);
fseek(fPtr, i, SEEK_SET);
fwrite(&c2,1,1, fPtr);

–loc;/*from then end,decrement one position in each loop */
++i;/*from the beginning, incrememt one position in each loop*/
}//end of while

return;
}//end of reverse

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

/*if the number of arguments is not 2, give error message*/
if(argc != 2) {
printf("Usage: ./a.out <filename>.\n");
exit(1);
}

/*if the file cannot be opened, give error message*/
if((fPtr = fopen(argv[1], "r+")) == NULL) {
printf("Cannot open input file.\n");
exit(1);
}
reverse(fPtr);
fclose(fPtr);

return 0;
}
[/code]