insert a string into file at exact position

#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>

#define INI_FILE "test.ini"

void usage(char *s) {
  printf("Usage: %s <line> <pos> <str>\n", basename(s));
  exit(0);
}

int main (int argc, char* argv[])
{
  FILE *fp_in, *fp_out;
  int tline, tcnum;
  unsigned line =1;
  unsigned cnum =0;
  char infile[256];
  char insert[81];
  int ch;

  if (argc < 4)
	usage(argv[0]);

  tline = atoi(argv[1]);
  tcnum = atoi(argv[2]);

  strncpy(insert, argv[3], 80);
  strcpy(infile, INI_FILE);
  strcat(infile, ".bak");
  rename(INI_FILE, infile);

  if ((fp_in = fopen(infile, "r")) == NULL) {
	fprintf(stderr, "Unable to open file '%s'\n", infile);
	return EXIT_FAILURE;
  }

  if ((fp_out = fopen(INI_FILE, "w")) == NULL) {
	fprintf(stderr, "Unable to open file '%s'\n", INI_FILE);
	return EXIT_FAILURE;
  }

  while ((ch = fgetc(fp_in)) != EOF) {
	if (ch == '\n') {
	  cnum = 0;
	  line++;
	}
	else {
	  ++cnum;
	  if (line == tline && cnum == (tcnum+1)) {
		fprintf(fp_out, insert);
	  }
	}

	fputc(ch, fp_out);
  }

  fclose(fp_out);
  fclose(fp_in);

  return EXIT_SUCCESS;
}