文字列置換

#include <stdio.h>

/*
*  使用例
*  chtxt before.txt after.txt "<" &lt;
*/

/* ヌル文字を除くヌル文字までのバイト数を返す */
int length(char *s){
  int count = 0;

  for(;;){
    if(s[count] == '\0'){
      return count;
    }
    count++;
  }
}

/* FILE *fp の現在の位置からの文字列が s と等しければ 1 を返す 違えば 0 */
/*    注：文字列が等しい場合探索後の位置を戻さない 違う場合位置を戻す   */
int filecmp(FILE *fp, char *s){
  int c, i, l;
  fpos_t pos;

  fgetpos(fp, &pos);

  l = length(s);

  for(i = 0; i < l; i++){
    c = fgetc(fp);
    if(c != s[i]){
      fsetpos(fp, &pos);
      return 0;
    }
  }

  return 1;
}

/* FILE *bfp に含まれる文字 before_word を after_word に変えて FILE *afp へ*/
int chenge_text(FILE *bfp, FILE *afp, char *before_word, char *after_word){
  int c, i, l;

  l = length(after_word);

  for(;;){
    if((filecmp(bfp, before_word)) == 1){
      for(i = 0; i < l; i++){
        c = after_word[i];
        fputc(c, afp);
      }
      continue;
    }
    else{
      if((c = fgetc(bfp)) == EOF){
        break;
      }
      fputc(c, afp);
    }
  }

  return 1;
}

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

  if(argc != 5){
    printf("usage:chtxt before_file after_file before_word after_word\n");
    return -1;
  }

  if((ifp = fopen(argv[1], "r")) == NULL){
    printf("before_file is not found.\n");
    return -1;
  }

  if((ofp = fopen(argv[2], "w")) == NULL){
    printf("after_file is not found.\n");
    return -1;
  }

  chenge_text(ifp, ofp, argv[3], argv[4]);

  fclose(ifp);
  fclose(ofp);

  return 0;
}