c語言fopen追加模式下fseek失效

用到編程改變檔案某個欄位的值，結果發現此現象。上代碼：

int writepcap(char *filename,int begin,int len，char *content)
{
if(!pcap || !content)
return -1;
    FILE *fp = NULL;
    fp = fopen(filename,"ab");//rb+
    if(!fp)
return -2;
if(-1 == fseek(fp,begin,SEEK_SET))
  {
      fclose(fp);
return -3;
  }
//printf("curr:%ld\n",ftell(fp));
  if(fwrite(content, sizeof(unsigned char), len, fp) <= 0)
  {
       fclose(fp);
return -4;     
  }
  fclose(fp);
return 0;
}

 

結果發現ftell輸出的位置是正確的，但是寫的時候還是寫在了最後面。

在linux下，man fopen看到：

a+     Open for reading and appending (writing at end of file).  The file is created if it does not exist.  The initial file 

       position for  reading is at the beginning of the file, but output is always appended to the end of the file.

原來如此。

解決辦法：改為rb+即可。