c-c++-printf

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C/C++ printf 系列

sprintf

  • #include <stdio.h>, #include <cstdio>
  • int sprintf ( char * str, const char * format, ... );
  • Write formatted data to string
  • A terminating null character is automatically appended after the content.

    參數

  • str
    • Pointer to a buffer where the resulting C-string is stored.
    • The buffer should be large enough to contain the resulting string.
  • format
    • C string that contains a format string that follows the same specifications as format in printf (see printf for details).
  • … (additional arguments)
    • Depending on the format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a format specifier in the format string (or a pointer to a storage location, for n).

      return

  • On success, the total number of characters written is returned.
    • This count does not include the additional null-character automatically appended at the end of the string.
  • On failure, a negative number is returned.

reference

snprintf

  • #include <stdio.h>, #include <cstdio>
  • int snprintf ( char * s, size_t n, const char * format, ... );
  • Write formatted output to sized buffer
  • If the resulting string would be longer than n-1 characters, the remaining characters are discarded and not stored, but counted for the value returned by the function.
  • A terminating null character is automatically appended after the content written.

參數

  • s
    • Pointer to a buffer where the resulting C-string is stored.
    • The buffer should have a size of at least n characters.
  • n
    • Maximum number of bytes to be used in the buffer.
    • The generated string has a length of at most n-1, leaving space for the additional terminating null character.
    • size_t is an unsigned integral type.
  • format
    • C string that contains a format string that follows the same specifications as format in printf (see printf for details).
  • … (additional arguments)
    • Depending on the format string, the function may expect a sequence of additional arguments, each containing a value to be used to replace a format specifier in the format string (or a pointer to a storage location, for n).

reference

asprintf

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char* cstr;
int c = asprintf( &cstr, "%d * %d = %d", tmp, tmp, tmp*tmp );
  • 不過 asprintf() 這個函式應該不在 C 語言的標準內，而是 GNU 的 extension（可能要加上「#define _GNU_SOURCE」才能使用）
  • 還是可以透過執行兩次 snprintf() 來做到同樣的功能～實作的方法
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    char* cstr;
    int c = snprintf( NULL, 0, "%d * %d = %d", tmp, tmp, tmp*tmp );
    cstr = new char[ c + 1 ];
    snprintf( cstr, c + 1, "%d * %d = %d", tmp, tmp, tmp*tmp );

snprintf vs sprintf

  • 在 C 語言裡，要建立一個字元陣列的字串，常常會使用 sprintf() 這個函數來做格式化的處理。
  • 但是實際上，這個函式卻不是那麼「安全」
  • 也就是在使用前，必須要先建立好一個字元陣列的空間，再用這個函式把內容填入
    • e.g.
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      int tmp = 10;
      char cstr[20];
      sprintf( cstr, "%d * %d = %d", tmp, tmp, tmp * tmp );
    • buffer overflow
  • 像如果是以 Visual C++ 2005 來編譯的話，如果程式裡有用到 sprintf()，他在編譯時就會顯示一個警告訊息： 
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    warning C4996: 'sprintf': This function or variable may be unsafe. Consider 
    using sprintf_s instead.
  • 微軟建議的 sprintf_s() 外，實際上在 C99 裡， 也多了一個 snprintf() 是用來取代現有的 sprintf()

    reference

  • 用 snprintf / asprintf 取代不安全的 sprintf – Heresy’s Space