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cformat-specifiers

Why does ] not displayed in C?

This is my code snippet in C:

      char *str = NULL;
      int len = -1;

      // Get length
      len = snprintf(NULL, 0, "[%d]", 2);

      // Allocate str
      str = (char *)malloc(len + 1);

      // Assign str
      snprintf(str, len, "[%d]", 2);

      assert(len == 3);

      // Display str
      puts(str);

I expect this should display [2]. And len here is 3.

But running this code displays only [2

Why is this so?

Solution

  • The length of the buffer is len+1, but you only pass len to snprintf, try this:

    snprintf(str, len + 1, "[%d]", 2);

    from cplusplus.com:

    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.