Question

I’m creating a function in C to convert an index value into a string, which is a verbose description of the “field” represented by the index.

So, I have a nice array with all the verbose descriptions indexed by, well the index.

To dump it into a buffer I use code like this

#define BUFFER_SIZE 40
void format_verbose(uint32_t my_index,
                    char     my_buffer[BUFFER_SIZE])
  {
    snprintf(mY_buffer, BUFFER_SIZE, "%s", MY_ARRAY[my_index].description);
  }

The problem comes for some cases I need to insert some other strings into the string when formatting it. So what I want is something like this (where the description in this case contains a %s).

void format_verbose_with_data(uint32_t my_index,
                              char     my_buffer[BUFFER_SIZE])
  {
    // ...
    snprintf(mY_buffer, BUFFER_SIZE, MY_ARRAY[my_index].description,
             some_string);
  }

Our make file is set up to make this (dangerous) use of snprintf() warn, and warnings are treated as errors. So, it won’t compile. I would like to turn off the warning for just this line, where although it is somewhat dangerous, I will control the string, and I can test to ensure it works with every value it’s called with.

Alternatively, I would be happy to do this some other way, but I’m really not keen to use this solution

void format_verbose_with_data(uint32_t my_index,
                              char     my_buffer[BUFFER_SIZE])
  {
    // ...
    snprintf(mY_buffer, BUFFER_SIZE, "%s%s%s"
    MY_ARRAY[my_index].description1, some_string,
    MY_ARRAY[my_index].description2);
  }

Because it makes my description array ugly, especially for the ones where I don’t need to add extra values.

Answer 1

After a night of thought, I plan on manually dividing the input string up, by looking for the %s marker myself, and then sending the strings separetley into snprintf() with their own %s. For this case, where only 1 type of format string is allowed, this less onerous, however it would suck to try to completely re-implement a printf() style parser.

void format_verbose_with_data(uint32_t my_index,
                              char     my_buffer[BUFFER_SIZE])
  {
    char    pre_description[BUFFER_SIZE];
    char    post_description[BUFFER_SIZE];
    int32_t offset = -1;

    offset = find_string(MY_ARRAY[my_index].description, "%s");
    ASSERT(offset >=0, "No split location!");
    // Use offset to copy the pre and post descriptions
    // Exercise left to the reader :-)
    snprintf(mY_buffer, BUFFER_SIZE, "%s%s%s"
             pre_description, some_string, post_description);
  }