standard c library for escaping a string

Question

Is there a standard C library function to escape C-strings?

For example, if I had the C string:

char example[] = \"first line\\nsecond line: \\\"inne         


        

Answer 1

I felt like my previous answer was cheating because a function that writes to a buffer is much more useful than one that simply writes to stdout, so here's an alternative solution which works out how much memory one needs if dst is NULL, and also stops at dstLen as per the requirement.

It's probably a little inefficient with all the if(dst) checking.

#include 
#include 
#include 

size_t str_escape(char *dst, const char *src, size_t dstLen)
{
    const char complexCharMap[] = "abtnvfr";

    size_t i;
    size_t srcLen = strlen(src);    
    size_t dstIdx = 0;

    // If caller wants to determine required length (supplying NULL for dst)
    // then we set dstLen to SIZE_MAX and pretend the buffer is the largest
    // possible, but we never write to it. Caller can also provide dstLen
    // as 0 if no limit is wanted.
    if (dst == NULL || dstLen == 0) dstLen = SIZE_MAX;

    for (i = 0; i < srcLen && dstIdx < dstLen; i++)
    {
        size_t complexIdx = 0;

        switch (src[i])
        {
            case '\'':
            case '\"':
            case '\\':
                if (dst && dstIdx <= dstLen - 2)
                {
                    dst[dstIdx++] = '\\';
                    dst[dstIdx++] = src[i];
                }
                else dstIdx += 2;
                break;

            case '\r': complexIdx++;
            case '\f': complexIdx++;
            case '\v': complexIdx++;
            case '\n': complexIdx++;
            case '\t': complexIdx++;
            case '\b': complexIdx++;
            case '\a':
                if (dst && dstIdx <= dstLen - 2)
                {
                    dst[dstIdx++] = '\\';
                    dst[dstIdx++] = complexCharMap[complexIdx];
                }
                else dstIdx += 2;
                break;

            default:
                if (isprint(src[i]))
                {
                    // simply copy the character
                    if (dst)
                        dst[dstIdx++] = src[i];
                    else
                        dstIdx++;
                }
                else
                {
                    // produce octal escape sequence
                    if (dst && dstIdx <= dstLen - 4)
                    {
                        dst[dstIdx++] = '\\';
                        dst[dstIdx++] = ((src[i] & 0300) >> 6) + '0';
                        dst[dstIdx++] = ((src[i] & 0070) >> 3) + '0';
                        dst[dstIdx++] = ((src[i] & 0007) >> 0) + '0';
                    }
                    else
                    {
                        dstIdx += 4;
                    }
                }
        }
    }

    if (dst && dstIdx <= dstLen)
        dst[dstIdx] = '\0';

    return dstIdx;
}