String switch in C 2012-08-09

C’s switch statement is very powerful. However, it can’t be used with strings, only with constant integral types. This is understandable, since strings in C are merely arrays – they’re not first-class citizens.

There are cases where such statement would be useful for strings. Here’s a trick I’m employing in lwan to avoid calling the strcmp family of functions in some hot paths. This exploits the notion that strings are just an array of bytes – and by casting them to a pointer to a 32-bit integer, and dereferencing this pointer, we’ll be able to perform a switch statement on very small strings (such as file extensions, which are usually comprised of four characters, including the dot).

C also supports multicharacters integral constants. However, because of endianess concerns, GCC warns by default when -Wall -Wextra is used with these constants. My solution was to just use a macro in conjuntion with an enum, to create the constant integral types expected by the compiler.

The code below, copied directly from lwan, illustrates an usage of this STRING_SWITCH statement:

#define STRING_SWITCH_L(s) switch (*((int32_t *)(s)) | 0x20202020)
#define MULTICHAR_CONSTANT(a,b,c,d) ((int32_t)((a) | (b) << 8 | (c)
<< 16 | (d) << 24))

enum {
    EXT_JPG = MULTICHAR_CONSTANT_L('.','j','p','g'),
    EXT_PNG = MULTICHAR_CONSTANT_L('.','p','n','g'),
    EXT_HTM = MULTICHAR_CONSTANT_L('.','h','t','m'),
    EXT_CSS = MULTICHAR_CONSTANT_L('.','c','s','s'),
    EXT_TXT = MULTICHAR_CONSTANT_L('.','t','x','t'),
    EXT_JS  = MULTICHAR_CONSTANT_L('.','j','s',0),
} lwan_mime_ext_t;

const char* lwan_determine_mime_type_for_file_name(char *file_name)
    char *last_dot = strrchr(file_name, '.');
    if (UNLIKELY(!last_dot))
        goto fallback;

    STRING_SWITCH_L(last_dot) {
    case EXT_CSS: return "text/css";
    case EXT_HTM: return "text/html";
    case EXT_JPG: return "image/jpeg";
    case EXT_JS:  return "application/javascript";
    case EXT_PNG: return "image/png";
    case EXT_TXT: return "text/plain";

    return "application/octet-stream";

Note that STRING_SWITCH_L performs a bitwise OR with the 32-bit integral value – this is a fast means of lowering the case of four characters at once.

This kind of switch statement is used in lwan to match HTTP headers and HTTP methods, and also the naïve file extension to MIME-Type conversion code shown above.

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