Best way to convert whole file to lowercase in C

Question

I was wondering if theres a realy good (performant) solution how to Convert a whole file to lower Case in C. I use fgetc convert the char to lower case and write it in another t

Answer 1

This doesn't really answer the question (community wiki), but here's an (over?)-optimized function to convert text to lowercase:

#include 
#include 
#include 

int fast_lowercase(FILE *in, FILE *out)
{
    char buffer[65536];
    size_t readlen, wrotelen;
    char *p, *e;
    char conversion_table[256];
    int i;

    for (i = 0; i < 256; i++)
        conversion_table[i] = tolower(i);

    for (;;) {
        readlen = fread(buffer, 1, sizeof(buffer), in);
        if (readlen == 0) {
            if (ferror(in))
                return 1;
            assert(feof(in));
            return 0;
        }

        for (p = buffer, e = buffer + readlen; p < e; p++)
            *p = conversion_table[(unsigned char) *p];

        wrotelen = fwrite(buffer, 1, readlen, out);
        if (wrotelen != readlen)
            return 1;
    }
}

This isn't Unicode-aware, of course.

I benchmarked this on an Intel Core 2 T5500 (1.66GHz), using GCC 4.6.0 and i686 (32-bit) Linux. Some interesting observations:

• It's about 75% as fast when buffer is allocated with malloc rather than on the stack.
• It's about 65% as fast using a conditional rather than a conversion table.