Declaring Pascal-style strings in C

Question

In C, is there a good way to define length first, Pascal-style strings as constants, so they can be placed in ROM? (I\'m working with a small embedded system with a non-GCC

Answer 1

This is why Variable Length Arrays were introduced in c99 (and to avoid the use of the "struct hack") IIRC, Pascal-strings were limited to a maximal length of 255.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h> // For CHAR_BIT

struct pstring {
        unsigned char len;
        char dat[];
        };

struct pstring *pstring_new(char *src, size_t len)
{
struct pstring *this;
if (!len) len = strlen(src);

    /* if the size does not fit in the ->len field: just truncate ... */
if (len >=(1u << (CHAR_BIT * sizeof this->len))) len = (1u << (CHAR_BIT * sizeof this->len))-1;

this = malloc(sizeof *this + len);
if (!this) return NULL;

this->len = len;
memcpy (this->dat, src, len);
return this;
}

int main(void)
{
struct pstring *pp;

pp = pstring_new("Hello, world!", 0);

printf("%p:[%u], %*.*s\n", (void*) pp
        , (unsigned int) pp->len
        , (unsigned int) pp->len
        , (unsigned int) pp->len
        , pp->dat
        );
return 0;
}

Answer 2

You can still use a const char * literal and an escape sequence as its first character that indicates the length:

const char *pascal_string = "\x03foo";

It will still be null-terminated, but that probably doesn't matter.

Answer 3

GCC and clang (and possibly others) accept the -fpascal-strings option which allows you to declare pascal-style string literals by having the first thing that appears in the string be a \p, e.g. "\pfoo". Not exactly portable, but certainly nicer than funky macros or the runtime construction of them.

See here for more info.

Answer 4

You can define an array in the way you like, but note that this syntax is not adequate:

const char *s = {3, 'f', 'o', 'o'};

You need an array instead of a pointer:

const char s[] = {3, 'f', 'o', 'o'};

Note that a char will only store numbers up to 255 (considering it's not signed) and this will be your maximum string length.

Don't expect this to work where other strings would, however. A C string is expected to terminate with a null character not only by the compiler, but by everything else.