Question

How would I create a C macro to get the integer value of a string? The specific use-case is following on from a question here. I want to change code like this:

enum insn {
    sysenter = (uint64_t)'r' << 56 | (uint64_t)'e' << 48 |
               (uint64_t)'t' << 40 | (uint64_t)'n' << 32 |
               (uint64_t)'e' << 24 | (uint64_t)'s' << 16 |
               (uint64_t)'y' << 8  | (uint64_t)'s',
    mov = (uint64_t)'v' << 16 | (uint64_t)'o' << 8 |
          (uint64_t)'m'
};

To this:

enum insn {
    sysenter = INSN_TO_ENUM("sysenter"),
    mov      = INSN_TO_ENUM("mov")
};

Where INSN_TO_ENUM expands to the same code. The performance would be the same, but the readability would be boosted by a lot.

I’m suspecting that in this form it might not be possible because of a the C preprocessor’s inability for string processing, so this would also be an unpreferred but acceptable solution (variable argument macro):

enum insn {
    sysenter = INSN_TO_ENUM('s','y','s','e','n','t','e','r'),
    mov      = INSN_TO_ENUM('m','o','v')
};

Answer 1

Here’s a compile-time, pure C solution, which you indicated as acceptable. You may need to extend it for longer mnemonics. I’ll keep on thinking about the desired one (i.e. INSN_TO_ENUM("sysenter")). Interesting question 🙂

#include <stdio.h>

#define head(h, t...) h
#define tail(h, t...) t

#define A(n, c...) (((long long) (head(c))) << (n)) | B(n + 8, tail(c))
#define B(n, c...) (((long long) (head(c))) << (n)) | C(n + 8, tail(c))
#define C(n, c...) (((long long) (head(c))) << (n)) | D(n + 8, tail(c))
#define D(n, c...) (((long long) (head(c))) << (n)) | E(n + 8, tail(c))
#define E(n, c...) (((long long) (head(c))) << (n)) | F(n + 8, tail(c))
#define F(n, c...) (((long long) (head(c))) << (n)) | G(n + 8, tail(c))
#define G(n, c...) (((long long) (head(c))) << (n)) | H(n + 8, tail(c))
#define H(n, c...) (((long long) (head(c))) << (n)) /* extend here */

#define INSN_TO_ENUM(c...) A(0, c, 0, 0, 0, 0, 0, 0, 0)

enum insn {
    sysenter = INSN_TO_ENUM('s','y','s','e','n','t','e','r'),
    mov      = INSN_TO_ENUM('m','o','v')
};

int main()
{
    printf("sysenter = %llx\nmov = %x\n", sysenter, mov);
    return 0;
}