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Libbpf eBPF macro bpf_cpu_to_be64

v0.0.6

The bpf_cpu_to_be64 macro is used to convert a 64-bit number from host byte order to network byte order.

Definition

#define ___bpf_mvb(x, b, n, m) ((__u##b)(x) << (b-(n+1)*8) >> (b-8) << (m*8))

#define ___bpf_swab64(x) ((__u64)(          \
              ___bpf_mvb(x, 64, 0, 7) | \
              ___bpf_mvb(x, 64, 1, 6) | \
              ___bpf_mvb(x, 64, 2, 5) | \
              ___bpf_mvb(x, 64, 3, 4) | \
              ___bpf_mvb(x, 64, 4, 3) | \
              ___bpf_mvb(x, 64, 5, 2) | \
              ___bpf_mvb(x, 64, 6, 1) | \
              ___bpf_mvb(x, 64, 7, 0)))

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
# define __bpf_cpu_to_be64(x)       __builtin_bswap64(x)
# define __bpf_constant_cpu_to_be64(x)  ___bpf_swab64(x)
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
# define __bpf_cpu_to_be64(x)       (x)
# define __bpf_constant_cpu_to_be64(x)  (x)
#else
# error "Fix your compiler's __BYTE_ORDER__?!"
#endif

#define bpf_cpu_to_be64(x)          \
    (__builtin_constant_p(x) ?      \
     __bpf_constant_cpu_to_be64(x) : __bpf_cpu_to_be64(x))

Usage

Converts a 64-bit number (a long long) from host byte order to network byte order.

The implementation checks the endianness of the host system and if the number is a compile time constant or not. If the endianness of the system we are compiling on is already in network order, the macro simply returns the number as is. Otherwise if conversion is needed, and the number is a compile time constant, the conversion is done at compile time. If the number is not a compile time constant, a compiler builtin is used to emit byte swap instructions.

Example

Docs could be improved

This part of the docs is incomplete, contributions are very welcome