NN_CMSG(3)

💖 NN_CMSG(3) 💖

okay bestie, welcome to nanomsg (vibes version 1.2.2, if u were wondering) 💅 this page is all about nn_cmsg and the little functions that let u peek at the extra lil goodies glued onto ur messages. yes it's giving ✨access control information✨, no cap.

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🎀 NAME 🎀

nn_cmsg — access control information. literally the tools for reading the bonus data hanging off ur messages 🫶

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✨ SYNOPSIS ✨

#include <nanomsg/nn.h>

struct nn_cmsghdr *NN_CMSG_FIRSTHDR(struct nn_msghdr *hdr);

struct nn_cmsghdr *NN_CMSG_NXTHDR(struct nn_msghdr *hdr, struct
nn_cmsghdr *cmsg);

unsigned char *NN_CMSG_DATA(struct nn_cmsghdr *cmsg);

size_t NN_CMSG_SPACE(size_t len);

size_t NN_CMSG_LEN(size_t len);

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🦋 DESCRIPTION 🦋

so these functions? they're how u iterate over ancillary data attached to a message. think of it like the side quests of ur message — extra properties riding along for the trip 🌈

the struct, bestie 👑

The structure nn_cmsghdr represents a single ancillary property and it's got these members:

size_t cmsg_len;
int cmsg_level;
int cmsg_type;

• cmsg_len — the size of the property data, including the nn_cmsghdr structure that comes right before it. yes it counts itself, iconic 💅
• cmsg_level — the level of the property. same values as u'd use with nn_getsockopt(3) and nn_setsockopt(3), slay ✨
• cmsg_type — the name of the property. these names are specific to each level, so don't go mixin' 'em up 🙈

the functions, one by one 🌸

• NN_CMSG_FIRSTHDR — returns a pointer to the first nn_cmsghdr in the control buffer inside the nn_msghdr structure u handed over. ur entry point, ur main character moment 👑
• NN_CMSG_NXTHDR — returns the next nn_cmsghdr after the one u supplied. if there isn't enough space left in the buffer it returns NULL. that's ur stop signal, no drama 💀
• NN_CMSG_DATA — returns a pointer to the data associated with the nn_cmsghdr u gave it. the actual payload, the real one 🍓
• NN_CMSG_SPACE — returns the number of bytes occupied by nn_cmsghdr with a payload of the specified length. math girlies rise 🧚
• NN_CMSG_LEN — returns the value to store in the cmsg_len member of the cmsghdr structure, taking into account any necessary alignment. alignment matters, don't skip it fr fr ⭐

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🔥 EXAMPLE 🔥

iterating over ancillary properties, like a lil loop-de-loop:

struct nn_cmsghdr *hdr = NN_CMSG_FIRSTHDR (&msg);
while (hdr != NULL) {
    size_t len = hdr->cmsg_len - sizeof (nn_cmsghdr);
    printf ("level: %d property: %d length: %dB data: ",
        (int) hdr->cmsg_level,
        (int) hdr->cmsg_type,
        (int) len);
    unsigned char *data = NN_CMSG_DATA(hdr);
    while (len) {
        printf ("%02X", *data);
        ++data;
        --len;
    }
    printf ("\n");
    hdr = NN_CMSG_NXTHDR (&msg, hdr);
}

see how it just keeps going until hdr is NULL? that's the whole vibe check right there 💜

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🌈 SEE ALSO 🌈

nn_sendmsg(3) nn_recvmsg(3) nn_getsockopt(3) nn_setsockopt(3) nanomsg(7)

go read those too, they're all connected, iykyk 🦋

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💜 AUTHORS 💜

Martin Sustrik <sustrik@250bpm.com> — the og, we stan 🫶

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kay that's the whole page bestie!! go forth and iterate ur ancillary data with confidence 💖✨

nanomsg 1.2.2, dated 2026-01-16 — mwah, love u, bye 💅🎀