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NOTE: This documentation is for version 0.6-beta of nanomsg, but the latest released version is 1.1.2. Please see the documentation for 1.1.2 for the most up-to-date information.


#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);


These functions can be used to iterate over ancillary data attached to a message.

Structure nn_cmsghdr represents a single ancillary property and contains following members:

size_t cmsg_len;
int cmsg_level;
int cmsg_type;

cmsg_len is the size of the property data, including the preceding nn_cmsghdr structure. cmsg_level is the level of the property; same values can be used as with nn_getsockopt(3) and nn_setsockopt(3). cmsg_type is the name of the property. These names are specific for each level.

NN_CMSG_FIRSTHDR returns a pointer to the first nn_cmsghdr in the control buffer in the supplied nn_msghdr structure.

NN_CMSG_NXTHDR returns the next nn_cmsghdr after the supplied nn_cmsghdr. Returns NULL if there isn’t enough space in the buffer.

NN_CMSG_DATA returns a pointer to the data associated with supplied nn_cmsghdr.

NN_CMSG_SPACE returns the number of bytes occupied by nn_cmsghdr with payload of the specified length.

NN_CMSG_LEN returns the value to store in the cmsg_len member of the cmsghdr structure, taking into account any necessary alignment.


Iterating over ancillary properties:

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);
    printf ("\n");
    hdr = NN_CMSG_NXTHDR (&msg, hdr);



Martin Sustrik <>

"nanomsg" is a trademark of Garrett D'Amore.