c - AF_UNIX socket overhead? -

i'm seeing couple strange things pair of af_unix sockets created call such as:

 socketpair(af_unix, sock_stream, 0, sfd);  

where sfd int[2] array file descriptors.

first, default buffer size seems 122k (124928 bytes), rather /proc/sys/net (such wmem_default set 128k). know cause of strange buffer size?

second, when writing small messages through socket (8 bytes). can write 423 of them before write blocks, 8*423 = 3384 bytes, odd size. messages acting though they're taking 295 + little bytes each. what's source of overhead?

running on rhel6 (2.6.32, 64-bit)

i wrote program try different sizes of data compare overhead costs:

#include <errno.h> #include <stdio.h> #include <stdint.h> #include <stdlib.h> #include <unistd.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h>  #define data_size 4  void run(size_t size) {     int sfd[2];     if (socketpair(af_unix, sock_stream, 0, sfd) == -1) {         perror("error");     }       int sndbuf, sbsize = sizeof(sndbuf);     getsockopt(sfd[0], sol_socket, so_sndbuf, &sndbuf, (socklen_t*)&sbsize);      printf("data size: %zd\n", size);     char buff[size];        size_t wrote=0;     (size_t ii=0; ii < 32768; ii++) {         if ((send(sfd[0], buff, size, msg_dontwait) == -1) && (errno == eagain)) {             wrote = ii;             break;         }     }      printf("wrote:     %zd\n", wrote);      if (wrote != 0) {          int bpm = sndbuf/wrote;         int oh  = bpm - size;          printf("bytes/msg: %i\n",  bpm);         printf("overhead:  %i\n",  oh);         printf("\n");     }      close(sfd[0]); close(sfd[1]); }  int main() {     int sfd[2];     socketpair(af_unix, sock_stream, 0, sfd);      int sndbuf, sbsize = sizeof(sndbuf);     getsockopt(sfd[0], sol_socket, so_sndbuf, &sndbuf, (socklen_t*)&sbsize);      printf("buffer size: %i\n\n", sndbuf);     close(sfd[0]); close(sfd[1]);      (size_t ii=4; ii <= 4096; ii *= 2) {         run(ii);     } } 

which gives:

buffer size: 124928  data size: 4 wrote:     423 bytes/msg: 295 overhead:  291  data size: 8 wrote:     423 bytes/msg: 295 overhead:  287  data size: 16 wrote:     423 bytes/msg: 295 overhead:  279  data size: 32 wrote:     423 bytes/msg: 295 overhead:  263  data size: 64 wrote:     423 bytes/msg: 295 overhead:  231  data size: 128 wrote:     348 bytes/msg: 358 overhead:  230  data size: 256 wrote:     256 bytes/msg: 488 overhead:  232  data size: 512 wrote:     168 bytes/msg: 743 overhead:  231  data size: 1024 wrote:     100 bytes/msg: 1249 overhead:  225  data size: 2048 wrote:     55 bytes/msg: 2271 overhead:  223  data size: 4096 wrote:     29 bytes/msg: 4307 overhead:  211 

versus using pipe there's lot of overhead:

data size: 4 wrote:     16384 bytes/msg: 4 overhead:  0  data size: 8 wrote:     8192 bytes/msg: 8 overhead:  0  data size: 16 wrote:     4096 bytes/msg: 16 overhead:  0  data size: 32 wrote:     2048 bytes/msg: 32 overhead:  0  data size: 64 wrote:     1024 bytes/msg: 64 overhead:  0  data size: 128 wrote:     512 bytes/msg: 128 overhead:  0  data size: 256 wrote:     256 bytes/msg: 256 overhead:  0  data size: 512 wrote:     128 bytes/msg: 512 overhead:  0  data size: 1024 wrote:     64 bytes/msg: 1024 overhead:  0  data size: 2048 wrote:     32 bytes/msg: 2048 overhead:  0  data size: 4096 wrote:     16 bytes/msg: 4096 overhead:  0 

take @ socket(7) man page. there section reads:

so_sndbuf sets or gets maximum socket send buffer in bytes. kernel doubles value (to allow space bookkeeping overhead) when set using setsockopt(2), , doubled value returned getsockopt(2). default value set /proc/sys/net/core/wmem_default file , maximum allowed value set /proc/sys/net/core/wmem_max file. minimum (doubled) value option 2048.

so appears overhead hold bookkeeping information kernel.