可能是umask。

援引的手册页shm_open：

   O_CREAT    Create  the  shared memory object if it does not exist.  The user and
              group ownership of the object are taken from the corresponding effec‐
              tive IDs of the calling process, and the object's permission bits are
              set according to the low-order 9 bits of mode, except that those bits
              set in the process file mode creation mask (see umask(2)) are cleared
              for the new object.  A set of macro constants which can  be  used  to
              define  mode  is  listed  in open(2).  (Symbolic definitions of these
              constants can be obtained by including <sys/stat.h>.)

因此，为了允许创建可全局写入的文件，您需要设置允许它的umask，例如：

umask(0);

进行这样的设置，umask将不再影响已创建文件的任何权限。但是，您应该注意，如果您随后创建另一个文件而未明确指定权限，那么该文件也是可写的。

因此，您可能只想暂时清除umask，然后将其还原：

#include <sys/types.h>
#include <sys/stat.h>

...

void yourfunc()
{
    // store old
    mode_t old_umask = umask(0);

    int fd = shm_open(SHARE_MEM_NAME,O_RDWR | O_CREAT,0606);

    // restore old
    umask(old_umask);
}