bindresvport — bind a socket to a privileged IP port


#include <sys/types.h>
#include <netinet/in.h>
int bindresvport( int sockfd,
  struct sockaddr_in *sin);


bindresvport() is used to bind the socket referred to by the file descriptor sockfd to a privileged anonymous IP port, that is, a port number arbitrarily selected from the range 512 to 1023.

If the bind(2) performed by bindresvport() is successful, and sin is not NULL, then sin−>sin_port returns the port number actually allocated.

sin can be NULL, in which case sin−>sin_family is implicitly taken to be AF_INET. However, in this case, bindresvport() has no way to return the port number actually allocated. (This information can later be obtained using getsockname(2).)


bindresvport() returns 0 on success; otherwise −1 is returned and errno set to indicate the cause of the error.


bindresvport() can fail for any of the same reasons as bind(2). In addition, the following errors may occur:


The calling process was not privileged (on Linux: the calling process did not have the CAP_NET_BIND_SERVICE capability in the user namespace governing its network namespace).


All privileged ports are in use.

EAFNOSUPPORT (EPFNOSUPPORT in glibc 2.7 and earlier)

sin is not NULL and sin->sin_family is not AF_INET.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
bindresvport() Thread safety

glibc >= 2.17: MT-Safe .\" commit f6da27e53695ad1cc0e2a9490358decbbfdff5e5

glibc < 2.17: MT-Unsafe

The bindresvport() function uses a static variable that was not protected by a lock before glibc 2.17, rendering the function MT-Unsafe.


Not in POSIX.1. Present on the BSDs, Solaris, and many other systems.


Unlike some bindresvport() implementations, the glibc implementation ignores any value that the caller supplies in sin−>sin_port.


bind(2), getsockname(2)


This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright (C) 2007, Michael Kerrisk <>
and Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk

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2007-05-31, mtk: Rewrite and substantial additional text.
2008-12-03, mtk: Rewrote some pieces and fixed some errors