BINDRESVPORT(3) Linux Programmer's Manual BINDRESVPORT(3)

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 . 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)