[libc] Implement if_nametoindex and if_indextoname (#206082)

This patch implements if_nametoindex and if_indextoname for Linux. Both functions work by creating a temporary AF_UNIX socket and issuing ioctl calls (SIOCGIFINDEX and SIOCGIFNAME, respectively). The main implementation details: - for if_nametoindex, I check that the interface name fits within IF_NAMESIZE before issuing the ioctl - for if_indextoname, if the kernel returns ENODEV for an unknown index, I map it to ENXIO to comply with POSIX requirements - added the definition of struct ifreq to support these operations. Similar to the other net structure definitions, I'm using an anonymous union to avoid #defining members. Assisted by Gemini.
2026-07-07 13:26:10 +02:00 · 2026-07-07 13:26:10 +02:00 · 12c36c7abb
commit 12c36c7abb
parent b4c4fd9ee3
27 changed files with 585 additions and 0 deletions
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@ -1033,6 +1033,10 @@ if(LLVM_LIBC_FULL_BUILD)
    libc.src.arpa.inet.ntohl
    libc.src.arpa.inet.ntohs

+    # net/if.h entrypoints
+    libc.src.net.if_indextoname
+    libc.src.net.if_nametoindex
+
    # netinet/in.h entrypoints
    libc.src.netinet.in6addr_any
    libc.src.netinet.in6addr_loopback
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/libc/config/linux/riscv/entrypoints.txt
@ -1167,6 +1167,10 @@ if(LLVM_LIBC_FULL_BUILD)
    libc.src.arpa.inet.ntohl
    libc.src.arpa.inet.ntohs

+    # net/if.h entrypoints
+    libc.src.net.if_indextoname
+    libc.src.net.if_nametoindex
+
    # netinet/in.h entrypoints
    libc.src.netinet.in6addr_any
    libc.src.netinet.in6addr_loopback
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@ -42,6 +42,10 @@ set(TARGET_LIBC_ENTRYPOINTS
    libc.src.fcntl.open
    libc.src.fcntl.openat

+    # net/if.h entrypoints
+    libc.src.net.if_indextoname
+    libc.src.net.if_nametoindex
+
    # netinet/in.h entrypoints
    libc.src.netinet.in6addr_any
    libc.src.netinet.in6addr_loopback
--- a/libc/hdr/CMakeLists.txt
+++ b/libc/hdr/CMakeLists.txt
@ -86,6 +86,15 @@ add_proxy_header_library(
    libc.include.llvm-libc-macros.inet_address_macros
 )

+add_proxy_header_library(
+  net_if_macros
+  HDRS
+    net_if_macros.h
+  FULL_BUILD_DEPENDS
+    libc.include.net_if
+    libc.include.llvm-libc-macros.net_if_macros
+)
+
 add_proxy_header_library(
  netinet_in_macros
  HDRS
--- a/libc/hdr/net_if_macros.h
+++ b/libc/hdr/net_if_macros.h
@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Definition of macros from net/if.h.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_HDR_NET_IF_MACROS_H
+#define LLVM_LIBC_HDR_NET_IF_MACROS_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-macros/net-if-macros.h"
+
+#else // Overlay mode
+
+#include <net/if.h>
+
+#endif // LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_NET_IF_MACROS_H
--- a/libc/hdr/types/CMakeLists.txt
+++ b/libc/hdr/types/CMakeLists.txt
@ -538,6 +538,15 @@ add_proxy_header_library(
    libc.include.arpa_inet
 )

+add_proxy_header_library(
+  struct_ifreq
+  HDRS
+    struct_ifreq.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.struct_ifreq
+    libc.include.net_if
+)
+
 add_proxy_header_library(
  struct_sockaddr
  HDRS
--- a/libc/hdr/types/struct_ifreq.h
+++ b/libc/hdr/types/struct_ifreq.h
@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Proxy for struct ifreq.
+///
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_LIBC_HDR_TYPES_STRUCT_IFREQ_H
+#define LLVM_LIBC_HDR_TYPES_STRUCT_IFREQ_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/struct_ifreq.h"
+
+#else
+
+#include <net/if.h>
+
+#endif // LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_STRUCT_IFREQ_H
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@ -896,6 +896,18 @@ add_header_macro(
    .llvm-libc-types.siginfo_t
 )

+file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/net)
+
+add_header_macro(
+  net_if
+  ../libc/include/net/if.yaml
+  net/if.h
+  DEPENDS
+    .llvm_libc_common_h
+    .llvm-libc-macros.net_if_macros
+    .llvm-libc-types.struct_ifreq
+)
+
 # TODO: This should be conditional on POSIX networking being included.
 file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/netinet)

--- a/libc/include/llvm-libc-macros/CMakeLists.txt
+++ b/libc/include/llvm-libc-macros/CMakeLists.txt
@ -314,6 +314,12 @@ add_macro_header(
    sys-socket-macros.h
 )

+add_macro_header(
+  net_if_macros
+  HDR
+    net-if-macros.h
+)
+
 add_macro_header(
  sys_time_macros
  HDR
--- a/libc/include/llvm-libc-macros/linux/CMakeLists.txt
+++ b/libc/include/llvm-libc-macros/linux/CMakeLists.txt
@ -64,6 +64,12 @@ add_header(
    sys-socket-macros.h
 )

+add_header(
+  net_if_macros
+  HDR
+    net-if-macros.h
+)
+
 add_header(
  sys_wait_macros
  HDR
--- a/libc/include/llvm-libc-macros/linux/net-if-macros.h
+++ b/libc/include/llvm-libc-macros/linux/net-if-macros.h
@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Macros defined in net/if.h header file.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_MACROS_LINUX_NET_IF_MACROS_H
+#define LLVM_LIBC_MACROS_LINUX_NET_IF_MACROS_H
+
+#define IF_NAMESIZE 16
+
+#endif // LLVM_LIBC_MACROS_LINUX_NET_IF_MACROS_H
--- a/libc/include/llvm-libc-macros/net-if-macros.h
+++ b/libc/include/llvm-libc-macros/net-if-macros.h
@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Macros defined in net/if.h header file.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_MACROS_NET_IF_MACROS_H
+#define LLVM_LIBC_MACROS_NET_IF_MACROS_H
+
+#ifdef __linux__
+#include "linux/net-if-macros.h"
+#endif
+
+#endif // LLVM_LIBC_MACROS_NET_IF_MACROS_H
--- a/libc/include/llvm-libc-types/CMakeLists.txt
+++ b/libc/include/llvm-libc-types/CMakeLists.txt
@ -226,6 +226,7 @@ add_header(
 )
 add_header(struct_sockaddr_storage HDR struct_sockaddr_storage.h DEPENDS .sa_family_t)
 add_header(struct_sockaddr_un HDR struct_sockaddr_un.h DEPENDS .sa_family_t)
+add_header(struct_ifreq HDR struct_ifreq.h DEPENDS libc.include.llvm-libc-macros.net_if_macros .struct_sockaddr)
 add_header(struct_iovec HDR struct_iovec.h DEPENDS .size_t)
 add_header(struct_linger HDR struct_linger.h)
 add_header(struct_cmsghdr HDR struct_cmsghdr.h DEPENDS .size_t)
--- a/libc/include/llvm-libc-types/struct_ifreq.h
+++ b/libc/include/llvm-libc-types/struct_ifreq.h
@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Definition of struct ifreq.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES_STRUCT_IFREQ_H
+#define LLVM_LIBC_TYPES_STRUCT_IFREQ_H
+
+#include "../llvm-libc-macros/net-if-macros.h"
+#include "struct_sockaddr.h"
+
+struct ifreq {
+  char ifr_name[IF_NAMESIZE];
+  __extension__ union {
+    struct sockaddr ifr_hwaddr;
+    struct sockaddr ifr_addr;
+    struct sockaddr ifr_dstaddr;
+    struct sockaddr ifr_broadaddr;
+    struct sockaddr ifr_netmask;
+    short int ifr_flags;
+    int ifr_metric;
+    int ifr_mtu;
+    int ifr_ifindex;
+    int ifr_bandwidth;
+    int ifr_qlen;
+    char ifr_newname[IF_NAMESIZE];
+    char ifr_slave[IF_NAMESIZE];
+    char *ifr_data;
+  };
+};
+
+#endif // LLVM_LIBC_TYPES_STRUCT_IFREQ_H
--- a/libc/include/net/if.yaml
+++ b/libc/include/net/if.yaml
@ -0,0 +1,24 @@
+header: net/if.h
+standards:
+  - posix
+macros:
+  - macro_name: IF_NAMESIZE
+    macro_header: net-if-macros.h
+types:
+  - type_name: struct_ifreq
+enums: []
+objects: []
+functions:
+  - name: if_indextoname
+    standards:
+      - posix
+    return_type: char *
+    arguments:
+      - type: unsigned int
+      - type: char *
+  - name: if_nametoindex
+    standards:
+      - posix
+    return_type: unsigned int
+    arguments:
+      - type: const char *
--- a/libc/src/CMakeLists.txt
+++ b/libc/src/CMakeLists.txt
@ -10,6 +10,7 @@ add_subdirectory(inttypes)
 add_subdirectory(libgen)
 add_subdirectory(link)
 add_subdirectory(math)
+add_subdirectory(net)
 add_subdirectory(netinet)
 add_subdirectory(stdbit)
 add_subdirectory(stdfix)
--- a/libc/src/net/CMakeLists.txt
+++ b/libc/src/net/CMakeLists.txt
@ -0,0 +1,17 @@
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+endif()
+
+add_entrypoint_object(
+  if_indextoname
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.if_indextoname
+)
+
+add_entrypoint_object(
+  if_nametoindex
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.if_nametoindex
+)
--- a/libc/src/net/if_indextoname.h
+++ b/libc/src/net/if_indextoname.h
@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Implementation header for if_indextoname.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_NET_IF_INDEXTONAME_H
+#define LLVM_LIBC_SRC_NET_IF_INDEXTONAME_H
+
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+char *if_indextoname(unsigned int ifindex, char *ifname);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_NET_IF_INDEXTONAME_H
--- a/libc/src/net/if_nametoindex.h
+++ b/libc/src/net/if_nametoindex.h
@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Implementation header for if_nametoindex.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_NET_IF_NAMETOINDEX_H
+#define LLVM_LIBC_SRC_NET_IF_NAMETOINDEX_H
+
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+unsigned int if_nametoindex(const char *ifname);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_NET_IF_NAMETOINDEX_H
--- a/libc/src/net/linux/CMakeLists.txt
+++ b/libc/src/net/linux/CMakeLists.txt
@ -0,0 +1,44 @@
+add_entrypoint_object(
+  if_indextoname
+  SRCS
+    if_indextoname.cpp
+  HDRS
+    ../if_indextoname.h
+  DEPENDS
+    libc.hdr.net_if_macros
+    libc.hdr.errno_macros
+    libc.hdr.sys_ioctl_macros
+    libc.hdr.sys_socket_macros
+    libc.hdr.types.struct_ifreq
+    libc.src.__support.CPP.string_view
+    libc.src.__support.common
+    libc.src.__support.macros.null_check
+    libc.src.__support.OSUtil.linux.syscall_wrappers.socket
+    libc.src.__support.OSUtil.linux.syscall_wrappers.ioctl
+    libc.src.__support.OSUtil.linux.syscall_wrappers.close
+    libc.src.errno.errno
+    libc.src.string.memory_utils.inline_memcpy
+    libc.src.string.string_utils
+)
+
+add_entrypoint_object(
+  if_nametoindex
+  SRCS
+    if_nametoindex.cpp
+  HDRS
+    ../if_nametoindex.h
+  DEPENDS
+    libc.hdr.net_if_macros
+    libc.hdr.errno_macros
+    libc.hdr.sys_ioctl_macros
+    libc.hdr.sys_socket_macros
+    libc.hdr.types.struct_ifreq
+    libc.src.__support.CPP.string_view
+    libc.src.__support.common
+    libc.src.__support.macros.null_check
+    libc.src.__support.OSUtil.linux.syscall_wrappers.socket
+    libc.src.__support.OSUtil.linux.syscall_wrappers.ioctl
+    libc.src.__support.OSUtil.linux.syscall_wrappers.close
+    libc.src.errno.errno
+    libc.src.string.memory_utils.inline_memcpy
+)
--- a/libc/src/net/linux/if_indextoname.cpp
+++ b/libc/src/net/linux/if_indextoname.cpp
@ -0,0 +1,71 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Linux implementation of if_indextoname.
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/net/if_indextoname.h"
+#include "hdr/errno_macros.h"
+#include "hdr/net_if_macros.h"
+#include "hdr/sys_ioctl_macros.h"
+#include "hdr/sys_socket_macros.h"
+#include "hdr/types/struct_ifreq.h"
+#include "src/__support/CPP/string_view.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/close.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/ioctl.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/socket.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/null_check.h"
+#include "src/string/memory_utils/inline_memcpy.h"
+#include "src/string/string_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(char *, if_indextoname,
+                   (unsigned int ifindex, char *ifname)) {
+  LIBC_CRASH_ON_NULLPTR(ifname);
+
+  ErrorOr<int> fd =
+      linux_syscalls::socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+  if (!fd.has_value()) {
+    libc_errno = fd.error();
+    return nullptr;
+  }
+
+  struct ifreq ifr;
+  ifr.ifr_ifindex = static_cast<int>(ifindex);
+
+  ErrorOr<int> ioctl_res = linux_syscalls::ioctl(*fd, SIOCGIFNAME, &ifr);
+
+  if (ErrorOr<int> res = linux_syscalls::close(*fd); !res.has_value()) {
+    libc_errno = res.error();
+    return nullptr;
+  }
+
+  if (!ioctl_res.has_value()) {
+    // Map kernel ENODEV to POSIX-mandated ENXIO.
+    libc_errno = (ioctl_res.error() == ENODEV) ? ENXIO : ioctl_res.error();
+    return nullptr;
+  }
+
+  cpp::string_view k_name(
+      ifr.ifr_name, internal::strnlen(ifr.ifr_name, sizeof(ifr.ifr_name)));
+  inline_memcpy(ifname, k_name.data(), k_name.size());
+  // Linux kernel does allow network devices exactly IF_NAMESIZE long (it leaves
+  // room for the terminating \0). But if that happens, let's not overrun the
+  // user provided buffer.
+  if (k_name.size() < IF_NAMESIZE)
+    ifname[k_name.size()] = '\0';
+
+  return ifname;
+}
+
+} // namespace LIBC_NAMESPACE_DECL
--- a/libc/src/net/linux/if_nametoindex.cpp
+++ b/libc/src/net/linux/if_nametoindex.cpp
@ -0,0 +1,65 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Linux implementation of if_nametoindex.
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/net/if_nametoindex.h"
+#include "hdr/errno_macros.h"
+#include "hdr/net_if_macros.h"
+#include "hdr/sys_ioctl_macros.h"
+#include "hdr/sys_socket_macros.h"
+#include "hdr/types/struct_ifreq.h"
+#include "src/__support/CPP/string_view.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/close.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/ioctl.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/socket.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/null_check.h"
+#include "src/string/memory_utils/inline_memcpy.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(unsigned int, if_nametoindex, (const char *ifname)) {
+  LIBC_CRASH_ON_NULLPTR(ifname);
+
+  cpp::string_view name(ifname);
+  if (name.empty() || name.size() >= IF_NAMESIZE) {
+    libc_errno = ENODEV;
+    return 0;
+  }
+
+  ErrorOr<int> fd =
+      linux_syscalls::socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+  if (!fd.has_value()) {
+    libc_errno = fd.error();
+    return 0;
+  }
+
+  struct ifreq ifr;
+  inline_memcpy(ifr.ifr_name, name.data(), name.size() + 1);
+
+  ErrorOr<int> ioctl_res = linux_syscalls::ioctl(*fd, SIOCGIFINDEX, &ifr);
+
+  if (ErrorOr<int> res = linux_syscalls::close(*fd); !res.has_value()) {
+    libc_errno = res.error();
+    return 0;
+  }
+
+  if (!ioctl_res.has_value()) {
+    libc_errno = ioctl_res.error();
+    return 0;
+  }
+
+  return static_cast<unsigned int>(ifr.ifr_ifindex);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
--- a/libc/test/src/CMakeLists.txt
+++ b/libc/test/src/CMakeLists.txt
@ -66,6 +66,7 @@ add_subdirectory(fenv)
 add_subdirectory(libgen)
 add_subdirectory(link)
 add_subdirectory(math)
+add_subdirectory(net)
 add_subdirectory(netinet)
 add_subdirectory(search)
 add_subdirectory(setjmp)
--- a/libc/test/src/net/CMakeLists.txt
+++ b/libc/test/src/net/CMakeLists.txt
@ -0,0 +1,4 @@
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  add_subdirectory(${LIBC_TARGET_OS})
+endif()
+
--- a/libc/test/src/net/linux/CMakeLists.txt
+++ b/libc/test/src/net/linux/CMakeLists.txt
@ -0,0 +1,27 @@
+add_custom_target(libc_net_unittests)
+
+add_libc_unittest(
+  if_indextoname_test
+  SUITE
+    libc_net_unittests
+  SRCS
+    if_indextoname_test.cpp
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.hdr.net_if_macros
+    libc.src.net.if_indextoname
+    libc.src.net.if_nametoindex
+    libc.test.UnitTest.ErrnoCheckingTest
+)
+
+add_libc_unittest(
+  if_nametoindex_test
+  SUITE
+    libc_net_unittests
+  SRCS
+    if_nametoindex_test.cpp
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.src.net.if_nametoindex
+    libc.test.UnitTest.ErrnoCheckingTest
+)
--- a/libc/test/src/net/linux/if_indextoname_test.cpp
+++ b/libc/test/src/net/linux/if_indextoname_test.cpp
@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Unittests for if_indextoname.
+///
+//===----------------------------------------------------------------------===//
+
+#include "hdr/errno_macros.h"
+#include "hdr/net_if_macros.h"
+#include "src/net/if_indextoname.h"
+#include "src/net/if_nametoindex.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
+#include "test/UnitTest/Test.h"
+
+using LlvmLibcIfIndexToNameTest = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+
+TEST_F(LlvmLibcIfIndexToNameTest, Loopback) {
+  unsigned int lo_idx = LIBC_NAMESPACE::if_nametoindex("lo");
+  ASSERT_GT(lo_idx, 0u);
+
+  char buf[IF_NAMESIZE];
+  char *res = LIBC_NAMESPACE::if_indextoname(lo_idx, buf);
+  ASSERT_EQ(res, buf);
+  ASSERT_STREQ(buf, "lo");
+  ASSERT_ERRNO_SUCCESS();
+}
+
+TEST_F(LlvmLibcIfIndexToNameTest, InvalidIndex) {
+  char buf[IF_NAMESIZE];
+  char *res = LIBC_NAMESPACE::if_indextoname(-1u, buf);
+  ASSERT_EQ(res, static_cast<char *>(nullptr));
+  ASSERT_ERRNO_EQ(ENXIO);
+}
+
+TEST_F(LlvmLibcIfIndexToNameTest, ZeroIndex) {
+  char buf[IF_NAMESIZE];
+  char *res = LIBC_NAMESPACE::if_indextoname(0u, buf);
+  ASSERT_EQ(res, static_cast<char *>(nullptr));
+  ASSERT_ERRNO_EQ(ENXIO);
+}
+
+TEST_F(LlvmLibcIfIndexToNameTest, NullPtrDeath) {
+  ASSERT_DEATH([]() { LIBC_NAMESPACE::if_indextoname(1u, nullptr); },
+               WITH_SIGNAL(-1));
+}
--- a/libc/test/src/net/linux/if_nametoindex_test.cpp
+++ b/libc/test/src/net/linux/if_nametoindex_test.cpp
@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Unittests for if_nametoindex.
+///
+//===----------------------------------------------------------------------===//
+
+#include "hdr/errno_macros.h"
+#include "src/net/if_nametoindex.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
+#include "test/UnitTest/Test.h"
+
+using LlvmLibcIfNameToIndexTest = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+
+TEST_F(LlvmLibcIfNameToIndexTest, Loopback) {
+  unsigned int idx = LIBC_NAMESPACE::if_nametoindex("lo");
+  ASSERT_GT(idx, 0u);
+  ASSERT_ERRNO_SUCCESS();
+}
+
+TEST_F(LlvmLibcIfNameToIndexTest, InvalidName) {
+  unsigned int idx = LIBC_NAMESPACE::if_nametoindex("nonexistent_if");
+  ASSERT_EQ(idx, 0u);
+  ASSERT_ERRNO_EQ(ENODEV);
+}
+
+TEST_F(LlvmLibcIfNameToIndexTest, EmptyName) {
+  unsigned int idx = LIBC_NAMESPACE::if_nametoindex("");
+  ASSERT_EQ(idx, 0u);
+  ASSERT_ERRNO_EQ(ENODEV);
+}
+
+TEST_F(LlvmLibcIfNameToIndexTest, NullPtrDeath) {
+  ASSERT_DEATH([]() { LIBC_NAMESPACE::if_nametoindex(nullptr); },
+               WITH_SIGNAL(-1));
+}