FreeBSD Manual Pages
RTLD(1) BSD General Commands Manual RTLD(1) NAME ld-elf.so.1, ld.so, rtld -- run-time link-editor DESCRIPTION The ld-elf.so.1 utility is a self-contained shared object providing run- time support for loading and link-editing shared objects into a process' address space. It is also commonly known as the dynamic linker. It uses the data structures contained within dynamically linked programs to de- termine which shared libraries are needed and loads them using the mmap(2) system call. After all shared libraries have been successfully loaded, ld-elf.so.1 proceeds to resolve external references from both the main program and all objects loaded. A mechanism is provided for initialization routines to be called on a per-object basis, giving a shared object an opportunity to perform any extra set-up before execution of the program proper be- gins. This is useful for C++ libraries that contain static constructors. When resolving dependencies for the loaded objects, ld-elf.so.1 trans- lates dynamic token strings in rpath and soname. If the -z origin option of the static linker was set when linking the binary, the token expansion is performed at the object load time, see ld(1). The following strings are recognized now: $ORIGIN Translated to the full path of the loaded object. $OSNAME Translated to the name of the operating system implementation. $OSREL Translated to the release level of the operating system. $PLATFORM Translated to the machine hardware platform. The ld-elf.so.1 utility itself is loaded by the kernel together with any dynamically-linked program that is to be executed. The kernel transfers control to the dynamic linker. After the dynamic linker has finished loading, relocating, and initializing the program and its required shared objects, it transfers control to the entry point of the program. The following search order is used to locate required shared objects: 1. DT_RPATH of the referencing object unless that object also contains a DT_RUNPATH tag 2. DT_RPATH of the program unless the referencing object contains a DT_RUNPATH tag 3. Path indicated by LD_LIBRARY_PATH environment variable 4. DT_RUNPATH of the referencing object 5. Hints file produced by the ldconfig(8) utility 6. The /lib and /usr/lib directories, unless the referencing ob- ject was linked using the "-z nodefaultlib" option The ld-elf.so.1 utility recognizes a number of environment variables that can be used to modify its behaviour. On 64-bit architectures, the linker for 32-bit objects recognizes all the environment variables listed below, but is being prefixed with LD_32_, for example: LD_32_TRACE_LOADED_OBJECTS. LD_DUMP_REL_POST If set, ld-elf.so.1 will print a table containing all relocations after symbol binding and relocation. LD_DUMP_REL_PRE If set, ld-elf.so.1 will print a table containing all relocations before symbol binding and relocation. LD_LIBMAP A library replacement list in the same format as libmap.conf(5). For convenience, the characters `=' and `,' can be used instead of a space and a newline. This variable is parsed after libmap.conf(5), and will override its entries. This variable is unset for set- user-ID and set-group-ID programs. LD_LIBMAP_DISABLE If set, disables the use of libmap.conf(5) and LD_LIBMAP. This variable is unset for set-user-ID and set-group-ID programs. LD_ELF_HINTS_PATH This variable will override the default location of "hints" file. This variable is unset for set-user-ID and set-group-ID programs. LD_LIBRARY_PATH A colon separated list of directories, overriding the default search path for shared libraries. This vari- able is unset for set-user-ID and set-group-ID pro- grams. LD_LIBRARY_PATH_RPATH If the variable is specified and has a value starting with any of 'y', 'Y' or '1' symbols, the path speci- fied by LD_LIBRARY_PATH variable is allowed to over- ride the path from DT_RPATH for binaries which does not contain DT_RUNPATH tag. For such binaries, when the variable LD_LIBRARY_PATH_RPATH is set, "-z nodefaultlib" link-time option is ignored as well. LD_PRELOAD A list of shared libraries, separated by colons and/or white space, to be linked in before any other shared libraries. If the directory is not specified then the directories specified by LD_LIBRARY_PATH will be searched first followed by the set of built-in stan- dard directories. This variable is unset for set- user-ID and set-group-ID programs. LD_LIBRARY_PATH_FDS A colon separated list of file de- scriptor numbers for library directories. This is in- tended for use within capsicum(4) sandboxes, when global namespaces such as the filesystem are unavail- able. It is consulted just after LD_LIBRARY_PATH. This variable is unset for set-user-ID and set-group- ID programs. LD_BIND_NOT When set to a nonempty string, prevents modifications of the PLT slots when doing bindings. As result, each call of the PLT-resolved function is resolved. In combination with debug output, this provides complete account of all bind actions at runtime. This variable is unset for set-user-ID and set-group-ID programs. LD_BIND_NOW When set to a nonempty string, causes ld-elf.so.1 to relocate all external function calls before starting execution of the program. Normally, function calls are bound lazily, at the first call of each function. LD_BIND_NOW increases the start-up time of a program, but it avoids run-time surprises caused by unexpect- edly undefined functions. LD_TRACE_LOADED_OBJECTS When set to a nonempty string, causes ld-elf.so.1 to exit after loading the shared objects and printing a summary which includes the absolute pathnames of all objects, to standard output. LD_TRACE_LOADED_OBJECTS_ALL When set to a nonempty string, causes ld-elf.so.1 to expand the summary to indicate which objects caused each object to be loaded. LD_TRACE_LOADED_OBJECTS_FMT1 LD_TRACE_LOADED_OBJECTS_FMT2 When set, these variables are interpreted as format strings a la printf(3) to customize the trace output and are used by ldd(1)'s -f option and allows ldd(1) to be operated as a filter more conveniently. If the dependency name starts with string lib, LD_TRACE_LOADED_OBJECTS_FMT1 is used, otherwise LD_TRACE_LOADED_OBJECTS_FMT2 is used. The following conversions can be used: %a The main program's name (also known as "__progname"). %A The value of the environment variable LD_TRACE_LOADED_OBJECTS_PROGNAME. Typically used to print both the names of programs and shared libraries being inspected using ldd(1). %o The library name. %p The full pathname as determined by rtld's li- brary search rules. %x The library's load address. Additionally, `\n' and `\t' are recognized and have their usual meaning. LD_UTRACE If set, ld-elf.so.1 will log events such as the load- ing and unloading of shared objects via utrace(2). LD_LOADFLTR If set, ld-elf.so.1 will process the filtee dependen- cies of the loaded objects immediately, instead of postponing it until required. Normally, the filtees are opened at the time of the first symbol resolution from the filter object. DIRECT EXECUTION MODE ld-elf.so.1 is typically used implicitly, loaded by the kernel as re- quested by the PT_INTERP program header of the executed binary. FreeBSD also supports a direct execution mode for the dynamic linker. In this mode, the user explicitly executes ld-elf.so.1 and provides the path of the program to be linked and executed as an argument. This mode allows use of a non-standard dynamic linker for a program activation without changing the binary or without changing the installed dynamic linker. Execution options may be specified. The syntax of the direct invocation is /libexec/ld-elf.so.1 [-f fd] [-p] [--] image_path [image arguments] The options are as follows: -f fd File descriptor fd references the binary to be activated by ld-elf.so.1. It must already be opened in the process when exe- cuting ld-elf.so.1. If this option is specified, image_path is only used to provide the argv[0] value to the program. -p If the image_path argument specifies a name which does not con- tain a slash "/" character, ld-elf.so.1 uses the search path pro- vided by the environment variable PATH to find the binary to exe- cute. -- Ends the ld-elf.so.1 options. The argument following -- is in- terpreted as the path of binary to execute. To conform to user expectation to not break some naively restricted exe- cution environments, in the direct execution mode ld-elf.so.1 emulates verification of the binary execute permission for current user. The ver- ification only uses Unix DACs, ignores ACLs and is racy by its nature. The environments which rely on such restrictions are weak and breakable on its own. FILES /var/run/ld-elf.so.hints Hints file. /var/run/ld-elf32.so.hints Hints file for 32-bit binaries on 64-bit sys- tem. /etc/libmap.conf The libmap configuration file. /etc/libmap32.conf The libmap configuration file for 32-bit bi- naries on 64-bit system. SEE ALSO ld(1), ldd(1), capsicum(4), elf(5), libmap.conf(5), ldconfig(8) BSD May 20, 2017 BSD
NAME | DESCRIPTION | DIRECT EXECUTION MODE | FILES | SEE ALSO
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=rtld&sektion=1&manpath=FreeBSD+11.1-RELEASE+and+Ports>