dld.so.5 (2010 09)

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dld.so(5)

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dld.so(5)

The LD_PRELOAD Environment Variable

NOTE: The LD_PRELOAD feature is disabled for seteuid/setegid programs, such as

passwd. See ld(1)

for more details. This feature is not available to fully-bound static executables.

The

LD_PRELOAD environment variable allows you to load additional shared libraries at program

startup. LD_PRELOAD provides a colon-separated or space-separated list of shared libraries that the

dynamic loader can interpret. The dynamic loader, dld.so

, loads the speciﬁed shared libraries as if the

program had been linked explicitly with the shared libraries in LD_PRELOAD before any other depen-

dents of the program.

At startup time, the dynamic loader implicitly loads one or more libraries, if found, speciﬁed in the

LD_PRELOAD environment. It uses the same load order and symbol resolution order as if the library had

been explicitly linked as the ﬁrst library in the link line when building the executable. For example,

given an executable built with the following link line:

$ ld ... lib2.so lib3.so lib4.so

If LD_PRELOAD=""/var/tmp/lib1.so"",

the dynamic loader uses the same load order and symbol

resolution order as if

lib1.so had been speciﬁed as the ﬁrst library in the link line:

$ ld ... /var/tmp/lib1.so lib2.so lib3.so lib4.so

In a typical command line use (with /usr/bin/sh

), where LD_PRELOAD is deﬁned as follows:

$ LD_PRELOAD=mysl.so

application

The dynamic loader searches application according to

$PATH, but searches mysl.so according to

SHLIB_PATH and/or LD_LIBRARY_PATH

, and/or the embedded path (if enabled).

NOTE: Because the dynamic loader checks the

LD_PRELOAD environment variable when running any

executable (except seteuid/setegid programs), if you export

LD_PRELOAD, you should unset it after run-

ning your executable, or run the executable as in the command listed above or in a script.

You can use the

LD_PRELOAD environment variable to load a shared library built with static thread-local

storage model that contains TLS to avoid the following error when loading the library dynamically:

/usr/lib/hpux[32|64]/dld.so: Can’t shl_load() a library containing

Thread Local Storage: /usr/lib/hpux[32|64]/libcps.so.1

You can use +tls=dynamic compiler option to re-compile the library to avoid the above error message.

The load order and symbol resolution order may be different in a PA-RISC 32-bit compatibility mode pro-

gram because the dynamic loader uses depth-ﬁrst search order in PA-RISC 32-bit mode and breadth-ﬁrst

search order in standard mode. See Symbol Searching and Dependent Libraries in the

+help option to

ld(1) or the HP-UX Linker and Libraries User’s Guide for more information.

The dynamic loader uses the

LD_PRELOAD environment variable even if you use the +noenvvar in the

link line. This insures that LD_PRELOAD is enabled even in a +compat link. The LD_PRELOAD vari-

able is always enabled except for setuid and setgid programs.

NOTE: Using

LD_PRELOAD can cause a core dump when used with applications which mix shared and

archived libraries, especially when both the shared library and the application are built with aC++ or use

libc.

You can specify multiple libraries as part of the

LD_PRELOAD environment variable. Separate the

libraries by spaces or colons as in LD_LIBRARY_PATH. (Multi-byte support is not provided as part of

parsing the LD_PRELOAD library list). You can specify LD_PRELOAD libraries with absolute paths or

relative paths. The LD_PRELOAD libraries can also consist of just the library names, in which case the

dynamic loader uses the directory path list in the environment variables LD_LIBRARY_PATH and/or

SHLIB_PATH or the embedded path list (if enabled) to search for the libraries.

The dynamic loader does not issue an error or warning message if it cannot ﬁnd a library speciﬁed by

LD_PRELOAD. However, if it does not ﬁnd a dependent of the LD_PRELOAD libraries, the dynamic

loader issues the same error message as if the LD_PRELOAD library is speciﬁed in the link line.

The LD_PRELOAD_ONCE Environment Variable

The

LD_PRELOAD_ONCE feature is similar to LD_PRELOAD except that the dynamic loader, dld.so,

unsets LD_PRELOAD_ONCE after reading it, so that any applications invoked by the current application

do not have LD_PRELOAD_ONCE set. This is useful in situations where the current application needs

certain libraries preloaded while the child application is adversely affected if these are preloaded (for

example, ksh terminates abnormally if LD_PRELOAD contains

HP-UX 11i Version 3: September 2010 − 5 − Hewlett-Packard Company 5