dlopen(3X) — MISCELLANEOUS LIBRARY FUNCTIONS

NAME

dlopen − open a shared object

SYNOPSIS

cc [flag ...] file ... −ldl [library ...]

#include <dlfcn.h>

void ∗dlopen(char ∗pathname, int mode);

DESCRIPTION

dlopen is one of a family of routines that give the user direct access to the dynamic linking facilities. These routines are available in a library that is loaded if the option -ldl is used with cc or ld. The -ldl library (and the routines it contains) may not be used when a program is to be statically linked.

dlopen makes a shared object available to a running process. dlopen returns to the process a handle the process may use on subsequent calls to dlsym and dlclose. This value should not be interpreted in any way by the process. pathname is the path name of the object to be opened; it may be an absolute path or relative to the current directory. If the value of pathname is 0, dlopen makes the symbols contained in the original a.out, and all of the objects that were loaded at program startup with the a.out, available through dlsym.

When a shared object is brought into the address space of a process, it may contain references to symbols whose addresses are not known until the object is loaded. These references must be relocated before the symbols can be accessed. The mode parameter governs when these relocations take place and may have the following values:

RTLD_LAZY
Under this mode, only references to data symbols are relocated when the object is loaded. References to functions are not relocated until a given function is invoked for the first time. This mode should result in better performance, since a process may not reference all of the functions in any given shared object.

RTLD_NOW
Under this mode, all necessary relocations are performed when the object is first loaded. This may result in some wasted effort, if relocations are performed for functions that are never referenced, but is useful for applications that need to know as soon as an object is loaded that all symbols referenced during execution will be available.

DIAGNOSTICS

If pathname cannot be found, cannot be opened for reading, is not a shared object, or if an error occurs during the process of loading pathname or relocating its symbolic references, dlopen returns NULL. More detailed diagnostic information is available through dlerror.

NOTES

If other shared objects were link edited with pathname when pathname was built, those objects are automatically loaded by dlopen. The directory search path to be used to find both pathname and the other needed objects may be specified by setting the environment variable LD_LIBRARY_PATH. This environment variable should contain a colon-separated list of directories, in the same format as the PATH variable [see sh(1)]. LD_LIBRARY_PATH is ignored if the process is running setuid or setgid [see exec(2)] or if the name specified is not a simple file name (that is, contains a / character). Objects whose names resolve to the same absolute or relative path name may be opened any number of times using dlopen, however, the object referenced is loaded only once into the address space of the current process. The same object referenced by two different path names, however, may be loaded multiple times. For example, given the object /usr/home/me/mylibs/mylib.so, and assuming the current working directory is /usr/home/me/workdir,

. . .
void ∗handle1;
void ∗handle2;
handle1 = dlopen("../mylibs/mylib.so", RTLD_LAZY);
handle2 = dlopen("/usr/home/me/mylibs/mylib.so", RTLD_LAZY);
. . .

results in mylibs.so being loaded twice for the current process. On the other hand, given the same object and current working directory, if LD_LIBRARY_PATH=/usr/home/me/mylibs, then

. . .
void ∗handle1;
void ∗handle2;
handle1 = dlopen("mylib.so", RTLD_LAZY);
handle2 = dlopen("/usr/home/me/mylibs/mylib.so", RTLD_LAZY);
. . .

results in mylibs.so being loaded only once.

Objects loaded by a single invocation of dlopen may import symbols from one another or from any object loaded automatically during program startup, but objects loaded by one dlopen invocation may not directly reference symbols from objects loaded by a different dlopen invocation. Those symbols may, however, be referenced indirectly using dlsym.

Users who wish to gain access to the symbol table of the a.out itself using dlsym(0, mode) should be aware that some symbols defined in the a.out may not be available to the dynamic linker. The symbol table created by ld for use by the dynamic linker might contain only a subset of the symbols defined in the a.out: specifically those referenced by the shared objects with which the a.out is linked.

Any symbols in the executable that may be referenced from a shared object accessed via dlopen must also be referenced in a shared library that is linked in at link time.

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