DLDUMP(3C)DLDUMP(3C)NAMEdldump - create a new file from a dynamic object component of the call‐
ing process
SYNOPSIS
#include <dlfcn.h>
int dldump(const char * ipath, const char * opath, int flags);
DESCRIPTION
The dldump() function creates a new dynamic object opath from an exist‐
ing dynamic object ipath that is bound to the current process. An ipath
value of 0 is interpreted as the dynamic object that started the
process. The new object is constructed from the existing objects' disc
file. Relocations can be applied to the new object to pre-bind it to
other dynamic objects, or fix the object to a specific memory location.
In addition, data elements within the new object can be obtained from
the objects' memory image as this data exists in the calling process.
These techniques allow the new object to be executed with a lower
startup cost. This reduction can be because of less relocations being
required to load the object, or because of a reduction in the data pro‐
cessing requirements of the object. However, limitations can exist in
using these techniques. The application of relocations to the new
dynamic object opath can restrict its flexibility within a dynamically
changing environment. In addition, limitations in regards to data usage
can make dumping a memory image impractical. See EXAMPLES.
The runtime linker verifies that the dynamic object ipath is mapped as
part of the current process. Thus, the object must either be the
dynamic object that started the process, one of the process's dependen‐
cies, or an object that has been preloaded. See exec(2), and
ld.so.1(1).
As part of the runtime processing of a dynamic object, relocation
records within the object are interpreted and applied to offsets within
the object. These offsets are said to be relocated. Relocations can be
categorized into two basic types: non-symbolic and symbolic.
The non-symbolic relocation is a simple relative relocation that
requires the base address at which the object is mapped to perform the
relocation. The symbolic relocation requires the address of an associ‐
ated symbol, and results in a binding to the dynamic object that
defines this symbol. The symbol definition can originate from any of
the dynamic objects that make up the process, that is, the object that
started the process, one of the process's dependencies, an object that
has been preloaded, or the dynamic object being relocated.
The flags parameter controls the relocation processing and other
attributes of producing the new dynamic object opath. Without any
flags, the new object is constructed solely from the contents of the
ipath disc file without any relocations applied.
Various relocation flags can be or'ed into the flags parameter to
affect the relocations that are applied to the new object. Non-symbolic
relocations can be applied using the following:
RTLD_REL_RELATIVE
Relocation records from the object ipath, that
define relative relocations, are applied to the
object opath.
A variety of symbolic relocations can be applied using the following
flags (each of these flags also implies RTLD_REL_RELATIVE is in
effect):
RTLD_REL_EXEC
Symbolic relocations that result in binding ipath
to the dynamic object that started the process,
commonly a dynamic executable, are applied to the
object opath.
RTLD_REL_DEPENDS
Symbolic relocations that result in binding ipath
to any of the dynamic dependencies of the process
are applied to the object opath.
RTLD_REL_PRELOAD
Symbolic relocations that result in binding ipath
to any objects preloaded with the process are
applied to the object opath. See LD_PRELOAD in
ld.so.1(1).
RTLD_REL_SELF
Symbolic relocations that result in binding ipath
to itself, are applied to the object opath.
RTLD_REL_WEAK
Weak relocations that remain unresolved are applied
to the object opath as 0.
RTLD_REL_ALL
All relocation records defined in the object ipath
are applied to the new object opath. This is basi‐
cally a concatenation of all the above relocation
flags.
Note that for dynamic executables, RTLD_REL_RELATIVE, RTLD_REL_EXEC,
and RTLD_REL_SELF have no effect. See EXAMPLES.
If relocations, knowledgeable of the base address of the mapped object,
are applied to the new object opath, then the new object becomes fixed
to the location that the ipath image is mapped within the current
process.
Any relocations applied to the new object opath will have the original
relocation record removed so that the relocation will not be applied
more than once. Otherwise, the new object opath will retain the reloca‐
tion records as they exist in the ipath disc file.
The following additional attributes for creating the new dynamic object
opath can be specified using the flags parameter:
RTLD_MEMORY
The new object opath is constructed from the current
memory contents of the ipath image as it exists in the
calling process. This option allows data modified by the
calling process to be captured in the new object. Note
that not all data modifications may be applicable for
capture; significant restrictions exist in using this
technique. See EXAMPLES. By default, when processing a
dynamic executable, any allocated memory that follows
the end of the data segment is captured in the new
object (see malloc(3C) and brk(2)). This data, which
represents the process heap, is saved as a new
.SUNW_heap section in the object opath. The objects'
program headers and symbol entries, such as _end, are
adjusted accordingly. See also RTLD_NOHEAP. When using
this attribute, any relocations that have been applied
to the ipath memory image that do not fall into one of
the requested relocation categories are undone, that is,
the relocated element is returned to the value as it
existed in the ipath disc file.
RTLD_STRIP
Only collect allocatable sections within the object
opath. Sections that are not part of the dynamic
objects' memory image are removed. RTLD_STRIP reduces
the size of the opath disc file and is comparable to
having run the new object through strip(1).
RTLD_NOHEAP
Do not save any heap to the new object. This option is
only meaningful when processing a dynamic executable
with the RTLD_MEMORY attribute and allows for reducing
the size of the opath disc file. The executable must
confine its data initialization to data elements within
its data segment, and must not use any allocated data
elements that comprise the heap.
It should be emphasized, that an object created by dldump() is simply
an updated ELF object file. No additional state regarding the process
at the time dldump() is called is maintained in the new object.
dldump() does not provide a panacea for checkpoint and resume. A new
dynamic executable, for example, will not start where the original exe‐
cutable called dldump(). It will gain control at the executable's nor‐
mal entry point. See EXAMPLES.
RETURN VALUES
On successful creation of the new object, dldump() returns 0. Other‐
wise, a non-zero value is returned and more detailed diagnostic infor‐
mation is available through dlerror().
EXAMPLES
Example 1 Sample code using dldump().
The following code fragment, which can be part of a dynamic executable
a.out, can be used to create a new shared object from one of the
dynamic executables' dependencies libfoo.so.1:
const char * ipath = "libfoo.so.1";
const char * opath = "./tmp/libfoo.so.1";
...
if (dldump(ipath, opath, RTLD_REL_RELATIVE) != 0)
(void) printf("dldump failed: %s\n", dlerror());
The new shared object opath is fixed to the address of the mapped ipath
bound to the dynamic executable a.out. All relative relocations are
applied to this new shared object, which will reduce its relocation
overhead when it is used as part of another process.
By performing only relative relocations, any symbolic relocation
records remain defined within the new object, and thus the dynamic
binding to external symbols will be preserved when the new object is
used.
Use of the other relocation flags can fix specific relocations in the
new object and thus can reduce even more the runtime relocation startup
cost of the new object. However, this will also restrict the flexibil‐
ity of using the new object within a dynamically changing environment,
as it will bind the new object to some or all of the dynamic objects
presently mapped as part of the process.
For example, the use of RTLD_REL_SELF will cause any references to sym‐
bols from ipath to be bound to definitions within itself if no other
preceding object defined the same symbol. In other words, a call to
foo() within ipath will bind to the definition foo within the same
object. Therefore, opath will have one less binding that must be com‐
puted at runtime. This reduces the startup cost of using opath by other
applications; however, interposition of the symbol foo will no longer
be possible.
Using a dumped shared object with applied relocations as an applica‐
tions dependency normally requires that the application have the same
dependencies as the application that produced the dumped image. Dumping
shared objects, and the various flags associated with relocation pro‐
cessing, have some specialized uses. However, the technique is intended
as a building block for future technology.
The following code fragment, which is part of the dynamic executable
a.out, can be used to create a new version of the dynamic executable:
static char * dumped = 0;
const char * opath = "./a.out.new";
...
if (dumped == 0) {
char buffer[100];
int size;
time_t seconds;
...
/* Perform data initialization */
seconds = time((time_t *)0);
size = cftime(buffer, (char *)0, &seconds);
if ((dumped = (char *)malloc(size + 1)) == 0) {
(void) printf("malloc failed: %s\n", strerror(errno));
return (1);
}
(void) strcpy(dumped, buffer);
...
/*
* Tear down any undesirable data initializations and
* dump the dynamic executables memory image.
*/
_exithandle();
_exit(dldump(0, opath, RTLD_MEMORY));
}
(void) printf("Dumped: %s\n", dumped);
Any modifications made to the dynamic executable, up to the point the
dldump() call is made, are saved in the new object a.out.new. This
mechanism allows the executable to update parts of its data segment and
heap prior to creating the new object. In this case, the date the exe‐
cutable is dumped is saved in the new object. The new object can then
be executed without having to carry out the same (presumably expensive)
initialization.
For greatest flexibility, this example does not save any relocated
information. The elements of the dynamic executable ipath that have
been modified by relocations at process startup, that is, references to
external functions, are returned to the values of these elements as
they existed in the ipath disc file. This preservation of relocation
records allows the new dynamic executable to be flexible, and correctly
bind and initialize to its dependencies when executed on the same or
newer upgrades of the OS.
Fixing relocations by applying some of the relocation flags would bind
the new object to the dependencies presently mapped as part of the
process calling dldump(). It may also remove necessary copy relocation
processing required for the correct initialization of its shared object
dependencies. Therefore, if the new dynamic executables' dependencies
have no specialized initialization requirements, the executable may
still only interact correctly with the dependencies to which it binds
if they were mapped to the same locations as they were when dldump()
was called.
Note that for dynamic executables, RTLD_REL_RELATIVE, RTLD_REL_EXEC,
and RTLD_REL_SELF have no effect, as relocations within the dynamic
executable will have been fixed when it was created by ld(1).
When RTLD_MEMORY is used, care should be taken to insure that dumped
data sections that reference external objects are not reused without
appropriate re-initialization. For example, if a data item contains a
file descriptor, a variable returned from a shared object, or some
other external data, and this data item has been initialized prior to
the dldump() call, its value will have no meaning in the new dumped
image.
When RTLD_MEMORY is used, any modification to a data item that is ini‐
tialized via a relocation whose relocation record will be retained in
the new image will effectively be lost or invalidated within the new
image. For example, if a pointer to an external object is incremented
prior to the dldump() call, this data item will be reset to its disc
file contents so that it can be relocated when the new image is used;
hence, the previous increment is lost.
Non-idempotent data initializations may prevent the use of RTLD_MEMORY.
For example, the addition of elements to a linked-list via init sec‐
tions can result in the linked-list data being captured in the new
image. Running this new image may result in init sections continuing to
add new elements to the list without the prerequisite initialization of
the list head. It is recommended that _exithandle(3C) be called before
dldump() to tear down any data initializations established via initial‐
ization code. Note that this may invalidate the calling image; thus,
following the call to dldump(), only a call to _Exit(2) should be made.
USAGE
The dldump() function is one of a family of functions that give the
user direct access to the dynamic linking facilities. These facilities
are available to dynamically-linked processes only. See Linker and
Libraries Guide).
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌───────────────┬─────────────────┐
│ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├───────────────┼─────────────────┤
│MT-Level │ MT-Safe │
└───────────────┴─────────────────┘
SEE ALSOld(1), ld.so.1(1), strip(1), _Exit(2), brk(2), exec(2),
_exithandle(3C), dladdr(3C), dlclose(3C), dlerror(3C), dlopen(3C),
dlsym(3C), end(3C), malloc(3C), attributes(5)
Linker and Libraries Guide
NOTES
These functions are available to dynamically-linked processes only.
Any NOBITS sections within the ipath are expanded to PROGBITS sections
within the opath. NOBITS sections occupy no space within an ELF file
image. NOBITS sections declare memory that must be created and zero-
filled when the object is mapped into the runtime environment. .bss is
a typical example of this section type. PROGBITS sections, on the other
hand, hold information defined by the object within the ELF file image.
This section conversion reduces the runtime initialization cost of the
new dumped object but increases the objects' disc space requirement.
When a shared object is dumped, and relocations are applied which are
knowledgeable of the base address of the mapped object, the new object
is fixed to this new base address. The dumped object has its ELF type
reclassified to be a dynamic executable. The dumped object can be pro‐
cessed by the runtime linker, but is not valid as input to the link-
editor.
If relocations are applied to the new object, any remaining relocation
records are reorganized for better locality of reference. The reloca‐
tion sections are renamed to .SUNW_reloc and the association with the
section to relocate, is lost. Only the offset of the relocation record
is meaningful. .SUNW_reloc relocations do not make the new object
invalid to either the runtime linker or link-editor, but can reduce the
objects analysis with some ELF readers.
Mar 1, 2004 DLDUMP(3C)
