Standard C Library Functions dldump(3C)
NAME
dldump - create a new file from a dynamic object component
of the calling process
SYNOPSIS
#include
int dldump(const char * ipath, const char * opath, int flags);
DESCRIPTION
The dldump() function creates a new dynamic object opath
from an existing 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. Reloca-
tions 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 processing requirements of the
object. However, limitations can exist in using these tech-
niques. The application of relocations to the new dynamic
object opath can restrict its flexibility within a dynami-
cally 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 dependencies, 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, relo-
cation 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
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Standard C Library Functions dldump(3C)
the address of an associated symbol, and results in a bind-
ing to the dynamic object that defines this symbol. The sym-
bol 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 relo-
cated.
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 parame-
ter to affect the relocations that are applied to the new
object. Non-symbolic relocations can be applied using the
following:
RTLDRELRELATIVE Relocation records from the object
ipath, that define relative reloca-
tions, are applied to the object opath.
A variety of symbolic relocations can be applied using the
following flags (each of these flags also implies
RTLDRELRELATIVE is in effect):
RTLDRELEXEC 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.
RTLDRELDEPENDS Symbolic relocations that result in
binding ipath to any of the dynamic
dependencies of the process are applied
to the object opath.
RTLDRELPRELOAD Symbolic relocations that result in
binding ipath to any objects preloaded
with the process are applied to the
object opath. See LDPRELOAD in
ld.so.1(1).
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RTLDRELSELF Symbolic relocations that result in
binding ipath to itself, are applied to
the object opath.
RTLDRELWEAK Weak relocations that remain unresolved
are applied to the object opath as 0.
RTLDRELAL All relocation records defined in the
object ipath are applied to the new
object opath. This is basically a con-
catenation of all the above relocation
flags.
Note that for dynamic executables, RTLDRELRELATIVE,
RTLDRELEXEC, and RTLDRELSELF have no effect. See EXAM-
PLES.
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 reloca-
tion will not be applied more than once. Otherwise, the new
object opath will retain the relocation 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 param-
eter:
RTLDMEMORY 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 execut-
able, 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
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Standard C Library Functions dldump(3C)
saved as a new .SUNWheap section in the
object opath. The objects' program headers
and symbol entries, such as end, are
adjusted accordingly. See also RTLDNOHEAP.
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.
RTLDSTRIP Only collect allocatable sections within the
object opath. Sections that are not part of
the dynamic objects' memory image are
removed. RTLDSTRIP reduces the size of the
opath disc file and is comparable to having
run the new object through strip(1).
RTLDNOHEAP Do not save any heap to the new object. This
option is only meaningful when processing a
dynamic executable with the RTLDMEMORY
attribute and allows for reducing the size of
the opath disc file. The executable must con-
fine 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 executable
called dldump(). It will gain control at the executable's
normal entry point. See EXAMPLES.
RETURN VALUES
On successful creation of the new object, dldump() returns
0. Otherwise, a non-zero value is returned and more detailed
diagnostic information is available through dlerror().
EXAMPLES
Example 1 Sample code using dldump().
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Standard C Library Functions dldump(3C)
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, RTLDRELRELATIVE) != 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 relo-
cation 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 reloca-
tions 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 flexibility 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 RTLDRELSELF will cause any refer-
ences to symbols 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. There-
fore, opath will have one less binding that must be computed
at runtime. This reduces the startup cost of using opath by
other applications; however, interposition of the symbol foo
will no longer be possible.
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Standard C Library Functions dldump(3C)
Using a dumped shared object with applied relocations as an
applications dependency normally requires that the applica-
tion have the same dependencies as the application that pro-
duced the dumped image. Dumping shared objects, and the
various flags associated with relocation processing, 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;
timet seconds;
...
/* Perform data initialization */
seconds = time((timet *)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, RTLDMEMORY));
}
(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 executable 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.
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Standard C Library Functions dldump(3C)
For greatest flexibility, this example does not save any
relocated information. The elements of the dynamic execut-
able 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 exe-
cuted 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, RTLDRELRELATIVE,
RTLDRELEXEC, and RTLDRELSELF have no effect, as reloca-
tions within the dynamic executable will have been fixed
when it was created by ld(1).
When RTLDMEMORY 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 vari-
able 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 RTLDMEMORY is used, any modification to a data item
that is initialized 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.
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Standard C Library Functions dldump(3C)
Non-idempotent data initializations may prevent the use of
RTLDMEMORY. For example, the addition of elements to a
linked-list via init sections 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 esta-
blished via initialization code. Note that this may invali-
date 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 facili-
ties. These facilities are available to dynamically-linked
processes only. See Linker and Libraries Guide).
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWcsu
MT-Level MT-Safe
SEE ALSO
ld(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 PROG-
BITS 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
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Standard C Library Functions dldump(3C)
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 run-
time 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 processed 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 relocation sections are renamed to
.SUNWreloc and the association with the section to relo-
cate, is lost. Only the offset of the relocation record is
meaningful. .SUNWreloc 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.
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