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man pages section 3: Basic Library Functions

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Updated: Thursday, June 13, 2019
 
 

dldump(3C)

Name

dldump - create a new file from a dynamic object component of the calling 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 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' disk file.

These techniques allow the new object to be executed with a lower startup cost. This reduction can be because of a reduction in the data processing requirements of the object. However, 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).

The flags parameter controls attributes of producing the new dynamic object opath. Without any flags, the new object is constructed solely from the contents of the ipath disk file.

The following 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 are undone, that is, the relocated element is returned to the value as it existed in the ipath disk 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 disk 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 disk 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 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().

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 = NULL;
const char   *opath = "./a.out.new";
...
if (dumped == NULL) {
    char        buffer[100];
    int         size;
    time_t      seconds;
    ...
    /* Perform data initialization */
    seconds = time(NULL);
    size = cftime(buffer, NULL, &seconds);

    if ((dumped = malloc(size + 1)) == NULL) {
        (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 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.

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 disk 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.

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 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 disk 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 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 established via initialization 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 the Oracle Solaris 11.4 Linkers and Libraries Guide).

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
MT-Level
MT-Safe

See Also

ld(1), ld.so.1(1), strip(1), _Exit(2), brk(2), exec(2), dlsym(3C), _exithandle(3C), dladdr(3C), dlclose(3C), dlerror(3C), dlopen(3C), end(3C), malloc(3C), attributes(7)

Oracle Solaris 11.4 Linkers 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' disk space requirement.

Oracle Solaris 11.4 discontinued support for the following dldump() flags, which enabled various forms of relocation processing: RTLD_REL_RELATIVE, RTLD_REL_EXEC, RTLD_REL_DEPENDS, RTLD_REL_PRELOAD, RTLD_REL_SELF, RTLD_REL_WEAK, and RTLD_REL_ALL. An attempt to use any of these flags will cause dldump() to report an error by returning a non-zero value.