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dynfindsym, dynfreeimport, dynloadfd, dynloadgen, dynobjfree, dyntabsize - load object file dynamically


#include <lib9.h>
#include <a.out.h>
#include <dynld.h>

Dynsym* dynfindsym(char *name, Dynsym *syms, int nsym)

Dynobj* dynloadfd(int fd, Dynsym *exports, int nexport,
ulong maxsize)

Dynobj* dynloadgen(void *file, long (*read)(void*,void*,long),
vlong (*seek)(void*,vlong,int), void (*err)(char*),
Dynsym *exports, int nexport, ulong maxsize)

void* dynimport(Dynobj *o, char *name, ulong sig)

void dynfreeimport(Dynobj *o)

void dynobjfree(Dynobj *o)

int dyntabsize(Dynsym *t)

extern Dynsym _exporttab[];


These functions allow a process to load further code and data into the currently executing image. A dynamically-loadable file, called a module here, is a variant of the a.out(10.6) executable format with some extra components. The loader for the architecture (see 2l(1)) creates a module file from component object file(s) when given the -u option. A module contains text and data sections, an import table, an export table, and relocation data. The import table lists the symbols the module needs from the loading program; the export table lists symbols the module provides when loaded. A program that loads a module provides a table of its own symbols to match the symbols in the module's import table.

A symbol entry in a symbol table names a global function or data item, and has an associated signature value representing the type of the corresponding function or data in the source code. The Dynsym structure defines a symbol:

typedef struct {
	ulong	sig;
	ulong	addr;
	char*	name;
} Dynsym;

The structure is known to the loaders 2l(1). Name is the linkage name of the function or data. Addr is its address, which is relative to the start of the module before loading, and an address in the current address space after loading. The signature sig is the value produced by the C compiler's signof operator applied to the type. Symbol tables must be sorted by name.

An executable that wishes to load modules will normally be linked using the -x option to the appropriate loader 2l(1). The resulting executable contains an export table _exporttab that lists all the exported symbols of the program (by default, all external symbols). A nil name marks the end of the table. See 2l(1) for details. The table can be given to the functions below to allow a loaded module to access those symbols.

A loaded module is described by a Dynobj structure:

typedef struct {
	ulong	size;	/* total size in bytes */
	ulong	text;	/* bytes of text */
	ulong	data;	/* bytes of data */
	ulong	bss;		/* bytes of bss */
	uchar*	base;	/* start of text, data, bss */
	int		nexport;
	Dynsym*	export;	/* export table */
	int		nimport;
	Dynsym**	import;	/* import table */
} Dynobj;

Several fields give sizes of the module's components, as noted in comments above. Base gives the address at which the module has been loaded. All its internal references have been adjusted where needed to reflect its current address. Export points to a symbol table listing the symbols exported by the module; nexport gives the table's length. Import points to a list of symbols imported by the module; note that each entry actually points to an entry in a symbol table provided by the program that loaded the module (see below). Nimport gives the import table's length. If the import table is not required, call dynfreeimport on the module pointer to free it.

Dynfindysm looks up the entry for the given name in symbol table syms (of length nsym). It returns a pointer to the entry if found; nil otherwise. The symbol table must be sorted by name in ascending order.

Dyntabsize returns the length of symbol table t, defined to be the number of Dynsym values starting at t that have non-nil name fields. It is used to find the length of _exporttab.

Dynloadfd loads a module from the file open for reading on fd, and returns the resulting module pointer on success, or nil on error. If maxsize is non-zero the size of the dynamically-loaded module's code and data is limited to maxsize bytes. Exports is an array of nexport symbols in the current program that can be imported by the current module. It uses read(2) and seek(2) to access fd, and calls werrstr (see errstr(2)) to set the error string if necessary.

Dynloadgen is a more general function that can load a module from an arbitrary source, not just an open file descriptor. (In particular, it can be called by the kernel using functions internal to the kernel instead of making system calls.) Exports, nexport and maxsize are just as for dynloadfd. File is a pointer to a structure defined by the caller that represents the file containing the module. It is passed to read and seek. Read is invoked as (*read)(file,bufnbytes). Read should read nbytes of data from file into buf and return the number of bytes transferred. It should return -1 on error. Seek is invoked as (*seek)(file,ntype) where n and type are just as for seek(2); it should seek to the requested offset in file, or return -1 on error. Dynloadgen returns a pointer to the loaded module on success. On error, it returns nil after calling its err parameter to set the error string.

Dynimport returns a pointer to the value of the symbol name in loaded module o, or nil if o does not export a symbol with the given name. If sig is non-zero, the exported symbol's signature must equal sig, or dynimport again returns nil. For example:

Dev *d;
d = dynimport(obj, "XXXdevtab", signof(*d));
if(d == nil)
	error("not a dynamically-loadable driver");

Dynobjfree frees the module o. There is no reference counting: it is the caller's responsibility to decide whether a module is no longer needed.


2l(10.1), a.out(10.6)


Functions that return pointers return nil on error. Dynloadfd sets the error string and returns nil.

DYNLD(10.2 ) Rev:  Tue Mar 31 02:42:39 GMT 2015