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AIX Version 4.3 Base Operating System and Extensions Technical Reference, Volume 1

load Subroutine


Loads and binds an object module into the current process.


int *load (FilePath, Flags, LibraryPath)
char *FilePath;
uint Flags;
char *LibraryPath;


The load subroutine loads the specified module into the calling process's address space. A module is an object file that may be a member of an archive. Unlike the exec subroutine, the load subroutine does not replace the current program with a new one. Instead, it loads the new module into the process private segment at the current break value and the break value is updated to point past the new module.

The exec subroutine is similar to the load subroutine, except that the exec subroutine does not have an explicit library path parameter; it has only the LIBPATH environment variable. Also, the LIBPATH variable is ignored when the program using the exec subroutine has more privilege than the caller, for example, in the case of an suid program.

If the calling process later uses the unload subroutine to unload the object file, the space is unusable by the process except through another call to the load subroutine. If the kernel finds an unused space created by a previous unload, it reuses this space rather than loading the new module at the break value. Space for loaded programs is managed by the kernel and not by any user-level storage-management routine.

A large application can be split up into one or more module s in one of two ways that allow execution within the same process. The first way is to create each of the application's modules separately and use load to explicitly load a module when it is needed. The other way is to specify the relationship between the modules when they are created by defining imported and exported symbols.

Modules can import symbols from other modules. Whenever symbols are imported from one or more other modules, these modules are automatically loaded to resolve the symbol references if the required modules are not already loaded, and if the imported symbols are not specified as deferred imports .These modules can be archive members in libraries or separate object files and can have either shared or private object file characteristics that control how and where they are loaded.

Shared modules (typically members of a shared library archive) are loaded into the shared library region, when their access permissions allow sharing, that is, when they have read-other permission . Shared modules without the required permissions for sharing and private modules are loaded into the process private region.

When the loader resolves a symbol, it uses the file name recorded with that symbol to find the module that exports the symbol. If the file name contains any / (slash) characters, it is used directly and must name an appropriate object file (or archive). However, if the file name is a base name (contains no / characters), the loader searches the directories specified in the default library path for an object file (or archive) with that base name.

The LibraryPath is a string containing one or more directory path names separated by colon s . If the base name is not found, the search continues, using the library path specified in the object file containing the symbol being resolved (normally the library path specified to the ld command that created the object file). The first instance of the base name found is used. An error occurs if this module cannot be loaded or does not export a definition of the symbol being resolved.

The default library path may be specified using the LibraryPath parameter. If not explicitly set, the default library path may be obtained from the LIBPATH environment variable or from the module specified by the FilePath parameter. If the L_LIBPATH_EXEC flag is specified, then the library path used at process exec time is prepended to any other library path specified in the load call.

When a process is executing under ptrace() , its segments 1 and 13 are reconstructed when the load() subroutine is called. Code modification will not work in this case. If the running code has been modified dynamically (in segment 1), when load() is called it will be changed back to its original value.

If the program calling the load subroutine was linked on 4.2 or a later release, the load subroutine will call initialization routines (init routines) for the new module and any of its dependents if they were not already loaded.

Modules loaded by this subroutine are automatically unloaded when the process terminates or when the exec subroutine is executed. They are explicitly unloaded by calling the unload subroutine.


FilePath Points to the name of the object file to be loaded. If the FilePath name contains no / (slash) symbols, it is treated as a base name, and should be in one of the directories listed in the library path.

The library path is either the value of the LibraryPath parameter if not a null value, or the value of the LIBPATH environment variable (if set) or the library path used at process exec time (if the L_LIBPATH_EXEC is set). If no library path is provided, the object file should be in the current directory.

If the FilePath parameter is not a base name (if it contains at least one / character), the name is used as it is, and no library path searches are performed to locate the object file. However, the library path is used to locate dependent modules.

Flags Modifies the behavior of the load service as follows (see the ldr.h file). If no special behavior is required, set the value of the flags parameter to 0 (zero) . For compatibility, a value of 1 (one) may also be specified.
Specifies that the library path used at process exec time should be prepended to any library path specified in the load call (either as an argument or environment variable). It is recommended that this flag be specified in all calls to the load subroutine.
Specifies that any deferred imports must be explicitly resolved by use of the loadbind subroutine. This allows unresolved imports to be explicitly resolved at a later time with a specified module. If this flag is not specified, deferred imports (marked for deferred resolution) are resolved at the earliest opportunity when any module is loaded that has exported symbols matching unresolved imports.
LibraryPath Points to a character string that specifies the default library search path.

If the LibraryPath parameter is a null value and the LIBPATH environment variable is set, the LIBPATH value is used as the default load path. If neither default library path option is provided, the library path specified in the loader section of the object file specified in the FilePath parameter is used as the default library path. If the L_LIBPATH_EXEC flag is specified, then the library path used at process exec time is prepended to the above specified default library path.

Note the difference between setting the LibraryPath parameter to null, and having the LibraryPath parameter point to a null string (" "). A null string is a valid library path which consists of a single directory: the current directory.

If the module is not in the LibraryPath parameter or the LIBPATH environmental variable (if the LibraryPath parameter was null), then the library path specified in the loader section of the module importing the symbol is used to locate the module exporting the required symbol. The library path in the importing module was specified when the module was link-edited (by the ld command).

The library path search is not performed when either a relative or an absolute path name is specified for the module exporting the symbol.

Return Values

Upon successful completion, the load subroutine returns the pointer to function for the entry point of the module. If the module has no entry point, the address of the data section of the module is returned.

Error Codes

If the load subroutine fails, a null pointer is returned, the module is not loaded, and errno global variable is set to indicate the error. The load subroutine fails if one or more of the following are true of a module to be explicitly or automatically loaded:

EACCES Indicates the file is not an ordinary file, or the mode of the program file denies execution permission, or search permission is denied on a component of the path prefix.
EINVAL Indicates the file or archive member has a valid magic number in its header, but the header is damaged or is incorrect for the machine on which the file is to be run.
ELOOP Indicates too many symbolic links were encountered in translating the path name.
ENOEXEC Indicates an error occurred when loading or resolving symbols for the specified module. This can be due to an attempt to load a module with an invalid XCOFF header, a failure to resolve symbols that were not defined as deferred imports or several other load time related problems. The loadquery subroutine can be used to return more information about the load failure. If the main program was linked on a 4.2 or later system, and if runtime linking is used, the load subroutine will fail if the runtime linker could not resolve some symbols. In this case, errno will be set to ENOEXEC , but the loadquery subroutine will not return any additional information.
ENOMEM Indicates the program requires more memory than is allowed by the system-imposed maximum.
ETXTBSY Indicates the file is currently open for writing by some process.
ENAMETOOLONG Indicates a component of a path name exceeded 255 characters, or an entire path name exceeded 1023 characters.
ENOENT Indicates a component of the path prefix does not exist, or the path name is a null value.
ENOTDIR Indicates a component of the path prefix is not a directory.
ESTALE Indicates the process root or current directory is located in a virtual file system that has been unmounted.

Implementation Specifics

This subroutine is part of Base Operating System (BOS) Runtime.

Related Information

The dlopen subroutine, exec subroutine, loadbind subroutine, loadquery subroutine, ptrace subroutine, unload subroutine.

The ld command.

The Shared Library Overview and Subroutines Overview in AIX Version 4.3 General Programming Concepts: Writing and Debugging Programs.

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