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The HP-UX linker, ld,
produces a single executable file from one or more input object
files and libraries. In doing so, it matches external references
to global definitions contained in other object files or libraries.
It revises code and data to reflect new addresses, a process known
as relocation. If the input files contain
debugger information, ld
updates this information appropriately. The linker places the resulting
executable code in a file named, by default, a.out. In the C program example (see “Compiling Programs on HP-UX:
An
Example ”) main.o
contains an external reference to sum_n,
which has a global definition in func.o.
ld matches the
external reference to the global definition, allowing the main program
code in a.out
to access sum_n
(see Figure 2-3 “Matching
the External Reference to sum_n ”). If ld cannot
match an external reference to a global definition, it displays
a message to standard error output. If, for instance, you compile
main.c without
func.c, ld
cannot match the external reference to sum_n
and displays this output: $ cc -Aa main.c
/usr/ccs/bin/ld: Unsatisfied symbols:
sum_n (code)
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The crt0.o Startup File | |
Notice in the example in “Compiler-Linker Interaction ” that the first object file on the
linker command line is /opt/langtools/lib/crt0.o,
even though this file was not specified on the compiler command
line. This file, known as a startup file,
contains the program's entry point that is,
the location at which the program starts running after HP-UX loads
it into memory to begin execution. The startup code does such things
as retrieving command line arguments into the program at run time,
and activating the dynamic loader (dld.sl(5))
to load any required shared libraries. In the C language, it also
calls the routine _start
in libc which,
in turn, calls the main program as a function. The 64-bit linker uses the startup file, /opt/langtools/lib/pa_64/crt0.o,
when: The linker is in compatibility mode
(+compat). The linker is in default standard mode (+std)
with the -noshared option.
If the -p
profiling option is specified on the 32-bit mode compile line, the
compilers link with mcrt0.o
instead of crt0.o.
If the -G profiling
option is specified, the compilers link with gcrt0.o.
In 64-bit mode with the -p option,
the linker adds -lprof before the
-lc
option. With the -G option, the
linker adds -lgprof. If the linker option -I
is specified to create an executable file with profile-based optimization,
in 32-bit mode icrt0.o is used,
and in 64-bit mode the linker inserts /usr/ccs/lib/pa20_64/fdp_init.o.
If the linker options -I
and -b are specified
to create a shared library with profile-based optimization, in 32-bit
mode scrt0.o is used, and in 64-bit
mode, the linker inserts /usr/ccs/lib/pa20_64/fdp_init_sl.o.
In 64-bit mode, the linker uses the single 64-bit crt0.o
to support these option. For details on startup files, see crt0(3). The Program's Entry Point In 32-bit mode and in 64-bit statically-bound
(-noshared) executables, the entry
point is the location at which execution begins in the a.out
file. The entry point is defined by the symbol $START$
in crt0.o. In 64-bit mode for dynamically bound executables, the entry
point, defined by the symbol $START$
in the dynamic loader (dld.sl). The a.out File | |
The information contained in the resulting a.out
file depends on which architecture the file was created on and what
options were used to link the program. In any case, an executable
a.out file contains
information that HP-UX needs when loading and running the file,
for example: Is it a shared executable? Does it reference shared
libraries? Is it demand-loadable? Where do the code (text), data,
and bss (uninitialized data) segments reside in the file? For details
on the format of this file, see a.out(4). In 32-bit mode, the linker records a magic number
with each executable program that determines how the program should
be loaded. There are three possible values for an executable file's
magic number: Title not available (Magic Numbers ) - SHARE_MAGIC
The program's text (code) can be shared by processes;
its data cannot be shared. The first process to run the program
loads the entire program into virtual memory. If the program is
already loaded by another process, then a process shares the program
text with the other process. - DEMAND_MAGIC
As with SHARE_MAGIC
the program's text is shareable but its data is not. However, the
program's text is loaded only as needed — that is, only
as the pages are accessed. This can improve process startup time
since the entire program does not need to be loaded; however, it
can degrade performance throughout execution. - EXEC_MAGIC
Neither the program's text nor data is shareable.
In other words, the program is an unshared executable. Usually,
it is not desirable to create such unshared executables because
they place greater demands on memory resources.
By default, the linker creates executables whose magic number
is SHARE_MAGIC.
The following shows which linker option to use to specifically set
the magic number. Table 2-1 32-bit
Mode Magic Number Linker Options To set the magic number to: | Use this option: |
|---|
SHARE_MAGIC | -n | DEMAND_MAGIC | -q | EXEC_MAGIC | -N |
An
executable file's magic number can also be changed using the chatr
command (see “Changing a Program's Attributes
with chatr(1) ”).
However, chatr
can only toggle between SHARE_MAGIC
and DEMAND_MAGIC;
it cannot be used to change from or to EXEC_MAGIC.
This is because the file format of SHARE_MAGIC
and DEMAND_MAGIC
is exactly the same, while EXEC_MAGIC
files have a different format. For details on magic numbers, refer
to magic(4). In 64-bit mode, the linker sets the magic number to the predefined
type for ELF object files (\177ELF).
The value of the E_TYPE in the
ELF object file specifies how the file should be loaded. File Permissions | |
If no linker errors occur, the linker gives the a.out
file read/write/execute permissions to all users (owner, group,
and other). If errors occurred, the linker gives read/write permissions
to all users. Permissions are further modified if the umask
is set (see umask(1)). For example, on a
system with umask set to 022, a successful link produces an a.out
file with read/write/execute permissions for the owner, and read/execute
permissions for group and other: $ umask
022
$ ls -l a.out
-rwxr-xr-x 1 michael users 74440 Apr 4 14:38 a.out
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