|Home > Image Structure and Generation > The structure of an Arm ELF image > Image entry points|
An entry point in an image is the location that is loaded into the PC. It is the location where program execution starts. Although there can be more than one entry point in an image, you can specify only one when linking.
Not every ELF file has to have an entry point. Multiple entry points in a single ELF file are not permitted.
There are two distinct types of entry point:
The initial entry point for an image is a single value that is stored in the ELF header file. For programs loaded into RAM by an operating system or boot loader, the loader starts the image execution by transferring control to the initial entry point in the image.
An image can have only one initial entry point. The initial entry point can be, but
is not required to be, one of the entry points set by the
You can select one of many possible entry points for an image. An image can have only one entry point.
You create entry points in objects with the
ENTRY directive in an
assembler file. In embedded systems, typical use of this directive is to mark code
that is entered through the processor exception vectors, such as RESET, IRQ, and FIQ.
The directive marks the output code section with an
that instructs the linker not to remove the section when it performs unused section
For C and C++ programs, the
__main() function in the C library is
also an entry point.
If an embedded image is to be used by a loader, it must have a single initial entry
point specified in the header. Use the
--entry command-line option to
select the entry point.
There can be only one initial entry point for an image, otherwise linker warning
L6305W is output.
The initial entry point must meet the following conditions:
The image entry point must always lie within an execution region.
The execution region must not overlay another execution region, and must be a root execution region. That is, where the load address is the same as the execution address.
If you do not use the
--entry option to
specify the initial entry point, then:
If the input objects contain only one entry point set by the
directive, the linker uses that entry point as the initial entry point for the
The linker generates an image that does not contain an initial entry point when either:
More than one entry point is specified using the
No entry point is specified using the
For embedded applications with ROM at address zero use
for processors that are using high vectors.