Difference between the ELF vs MAP file

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The linker can output both the ELF and the MAP file. These files are especially relevant in the embedded systems world because the ELF file is usually used to read out the addresses of variables or functions. Additionally, the ELF file is used by different embedded measurement or analysis tools.

When I open a MAP file, then within it, I can see for each global variable and every external function the following information: allocated address, symbolic name, allocated bytes, memory unit, and memory section.

On the other hand, once I open the ELF file, it is a binary file and not human readable. However, some tools I use are able to read it out and interpret it. Those tools can interpret the ELF file, and obtain the information about the symbolic name of a variable/function and its address or even show a function prototype.

From my understanding the ELF and MAP files are basically containing the same information, it is just that the first one is binary and the latter one is the text file. So what are the actual differences between these two files from the content perspective?

Thank you in advance!

embedded elf linker
1 Answers

The primary output of the linker (i.e. its main purpose) is to produce the fully linked executable code. That is the ELF (Executable Linkable Format) file. An ELF file may as you have observed contain symbols - these are used for debug. It may also contain meta-data that associates the machine code with the source code from which it was generated. But the bulk of its content (and the part that is not optional) is the executable machine code and data objects that are your application.

The MAP file is an optional information only human readable output that contains information about the location and size of code and data objects in your application. The MAP file includes a summary that shows the total size and memory usage of your code.

In an embedded cross-development environment, the symbol information in the ELF file is used when the code is loaded into a source-level symbolic debugger. The debugger takes the binary code/data segments in the ELF file and loads them onto the target (typically using a JTAG or other debug/programming hardware tool), it loads the symbols and source-level debug meta-data into the debugger, then while the real machine code is executing on the target, that execution is reflected in the debugger in the original source code where you can view, step and break-point the code at the source level.

In short, the ELF file is your program. The MAP file is, as its name suggests, a map of your executable - it tells you where things are in the executable.

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