CODEWITHSUNDEEP
ccompilationdecompiling
Shubhamay Sarkar
Shubhamay Sarkar

Reputation: 39

How to get a c source code from the compiled code

I have the compiled C code in text format. I need to extract the source code by decompiling the machine code. How to do that?

Upvotes: 2

Views: 9672

Answers (3)

srdjan.veljkovic
srdjan.veljkovic

Reputation: 2548

"True" decompiling is, basically, impossible. Foremost, you can't "decompile" local names (in functions and source code files / modules). For those, you'll get something like, for int local variables: i1, i2... Of course, unless you also have debug information, which is not often the case.

Decompiling to "something" (which might not be very readable) is possible, but it usually relies on some heuristics, recognizing code patterns that compilers generate and can be fooled into generating strange (possibly even incorrect) C code. In practice that means that a decompiler usually works OK for a certain compiler with certain (default) compile options, but, not so nice with others.

Having said that, decompilers do exist and you can try your luck with, say Snowman

Upvotes: 3

thurizas
thurizas

Reputation: 2528

As Srdjan has said, in general decompilation of a C (or C++) program is not possible. There is too much information lost during the compilation process. For example consider a declaration such as int x this is 'lost' as it does not directly produce any machine level instruction. The compiler needs this information to do type checking only.

Now, however it is possible to disassembly which is taking the compiled executable back up a level to assembly language. However, interpretation of the assembly might (will ?) be difficult and certainly time consuming. There are several disassemblers available, if you have money IDA-Pro is probably the industry standard in disassemblers, and if you are doing this type work, well worth the several thousand dollars per license. There are a number of open source disassemblers available, google can find them.

Now, that being said there have been efforts to create a decompilers, IDA-Pro has one, and you can look at http://boomerang.sourceforge.net/ in addition to Snowman linked above.

Lastly, other languages are more friendly towards decompilation then C or C++. For example a C# programs is decompilable with tools like dotPeek or ilSpy. Similarly with Java there are a number of tools that can convert Java bytecode back into Java source.

Upvotes: 2

NCL
NCL

Reputation: 11

Please post a sample of the "compiled C code in text format."

Perhaps then it will be easier to see what you are trying to achieve.

Typically it is not practical to reverse engineer assembly language into C because much the human readable information in the form of Labels and variable names is permanently lost in the compilation process.

Upvotes: 0

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