Some system libraries like malloc strlen want or return size_t as parameter.
size_t is not a keyword by necessity. Different architectures often have different sizes for integral types. For example a 64 bit machine is likely to have an unsigned long long as size_t if they didn't decide to make int a 64 bit datatype.
size_t is commonly used for array indexing and loop counting. Programs that use other types, such as unsigned int, for array indexing may fail on, e.g. 64-bit systems when the index exceeds UINT_MAX or if it relies on 32-bit modular arithmetic.
ssize_t is not a signed size_t . It is only guaranteed to support a signed value of -1, and while it might work on Posix, it is an unsigned type on Windows.
LLVM is a compiler and a toolkit for building compilers, which are programs that convert instructions into a form that can be read and executed by a computer. The LLVM project is a collection of modular and reusable compiler and toolchain technologies.
At the LLVM level, size_t doesn't exist. It is a construct for the benefit of the developer that is typedef'd to a native type. The native types have a fixed size for the target architecture and that is how the compiler represents them in LLVM bit code. So on x86, size_t might by viewed by the front end as unsigned long, which it then writes to LLVM as i32 (since LLVM assembly doesn't have an unsigned type).
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