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Const-safety

As explained in Modern C

Const-safety is the property of an interface that preserves read-only qualification when accepting or returning pointers to objects.

Const-safety preserves read-only qualification across an interface that accepts or returns object pointers. For example, numberline declares its input as char const, while fgetline receives writable storage. section 14.1

Read-only input versus writable input

The declaration size_t* numberline(size_t size, char const lbuf[restrict size], size_t* restrict np, int base) communicates that lbuf is read-only to the function. By contrast, fgetline receives char s[...] and writes through it before using strchr to locate and remove a newline. The library interface's lack of const-safeness must therefore be assessed against the actual mutability of the argument.

How Modern C covers it

How deep it goes
Defines the property and applies it to declarations and mutation, but defers broader interface analysis to the programmer.
Examples
Uses numberline's read-only input and fgetline's writable buffer to contrast interface promises with implementation behavior.
What you can do after
Evaluate C function interfaces by checking pointer qualification against the operations performed on their arguments.
How it connects
Ties const qualification to pointers, array parameters, restrict, string mutation, and C library interfaces.
Cover of Modern C
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Modern C. The section links point to the parts of the book it draws on.