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Lacking documentation

Goal unclear

OP has "I want a 'view' not a copy.", yet uses malloc to make a copy.

Lacking documentation

It appears that the function is to slice an array of char. Yet "%.*s" terminates early if a null character is found.

A helper function that exists!?

Use an auto format.

Recall size_t is an unsigned type and i - *scan_from become a large value near SIZE_MAX.

I'd expect a return types char * and not const char * as code is providing a allocated buffer for caller's use. const char *

In extreme use (int)substr_length fails when substr_length > INT_MAX. Function description would mention that. Better code would work for all substr_length. Consider memcpy() instead.

It appears that the function is to slice an array of char. Yet "%.*s" terminates early if a null character is found. This is a bug for an array function.

A helper function that exits!?

Use an auto formatter.

Recall size_t is an unsigned type and i - *scan_from can become a large value near SIZE_MAX.

I'd expect a return types char * and not const char * as code is providing a allocated buffer for caller's use.

In extreme use (int)substr_length fails when substr_length > INT_MAX. Function description would mention that. Better code would work for all substr_length. Consider memcpy() instead for an array function.

"to create a non-generic slice function. I want ....", "The scan_from arg is ... " and "Use is for ..." deserve to be in code as a comment and/or code should be clear enough to explain itself.

Array or string?

exit(ENOMEM) is surprising and undocumented. Further, ENOMEM is not part of the standard C lib and reduces portability.

// assert(substr_length == snprintf(substr, substr_length + 1, // "%.*s", (int)substr_length, s + *scan_from); int len = snprintf(substr, substr_length + 1, "%.*s", (int)substr_length, s + *scan_from); assert(len == substr_length); 

Poor for code to have a buried memory allocation yet does not advise that the caller should free it.

"to create a non-generic slice function. I want ....", "The scan_from arg is ... " and "Use is for ..." deserve to be in code as a comment and/or code should be clear enough to explain itself.

Document at a high level as if caller has access to the function declaration and not the definition. (in the .h file)

Array or string?

exit(ENOMEM) is surprising and undocumented in function description. @Mr R Further, ENOMEM is not part of the standard C lib and reduces portability.

// assert(substr_length == snprintf(substr, substr_length + 1, // "%.*s", (int)substr_length, s + *scan_from); int len = snprintf(substr, substr_length + 1, "%.*s", (int)substr_length, s + *scan_from); assert(len >= 0 && (unsigned) len == substr_length); 

const return?

I'd expect a return types char * and not const char * as code is providing a allocated buffer for caller's use. const char *

Bug

In extreme use (int)substr_length fails when substr_length > INT_MAX. Function description would mention that. Better code would work for all substr_length. Consider memcpy() instead.