Write str_join, which allocates one new string holding the n strings of parts separated by the byte sep, and hands that block to the caller.

• parts points to n strings, each NUL-terminated. sep goes between two neighbours, never before the first part, never after the last one. Joining 4 parts writes 3 separators.
• Measure, then allocate. Sum the lengths of the n strings, add n - 1 separators when n is at least 1, add one byte for the terminator you write at the end, and call malloc once with that total. A second pass copies the bytes in.
• The block belongs to the caller. Every test calls free on what you return, so it has to come from malloc. Returning "" is not an allocated empty string, and freeing a literal is a crash.
• Leave parts alone. Neither the array of pointers nor the strings it points to are yours to write into. Every test that has at least one part prints those parts back after the call.
• An n of 0 returns an allocated empty string, never NULL. parts itself can be NULL in that case, so nothing there is safe to read.
• Watch the n - 1 when n is 0. On a size_t it wraps to SIZE_MAX instead of going negative, your + 1 for the terminator brings the total back to 0, malloc(0) hands you a block with no room in it, and the terminator you write lands one byte past the end.
• An empty string inside parts contributes zero bytes of text and still gets its separators: "", "b", "" joined with , gives ,b,.
• A part that already contains sep is copied byte for byte like any other. There is nothing to escape and nothing to collapse.
• sep can be the NUL byte. Write it like any other byte. The result then carries NUL bytes in the middle plus the one at the end, and printf("%s", ...) on it will only show the first part. Tests render those interior NUL bytes as ..
• You have malloc and nothing else. Write your own length measure and your own copy loop.
• If malloc fails, return NULL.
• Grademe replays every test under a memory checker and that cell is part of the verdict. A single byte of under-allocation can leave the text output green while the memory cell goes red, and a red memory cell fails the run.

Write str_join, which allocates one new string holding the n strings of parts separated by the byte sep, and hands that block to the caller.

• parts points to n strings, each NUL-terminated. sep goes between two neighbours, never before the first part, never after the last one. Joining 4 parts writes 3 separators.
• Measure, then allocate. Sum the lengths of the n strings, add n - 1 separators when n is at least 1, add one byte for the terminator you write at the end, and call malloc once with that total. A second pass copies the bytes in.
• The block belongs to the caller. Every test calls free on what you return, so it has to come from malloc. Returning "" is not an allocated empty string, and freeing a literal is a crash.
• Leave parts alone. Neither the array of pointers nor the strings it points to are yours to write into. Every test that has at least one part prints those parts back after the call.
• An n of 0 returns an allocated empty string, never NULL. parts itself can be NULL in that case, so nothing there is safe to read.
• Watch the n - 1 when n is 0. On a size_t it wraps to SIZE_MAX instead of going negative, your + 1 for the terminator brings the total back to 0, malloc(0) hands you a block with no room in it, and the terminator you write lands one byte past the end.
• An empty string inside parts contributes zero bytes of text and still gets its separators: "", "b", "" joined with , gives ,b,.
• A part that already contains sep is copied byte for byte like any other. There is nothing to escape and nothing to collapse.
• sep can be the NUL byte. Write it like any other byte. The result then carries NUL bytes in the middle plus the one at the end, and printf("%s", ...) on it will only show the first part. Tests render those interior NUL bytes as ..
• You have malloc and nothing else. Write your own length measure and your own copy loop.
• If malloc fails, return NULL.
• Grademe replays every test under a memory checker and that cell is part of the verdict. A single byte of under-allocation can leave the text output green while the memory cell goes red, and a red memory cell fails the run.