Repair build_label so every byte of the name#id string it returns is read from memory the function still holds, then hand that one block to the caller.

• The label is the bytes of name, then one #, then id in decimal. build_label("core", 7) returns core#7.
• Allocate exactly the bytes the label needs plus the NUL terminator. The caller owns the returned block and calls free on it once, so that block is the only allocation allowed to outlive the call.
• A negative id keeps its sign, and the - sits between the # and the digits: -1024 becomes #-1024. Zero and positive ids carry no sign.
• INT_MIN has to come out as -2147483648. Negating it as an int is undefined behavior, so build the magnitude in an unsigned int instead.
• name is copied byte for byte. A # inside name is data, not a second separator, and an empty name gives a label that starts at #.
• You have malloc and free and nothing else, so the int to decimal conversion is yours to write.
• The starter compiles and the labels it prints read correctly. Freed heap bytes usually stay intact for a while, which is exactly why the text rows can all go green while the code is wrong. The memory cell is the verdict on this one, so read it first.
• Return NULL when name is NULL, and when malloc fails. Neither path may leave anything allocated behind.

Repair build_label so every byte of the name#id string it returns is read from memory the function still holds, then hand that one block to the caller.

• The label is the bytes of name, then one #, then id in decimal. build_label("core", 7) returns core#7.
• Allocate exactly the bytes the label needs plus the NUL terminator. The caller owns the returned block and calls free on it once, so that block is the only allocation allowed to outlive the call.
• A negative id keeps its sign, and the - sits between the # and the digits: -1024 becomes #-1024. Zero and positive ids carry no sign.
• INT_MIN has to come out as -2147483648. Negating it as an int is undefined behavior, so build the magnitude in an unsigned int instead.
• name is copied byte for byte. A # inside name is data, not a second separator, and an empty name gives a label that starts at #.
• You have malloc and free and nothing else, so the int to decimal conversion is yours to write.
• The starter compiles and the labels it prints read correctly. Freed heap bytes usually stay intact for a while, which is exactly why the text rows can all go green while the code is wrong. The memory cell is the verdict on this one, so read it first.
• Return NULL when name is NULL, and when malloc fails. Neither path may leave anything allocated behind.