pointers-memory · intermediate · ~15 min

Concat three strings (no leaks)

Compute total size, allocate once, and copy — avoid the trap of growing buffers in a loop with `realloc` for no reason.

Challenge

Join three strings into one freshly allocated string, sizing the buffer correctly in a single allocation.

Task

Implement char *concat3(const char *a, const char *b, const char *c) that returns a newly allocated string equal to a followed by b followed by c. No main — the grader calls it.

Input

Three non-NULL, NUL-terminated strings a, b, c (any of them may be empty).

Output

A malloc-ed NUL-terminated string holding the concatenation. The caller frees it. Return NULL if allocation fails.

Example

concat3("foo", "-", "bar")     ->   "foo-bar"
concat3("", "", "")            ->   ""
concat3("abc", "def", "ghi")   ->   "abcdefghi"

Edge cases

All-empty inputs produce "" (still a valid allocation, just the NUL byte).

Rules

Compute the total length first and allocate once (do not grow the buffer in a loop); remember the +1 for the terminator.

Why this matters

Joining two strings into a freshly allocated buffer is the simplest non-trivial use of malloc + strlen + strcpy. It teaches you why every dynamic-string library exists.

Input format

Three non-NULL NUL-terminated strings a, b, c.

Output format

A malloc'd string a+b+c owned by the caller; NULL on allocation failure.

Constraints

Single allocation sized to the total length plus 1 for the NUL.

Starter code

#include <stdlib.h>
#include <string.h>

char *concat3(const char *a, const char *b, const char *c) {
    /* TODO */
    return NULL;
}

Common mistakes

Forgetting +1 for the NUL terminator (off-by-one buffer overflow). Using strcpy + strcat unbounded. Returning the buffer without telling the caller they own it.

Edge cases to handle

Either input is empty — concat still works. Both empty — returns a 1-byte buffer holding just the NUL.

Complexity

O(strlen(a) + strlen(b)).

Background lessons

Up next

Solve this exercise in the browser editor — compile and run against the test harness, no setup required.