Concatenation in C

strcat appends at the existing terminator, so the target must have room for both.

#include <string.h>

void join_three(char *out, size_t cap, const char *a, const char *b,
                const char *c) {
    if (cap == 0) {
        return;
    }
    out[0] = '\0';

    size_t used = 0;
    const char *parts[3] = {a, b, c};
    for (int i = 0; i < 3; i++) {
        size_t n = strlen(parts[i]);
        if (used + n + 1 > cap) {
            break;
        }
        memcpy(out + used, parts[i], n + 1);
        used += n;
    }
}

How it works

1. The destination needs space for its own length plus the source plus one.
2. Repeated strcat rescans from the start each time, which is quadratic.
3. Tracking the end yourself turns it back into linear work.

Keywords and builtins used here

• break
• char
• const
• for
• if
• int
• join_three
• return
• size_t
• void

The run, in numbers

Lines

20

Characters to type

345

Tokens

134

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 85 seconds.

Type this snippet

Step 3 of 4 in Copying & building, step 3 of 13 in Strings & text.

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