snprintf is the safe builder in C

snprintf bounds the write and tells you what the full length would have been.

#include <stdio.h>

int build_label(char *out, size_t cap, const char *lang, int tpm) {
    int needed = snprintf(out, cap, "%s at %d tpm", lang, tpm);
    if (needed < 0) {
        return -1;
    }
    return ((size_t) needed >= cap) ? 0 : 1;
}

void show(void) {
    char small[10];
    char roomy[64];

    printf("%d %s\n", build_label(small, sizeof small, "rust", 104), small);
    printf("%d %s\n", build_label(roomy, sizeof roomy, "rust", 104), roomy);
}

How it works

1. It always terminates, as long as the size is non-zero.
2. The return value is the length it wanted, so it detects truncation.
3. That makes it the right tool for building any string.

Keywords and builtins used here

• build_label
• char
• const
• if
• int
• return
• show
• size_t
• sizeof
• void

The run, in numbers

Lines

17

Characters to type

421

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 2 of 4 in Copying & building, step 2 of 13 in Strings & text.

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