Copying strings in C

strcpy trusts you completely; strncpy does not always terminate. Know both traps.

#include <string.h>

void copy_bounded(char *dst, size_t cap, const char *src) {
    if (cap == 0) {
        return;
    }
    strncpy(dst, src, cap - 1);
    dst[cap - 1] = '\0';
}

size_t copy_reporting(char *dst, size_t cap, const char *src) {
    size_t needed = strlen(src);
    if (cap > 0) {
        size_t fits = (needed < cap - 1) ? needed : cap - 1;
        memcpy(dst, src, fits);
        dst[fits] = '\0';
    }
    return needed;
}

How it works

1. strcpy writes until the source's NUL, however big the target is.
2. strncpy stops at n bytes and may leave no terminator.
3. Writing the terminator yourself is the habit that saves you.

Keywords and builtins used here

• char
• const
• copy_bounded
• copy_reporting
• if
• return
• size_t
• void

The run, in numbers

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Tokens

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95 tpm

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Step 1 of 4 in Copying & building, step 1 of 13 in Strings & text.

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