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Snprintf

As explained in Modern C

snprintf is a C standard library function that formats output into a character array subject to a specified size limit and reports the required output length.

Measure before writing

size_t need = snprintf(0, 0, "%zu", number); snprintf writes nothing when its size argument is 0, yet returns the number of characters that would have been written. A bounded call can then format into a suitably sized character array. section 14.1

snprintf formats output into a character array while limiting writes to n bytes. If the return value is at least n, the result was truncated; otherwise the buffer contains a string after a successful call. section 14.1

Use snprintf to:

  • Avoid the buffer-overflow vulnerability of sprintf when the output length is unknown. section 14.1

How Modern C covers it

How deep it goes
Explains the signature, size limit, truncation test, zero-size behavior, and length-measurement pattern, but defers full implementation details.
Examples
Uses a zero-sized call inside a loop to count formatted numbers before producing a number line.
What you can do after
Apply it to bounded formatting, detect truncation, and calculate required output length before allocating storage.
How it connects
Connects formatted output with sprintf, buffer overflow, character arrays, dynamic allocation, and string construction.
Cover of Modern C
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Modern C. The section links point to the parts of the book it draws on.