fread / fwrite - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Signature – size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream); size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

Semantics – element-oriented block I/O: nmemb elements of size bytes each. fread returns the number of complete elements read, which may be short on end-of-file or error; a trailing partial element is discarded, leaving the stream at the true end of file (manual, Block Input/Output). fwrite normally returns nmemb; anything less is an error (for example, out of space). Zero size or nmemb returns zero and touches nothing. The file position indicator advances by the bytes actually transferred (fread(3)).

Return and errors – fread cannot distinguish EOF from error; callers must test feof/ferror (fread(3)). errno reflects the underlying read/write failures (manual, EOF and Errors).

Edge cases – a short count from fwrite means buffered data may have been partially committed; the recovery advice in the manual (Error Recovery) is to seek to a known position rather than blindly retry. Both are cancellation points in glibc (headers).

References – man3/fread.3; glibc manual Block Input/Output.