nmemb

nmemb is a parameter name used in several C standard library functions to represent the number of elements or members to process. It is typically paired with size, which denotes the size in bytes of each element. Together nmemb and size determine the total amount of data involved, via the product nmemb * size, and the functions usually return the number of elements successfully processed rather than bytes.

Common uses include fread, fwrite, and calloc. In fread(ptr, size, nmemb, stream), up to nmemb elements of data, each of size bytes, are read from stream into ptr. The function returns the number of elements actually read, which can be less than nmemb at end-of-file or on error. Similarly, fwrite(ptr, size, nmemb, stream) writes nmemb elements from ptr to stream and returns the number of elements written. calloc(nmemb, size) allocates memory for nmemb elements, each of size bytes, and initializes the memory to zero; it returns a pointer or NULL on failure.

Care is required to avoid overflow when computing nmemb * size, since size_t is used for the count and total byte size. If the product cannot be represented in size_t, the operation can wrap and lead to errors. Additionally, the behavior for nmemb = 0 is defined differently across functions; for fread/fwrite it typically yields 0 with no data transfer, while calloc may have implementation-defined outcomes.

The term nmemb is commonly found in C documentation and source, reflecting its longstanding role as a descriptive name for a count of elements in memory and I/O operations.