In "printf" and "scanf", the "f" at the end means "formatted input/output". This is because these functions perform format conversion: they can handle integers, floats, characters, strings, and pointers, and they know how to convert between data types and character data accordingly. This contrasts with, say, getchar, which only has the ability to fetch one character at a time, and cannot interpret input data as numeric values or pointers or anything like that. It is an unformatted I/O function.

In "fflush", "fopen", "fread", "fwrite", and so on, the "f" at the beginning means "file". Such functions expect a FILE* as an argument and operate on the corresponding stream. In some cases, this distinguishes them from functions that assume standard input or standard output: getchar versus fgetc, gets versus fgets. In other cases it distinguishes them from Unix functions that operate on file descriptors (which are integers) rather than C FILE* streams: fopen versus open, fwrite versus write, and so on. Note that this can be combined with the suffix "f": fprintf means "formatted output to a FILE*".

Finally, all the math functions in math.h come in three versions: one for floats, one for doubles, and one for long doubles. Hence, for example, cosf, cos, cosl. The "f" in this case means the argument is of type float, and the suffixes are needed because functions cannot be overloaded in C (but see type-generic expressions, introduced in C11).

As a special case, the "f" in fabs, which operates on floating point types, distinguishes it from abs, which operates on integer types.