Why does fgets accept an int instead of a size_t?

Question

Functions such as strcpy(), malloc(), strlen() and various others accept their arguments or return values as a size_t instead of a int or an unsigned int for obvious reasons.

Some file functions such as fread() and fwrite() use size_t as well. By extension, it would be expected that char* fgets (char *str, int num, FILE *stream) should use a size_t and not an int as argument for its buffer size.

However, fgets() uses an int. Is there any objective explanation why?

Answer 1

The original K&R defined fgets() on p.155 with an int argument. The code presented in the book would have worked with an unsigned int as well (it uses a >0, but the loop is written so to never go below zero).

size_t got introduced later, in C89 (ANSI C), as the type of sizeof(). As this feature was specifically introduced for harmonizing memory allocation, memory management functions and string functions were updated accordingly. But file I/O wasn't: the only file functions that used size_t in C89 are those new ones introduced by C89 and did not exist in K&R such as for example fread()/fwrite(). Yes, K&R didn't have these functions and relied for bloc operations only on (non portable) unix read/write functions using file descriptors.

It shall be noted that the POSIX standard, which has harmonized the unix functions, was developed in parallel to the ANSI C standard and issued late 1988. This standard has harmonized many unix functions to use size_t so that read()/write() nowadays are defined with size_t. But for the C standard library functions such as fgets(), POSIX gives precedence to the C standard (wording of the current version of the standard):

The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional.

So in POSIX also, ironically, fgets() still inherited from its historical K&R int.

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Edit: additional reading

• Going from K&R to Ansi C - Dr.Dobb's - 1989:

stdio.h: This header defines and prototypes most of the functions listed in Chapter 7 of K&R. Few, if any, changes were made in the definitions found in K&R but several new functions have been added.