printf返回什么？

Question

Today in my interview, the interviewer asked: printf is a function and every function returns something; int, void, float, etc. Now what does printf return as it's a function?

Answer 1

int . On success, the total number of characters written is returned. On failure, a negative number is returned.

See reference here

Answer 2

Not every function returns something, which is indicated by using void :

void function_returns_nothing(void);

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printf is a function (declared in <stdio.h> ) and it returns an int , which is the number of characters outputted. If an error occurs, the number is negative.

Answer 3

printf() 's reference from MSDN :

Returns the number of characters printed, or a negative value if an error occurs.

Answer 4

To add a detail refinement to other fine answers:

printf() returns an int , yet does that indicate transmitted vs. printed/written characters?

The printf function returns the number of characters transmitted , or a negative value if an output or encoding error occurred. C11dr §7.21.6.3 3 (my emphasis)

On success, the number transmitted is returned. stdout is typically buffered , so the number of characters printed may not be realized or fail until later.

When int printf() has trouble for various reasons, it returns a negative number. The number of characters transmitted is not known.

If a following successful fflush(stdout) occurs, then the non-negative value from printf() is certainly the number printed .

int transmitted = printf(......);
int flush_retval = fflush(stdout);

int number_certainly_printed = -1; // Unknown
if (transmitted >= 0 && flush_retval == 0) {
  number_certainly_printed = transmitted;
}

Note that "printing" a '\\n' typically flushes stdout , but even that action is not specified.
What are the rules of automatic flushing stdout buffer in C?