CODEWITHSUNDEEP
cprintf
ram619
ram619

Reputation: 190

How does printf() works without argument count being explicitly mentioned

In function with variable number of arguments, the "first argument" is "total number of arguments" being passed. But in printf() we never mention argument count. So how does it get to know about the total argument list ? How does printf() works ?

Upvotes: 2

Views: 325

Answers (3)

John Bode
John Bode

Reputation: 123468

Conversion specifiers in the format string tell printf the number and types of arguments it should expect - for example, the format string "there are %d vowels in %s\n" tells printf to expect two arguments in addition to the format string, the first one being type int and the second being char *.

It's up to you to make sure the arguments match up with the format string. If you don't pass enough arguments, or the argument types don't match what the format string expects, then the behavior is undefined (most likely garbled output or a runtime error). If you pass too many arguments, the additional arguments are evaluated, but otherwise the function will operate normally as long as the format string is satisfied.

Edit

Chapter and verse

7.21.6.1 The fprintf function
...
2     The fprintf function writes output to the stream pointed to by stream, under control of the string pointed to by format that specifies how subsequent arguments are converted for output. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The fprintf function returns when the end of the format string is encountered.

Upvotes: 1

dear_tzvi
dear_tzvi

Reputation: 745

Let's look on printf declaration structure:

int printf(const char *format, ...)

format is actually the string that contains the text to be written to stdout.

The contained embedded format tags are later replaced by the values specified in subsequent additional arguments, and format is set accordingly as required.

Upvotes: 3

tofro
tofro

Reputation: 6063

You don't supply an argument count to printf - You do supply a format string, however - And that specifies how many arguments printfshould expect.

Very roughly speaking, the number of % signs in the format string is the argument count (although reality is a bit more complicated).

Upvotes: 3

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