sscanf behaviour / return value

Question

I'm a novice learning C and trying to understand the following code from an online lecture. It scans a string for an integer; if characters are encountered, the sscanf fails.

int n; char c;
if (sscanf(string, " %d %c", &n, &c) == 1)
    //return the integer

else
    // fail

I've read the the man pages for sscanf and am still confused about checking the return value and why this code works. They state that "These functions return the number of input items assigned".

If sscanf encounters characters only, it writes them to &c...but in that case &n won't have been written to. In this case, I would have thought that the return value of sscanf would still be 1?

Answer 1

Your sscanf(string, " %d %c") will return EOF, 0,1 or 2:

2: If your input matches the following
[Optional spaces][decimal digits¹][Optional spaces][any character][following data not read]

1: If your input failed above but matched the following
[Optional spaces][decimal digits¹][Optional spaces][no more data available]

0: If your input, after white-space and an optional sign, did not find a digit: examples: "z" or "-".

EOF: If input was empty "" or only white-space².

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¹ The decimal digits may be preceded by a single sign character: + or -.

² Or rare input error.