Confusion about how the sprintf function works in C

Question

I'm confused because I thought the sprintf function returned a string, and it shows on cplusplus.com that it returns an int? Why?

Basically, I'm having trouble with the following line, where I'm trying to pad some spacing and format a string simultaneously:

printf("%30s", sprintf("1.10f", modeTimeTotal/num_tests));

There error I get is:

cannot convert 'double' to 'const char*' for argument '2' to 'int sprintf(char*, const char*, ...)'

Check this post out: http://stackoverflow.com/questions/4627330/difference-between-fprintf-printf-sprintf — , Sep 12 '13 at 15:10
*"Confusion about how the sprintf function works in C"* **Why the `C++` tag?** — Manu343726, Sep 12 '13 at 15:11

score 5 · Answer 1 · answered Sep 12 '13 at 15:10

sprintf prints into a string an returns the number of characters so printed. The first argument to the sprintf call should be the string to print into:

char buffer[30];
int n = sprintf(buffer, "%1.10f", modeTimeTotal/num_tests);
printf("Printed %d characters.  String is '%s'\n", n, buffer);

You're getting the warning you are because you're trying to shoehorn the modeTimeTotal/num_tests into the format string argument to sprintf.

score 0 · Answer 2 · answered Sep 12 '13 at 15:10

0

sprintf() returns the number of characters printed (not including the NUL terminator byte).

You should use it like this:

char buf[512];
sprintf(buf, "%30s%1.10f", "", modeTimeTotal/num_tests);
printf("%s\n", buf);

answered Sep 12 '13 at 15:10

Jeremy Friesner

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[sprintf](http://www.cplusplus.com/reference/cstdio/sprintf/) "Return Value On success, the total number of characters written is returned. This count does not include the additional null-character automatically appended at the end of the string. On failure, a negative number is returned." – Jongware Sep 12 '13 at 15:12

Confusion about how the sprintf function works in C

2 Answers2