How can I use the va_list correctly?

Question

I want to have something like a cross platform snprintf function, so I'm trying to use this (perhaps there are other solutions, but I'm wondering exactly that):

void string_print(char *str, size_t size, const char *format, ...) {
    va_list args;
    va_start(args, format);
#ifdef _WIN32
    sprintf_s(str, size, format, args);
#else
    snprintf(str, size, format, args);
#endif
    va_end(args);
}

Example of usage:

// timeStepNumber == 1
char fileName[40];  
string_print(fileName, 40, "Flow%d.dat", timeStepNumber);

But in this case I have fileName == "Flow-14843.dat", although va_arg(args, int) == 1. Can anybody explain, what maybe wrong in the string_print function?

I assume you have verified that 'timeStepNumber == 1' by displaying the value prior to calling 'string_print()' ? — Mahonri Moriancumer
– Mahonri Moriancumer, Commented May 10, 2014 at 7:11
@MahonriMoriancumer Yes - prior to calling string_print, and also inside (before an actual calling snprintf) — erthalion
– erthalion, Commented May 10, 2014 at 7:14
why have you elected to use 'snprintf()' instead of using 'int vsnprintf(char *str, size_t size, const char *format, va_list ap);' ? — Mahonri Moriancumer
– Mahonri Moriancumer, Commented May 10, 2014 at 7:18
because of "copy-paste" =) snprintf was used originally in code - looks like it's an error I'm looking for. — erthalion
– erthalion, Commented May 10, 2014 at 7:23

imbtfab · Accepted Answer · 2014-05-10 07:16:34Z

1

You need to use vsnprintf/vsnprintf_s functions with vararg lists.

vsnprintf(str, size, format, args);

answered May 10, 2014 at 7:16

imbtfab

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Comments

Mahonri Moriancumer · Accepted Answer · 2017-08-09 18:15:37Z

1

As indicated by imbtfab, use vsnprintf() in place of snprintf() and _vsnprintf() in place of sprintf_s.

edited Aug 9, 2017 at 18:15

user21926

answered May 10, 2014 at 7:25

Mahonri Moriancumer

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