Getting sizeof variable number of arguments

Question

I want bug variable to be dynamic and depending upon sizeof(cmd+args) passed to it

void excmd(const char* cmd, ...) { char buf[100]; // I want it to be dynamic like size of(cmd+args) va_list args; va_start(args,cmd); vsnprintf(buf,sizeof(buf),cmd,args); system(buf); va_end(args); }

This looks like an XY problem. What actual problem are you trying to solve? — Andrew Henle
– Andrew Henle, Commented Feb 13, 2019 at 14:03
I recommend e.g. this vprintf (and family) reference. Pay close attention to what the vsnprintf function returns, especially if the buffer is a null pointer and the size is zero. — Some programmer dude
– Some programmer dude, Commented Feb 13, 2019 at 14:03
I need buf variable to be Dynamic it's hard to guess how long user gives the input string like a path to a file or something — XeXkek
– XeXkek, Commented Feb 13, 2019 at 14:05
@XeXkek Some programmer dude's comment is almost the answer you're looking for. — Jabberwocky
– Jabberwocky, Commented Feb 13, 2019 at 14:06
When I use char *but, and use vsprintf to not include passing sizeof buffer than program crashes and sends signal SIGSEGV — XeXkek
– XeXkek, Commented Feb 13, 2019 at 14:12

Jabberwocky · Accepted Answer · 2019-02-13 15:09:46Z

You probably want something like this:

void excmd(const char* cmd, ...) { char *buf = NULL; va_list args; va_start(args, cmd); int sizeneeded = vsnprintf(buf, 0, cmd, args) + 1; va_end(args); buf = malloc(sizeneeded); va_start(args, cmd); vsnprintf(buf, sizeneeded, cmd, args); va_end(args); system(buf); free(buf); }

or

void excmd(const char* cmd, ...) { va_list args; va_start(args, cmd); int sizeneeded = vsnprintf(NULL, 0, cmd, args) + 1; va_end(args); char buf[sizeneeded]; va_start(args, cmd); vsnprintf(buf, sizeneeded, cmd, args); va_end(args); system(buf); }

Nice one but one correction that's this should be int sizeneeded = vsnprintf(NULL, 0, cmd, args)+1; to also reserve space for terminator
@XeXkek you're right, answer edited. You can remove the comments. Thanks.
As an optimization, you can make the first call to vsnprintf() with a fixed-size buffer, something like 512, 1024, or maybe even 4096 bytes. That way, if it does fit, you don't have to allocate memory and make the vsnprintf() call a second time. You might as well use the results of the first call if you can do so without doing it again.

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