variable argument function in C

Question

I have a variable list function

/* vsprintf example */
 #include <stdio.h>
 #include <stdarg.h>

 void PrintFError (char * format, ...)
 {
    char buffer[50];
    va_list args;
    va_start (args, format);
    vsprintf (buffer,format, args);
    perror (buffer);
    va_end (args);
 }

 int main ()
 {
     FILE * pFile;
     char szFileName[]="myfile.txt";
     int firstchar = (int) '#';

     pFile = fopen (szFileName,"r");
     if (pFile == NULL)
        PrintFError ("Error opening '%s'",szFileName);
     else
     {
        // file successfully open
        fclose (pFile);
     }
     return 0;
 }

In above example how can we check the received message in "PrintError" function that we are not crossing the buffer size in this example 50 while using "vsprintf" in above example. This should be achieved in portable way.

If you need to just print the message (and not save it in some variables) use vprintf (or vfprintf if you want to print on standard error or other files) instead of vsprintf: it will directly print the error, instead of saving it in a variable. — peoro
– peoro, Commented Jan 13, 2011 at 15:09
And note that if vsprintf sets errno, then your error message will be inappropriate. — William Pursell
– William Pursell, Commented Jan 13, 2011 at 15:10
Why put the C++ tag on this question? C and C++ are completely different languages. — Binary Worrier
– Binary Worrier, Commented Jan 13, 2011 at 15:11
@Binary Worrier: Because such functions are valid in C++, although they're much less necessary and generally discouraged. — David Thornley
– David Thornley, Commented Jan 13, 2011 at 15:14

Comrade Yeti · Accepted Answer · 2011-01-13 15:08:10Z

4

You should use the safer vsnprintf, and limit it to 50 characters maximum.

int vsnprintf(char *str, size_t size, const char *format, va_list args);

answered Jan 13, 2011 at 15:08

Comrade Yeti

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Comments

plinth · Accepted Answer · 2011-01-13 15:07:31Z

1

You are correct to worry about buffer overflow. You can't do this with vsprintf, but you can with vsnprintf, which includes an argument which is the length of the buffer.

answered Jan 13, 2011 at 15:07

plinth

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Comments

makes · Accepted Answer · 2011-01-13 15:09:00Z

0

Use vsnprintf(). It allows you to specify the number of characters to output (n):

int vsnprintf(char *s, size_t n, const char *format, va_list ap);

answered Jan 13, 2011 at 15:09

makes

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Comments

bdonlan · Accepted Answer · 2011-01-13 15:23:44Z

0

You can use vsnprintf. This is, strictly speaking, non-standard unless you have a C99 compiler, but is supported on most environments. If you do not have an implementation of vsnprintf on your platform, you can simply add a portable implementation to your program.

edited Jan 13, 2011 at 15:23

answered Jan 13, 2011 at 15:07

bdonlan

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3 Comments

venkysmarty Over a year ago

is it posisble to do in portable way

Steve M Over a year ago

C99 is a ratified standard, so vsnprintf is, strictly speaking, standard.

bdonlan Over a year ago

@Steve, I suppose, but the kinds of compilers that, in C89, would not have implemented vsnprintf, probably haven't gotten around to implementing C99 today.

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