Charles Charles - 4 months ago 42
C Question

writing formatted data of unknown length to a string (C programming)

The following C function:

int sprintf ( char * str, const char * format, ... );


writes formatted data to a string. The size of the array passed as str should be enough to contain the entire formatted string. However, what if the length of the formatted string is not known ahead of time? How can this function (or another function like it) be used write formatted data which has a length which is unknown?

For example:

#include <stdio.h>

int main ()
{
char buffer [13];
int n, a=5, b=3;
n=sprintf (buffer, "%d plus %d is %d", a, b, a+b);
printf ("[%s] is a %d char long string\n",buffer,n);
return 0;
}


The buffer needs to be 13 or greater in order for this to work. If the string length was unknown, and the buffer, for example has been set for 5, this would not work. I need something that can dynamically allocate or reallocate the buffer for strings that happen to be bigger than the buffer.

Answer

What you want is one of these two functions: * snprintf (http://libslack.org/manpages/snprintf.3.html). It takes the length of the output buffer as its second argument, and if the buffer is too small for the result it will return the number of characters needed, allowing you to reallocate a larger buffer. * asprintf. It takes a char ** argument and allocates enough memory to hold the output, as long as that much contiguous virtual memory is available. You have to call free to remove it from memory if you're done with it before the program exits and may need the memory for something else.