Format strings into a expanding buffer

README.md

sbprintf

Format strings into a expanding buffer.

Sample

struct sbprintf_string_buffer sb = { NULL, NULL, 0 };

sbprintf( &sb, "%d: %s\n", 1, "one" );
sbprintf( &sb, "%d: %s\n", 2, "two" );
sbprintf( &sb, "%d: %s\n", 3, "tree" );
sbprintf( &sb, "%d: %s\n", 4, "four" );
sbprintf( &sb, "%d: %s\n", 5, "five" );

printf( "%s", sb.base );
free( sb.base );

Strategies

You may either call sbprintf() with a NULL'ed sbprintf_string_buffer structure (see sample above) and have it (re)allocate memory on every call or you may allocate a buffer of reasonable size beforehand:

struct sbprintf_string_buffer sb;

sb.length = 1024;
sb.base = sb.cursor = malloc( sb.length );

The first option is more memory efficient, the latter may perform better according on how many additional realloc()'s are required.

sbprintf.c

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

#include "sbprintf.h"

/**
 * Format data and automatically allocate memory to be of sufficient length
 *
 * @param sb - string buffer structure
 * @param format - printf format description
 * @param ... - arguments
 */
int sbprintf( struct sbprintf_string_buffer *sb, const char *format, ... )
{
	while( sb )
	{
		va_list ap;
		int pos = sb->cursor-sb->base;
		int left = sb->length-pos;
		int written;
		char *p;

		va_start( ap, format );
		written = vsnprintf( sb->cursor, left, format, ap );
		va_end( ap );

		if( written < 0 )
			break;

		if( sb->cursor &&
			written < left )
		{
			sb->cursor += written;
			return written;
		}

		left = pos+written+1;

		if( !(p = (char *)realloc( sb->base, left )) )
			break;

		sb->base = p;
		sb->cursor = sb->base+pos;
		sb->length = left;
	}

	return -1;
}

sbprintf.h

#ifndef _sbprintf_h_
#define _sbprintf_h_

struct sbprintf_string_buffer
{
	char *base;
	char *cursor;
	int length;
};

int sbprintf( struct sbprintf_string_buffer *, const char *, ... );

#endif