mg_printf - return -1 if strlen(fmt) > MG_BUF_LEN

[pavel-pimenov]

http://msdn.microsoft.com/en-us/library/1kt27hek%28v=vs.80%29.aspx
vsnprintf,_vsnprintf, and _vsnwprintf return the number of characters written if the number of characters to write is less than or equal to count; if the number of characters to write is greater than count, these functions return -1 indicating that output has been truncated. The return value does not include the terminating null, if one is written.

Test code:

include <stdio.h>

include <stdarg.h>

int mg_printf(const char *fmt, ...) {
char mem[10];
int len;
va_list ap;
// Print in a local buffer first, hoping that it is large enough to
// hold the whole message
va_start(ap, fmt);
len = vsnprintf(mem, sizeof(mem), fmt, ap);
va_end(ap);
printf("len = %d", len);
return len;
}

int main()
{
mg_printf(0,"%d - long stringlong stringlong stringlong string ",123);
return 0;
}

stdout:
len = -1

Temp fix:

int mg_printf_dynamic(struct mg_connection *conn, const int len_helper, const char *fmt, ...)
{
va_list ap;
int len = -1;
int l_fly_size = len_helper;
char *l_fly_buf = (char *)malloc(l_fly_size);
if(l_fly_buf)
{
l_fly_buf[0] = 0;
va_start(ap, fmt);
while(_vsnprintf(l_fly_buf, l_fly_size, fmt, ap) == -1){
free(l_fly_buf);
l_fly_size *= 2;
l_fly_buf = (char *)malloc(l_fly_size);
if(!l_fly_buf)
return -1;
l_fly_buf[0] = 0;
}
va_end(ap);
len = mg_write(conn, l_fly_buf, (size_t) l_fly_size);
free(l_fly_buf);
}
return len;
}

[jmucchiello]

That code is not right. You should not call mg_write with l_fly_size. You should call it with the return value from _vsnprintf!

...
va_start(ap, fmt);
int actual_size;
while ((actual_size = _vsnprintf(l_fly_buf, l_fly_size, fmt, ap)) == -1) {
...
}
va_end(ap);
len = mg_write(conn, l_fly_buff, (size_t)actual_size);
...

[pavel-pimenov]

thanks!

[cpq]

That is fixed in b589e0c
I've added alloc_vprintf() function. It looks like if the buffer is 0, windows snprintf() behaves correctly.
I assume this is the case for all versions of msvcrt.dll