Man page 9 : sprintf

sprintf - Format a string and place it in a buffer

int sprintf(char *  buf , const char *  fmt , ... );

buf
    The buffer to place the result into

fmt
    The format string to use @...: Arguments for the format string

...
    variable arguments

The function returns the number of characters written into buf. Use snprintf or scnprintf in order to avoid buffer overflows.

See the vsnprintf documentation for format string extensions over C99.