In order to use C function, you only have to declare it in SML# in the following syntax.

val $id$ = _import "$symbol$" : $type$

$symbol$ is the name of the C function. $type$ is its type. Next section explain how to write the type of a C function.

This declaration instructs SML# compiler to link the named function and bind the SML# variable $id$ to that function. A target function linked by this declaration can be any code as far as it is in a standard calling convention of the OS in which SML# runs. The linking to the function is performed at linking time. So, to produce an executable file of SML# program containing this _import declaration, it is required to specify either a library or an object file to the command line of SML# command to link it with the SML# program. Some of standard C libraries (including libc and libm in Unix family OS) are linked by default.

This declaration can appear whenever val declaration is allied. After this declaration, variable $id$ can be used as an ordinary variable defined in SML#.

As an example, consider the standard C library function.

int puts(char *);

This function takes a string, appends a newline code and outputs it to the standard output, and returns the number of characters actually printed. If printing fails, then it returns the integer representing EOF (which is $-1$ in Linux). This function can be used by writing the following declarations.

val puts = _import "puts" : string -> int

As seen in this example, C function can bound and used just by writing _import keyword followed by the name and the type of the desired function. The following is interactive session using puts.

# val puts = _import "puts" : string -> int;
val puts = _ : string -> int
# puts "My first call to a C library";
My first call to a C library
val it = 29 : int
# map puts ["I","became","fully","operational","in","April","6th","2012."];
I
became
fully
operational
in
April
2nd
2012.
val it = [2, 7, 6, 12, 3, 6, 4, 5] : int list

The imported C functions can be freely used according to the ML’s programming principle – “expressions are freely composed as far as they are type consistent”.