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based on an earlier version of the Wolfram Language.
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MathLink C Function

MLTestHead()

int MLTestHead(MLINK link, const char *head, int *n)
tests that the next object to be read from link is an expression with head head, and stores the number of arguments of the expression in n.
  • MLTestHead() fails if the current object on the link is not a function with a symbol as a head, or if the name of the symbol does not match head.
  • MLTestHead() returns 0 in the event of an error, and a nonzero value if the function succeeds.
  • Use MLError() to retrieve the error code if MLTestHead() fails.
  • MLTestHead() is declared in the MathLink header file mathlink.h.
#include "mathlink.h"

/* test whether the head of the incoming object is "ReturnPacket" */

void f(MLINK lp)
{
    int args;

    if(MLTestHead(lp, "ReturnPacket", &args))
        { /* read the contents of the ReturnPacket[] */ }
    else
        { /* the head of incoming packet is not "ReturnPacket" */ }
}