Printf and fprintf prints only first argument

Question

I have a problem I can't grasp with printf. It's the first time ever I have this problem, so I'm sure it's something naive, but no matter what, I can't solve it myself... maybe it's just because I'm tired: fprintf (and i've found it's true also for printf) correctly prints only the first argument, from the second it will print only "0" for numbers and "(null)" for strings

Here's the relevant code:

#include <math.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void printInputStream(MatePair* inputStream, char* leftFile, char* rightFile){

    MatePair* iterator = inputStream;
    FILE* outLeft = fopen(leftFile, "w");
    FILE* outRight = fopen(rightFile, "w");


    while (iterator->leftRow != MATEPAIR_STOP){

        fprintf(outLeft, "%d: \n", iterator->leftRow);
        fprintf(outLeft, "%s \n", iterator->leftDNA);
        fprintf(outLeft, "%d: %s \n", iterator->leftRow, iterator->leftDNA);

        iterator++;
    }

    fclose(outLeft);
    fclose(outRight);

}

Here's the beginning of the output:

48: 
NAATAGACCTATATCCTGTACCCAAACAGAAGACAGAGGATTAACCAAACTCTT 
48: (null) 
44: 
NTAGCCATCTTAGACACATGAATATCTTGGGTCACAACTCATACCTCAACAAAA 
44: (null) 
40: 
NAAAATAAGGGGTATACTCGCTTCGGGGCCCCATTTGGCCTCCAAAAGGGGGCG 
40: (null) 
36: 
NTCTATCTTGCTCGAGAGAAAGGGTTGCCTTAGGGTTTTTTGGGGGGGGCTGTA 
36: (null) 
32: 
NCTATAGAAATTTCCCATACCAACTAGACATTTATCTTCCTGTTTTTTTCCGCC 
32: (null) 

As you can see I print every member of the array twice: once per argument and both arguments together. The data is fine, in fact with the first method it's all ok, with the second one only the first argument is printed. Any idea? Thanks in advance

Answer 1

Does the following line, with a cast, "work"?

fprintf(outLeft, "%d: %s \n", (int)iterator->leftRow, iterator->leftDNA);

I suspect iterator->leftRow is not of int type (or some smaller type that gets converted to int automagically). If I'm right you invoke Undefined Behaviour; in the 1st case (the separate statements) there's no apparent "misbehaviour" (bad luck), in the 2nd case the "misbehaviour" is to print "(NULL)".