My C program prints two different things when it shouldn't

Question

It prints one thing if its run on my computer, and a second thing if its ran on any other computer.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct hw5_struct {
    char f_name[12];
    char l_name[12];
    int age;
    float height;
}HW5_struct;

typedef struct hw5_struct_updated {
    char *f_name;
    char *l_name;
    char *address;
    int age;
    int birthday;
    float height;
}HW5_struct_updated;

void printThisFile( FILE *file, int fileSize );
void putNewStructuresInFile( FILE *newFile, int matchingFileSize );

int main( int argc, char *argv[] ) {

    printf("\n");

    const char *fileLocation = argv[1];

    FILE *originalData = fopen( fileLocation, "rb" );

    fseek( originalData, 0L, SEEK_END );
    int originalDataFileSize = ftell( originalData );
    rewind( originalData );
    printf( "The size of the original file is: %d\n", originalDataFileSize );

    HW5_struct_updated *structArray;

    printf("test\n");

    int i = 0;
    HW5_struct tempHW5struct;

    for( i = 0 ; i < originalDataFileSize ; i += sizeof( HW5_struct ) ) { 
        fseek( originalData, i, SEEK_SET );
        fread( &tempHW5struct, sizeof( HW5_struct ), 1, originalData);
        printf( "f_name: %s\nl_name: %s\nage: %d\nheight: %f\n\n", tempHW5struct.f_name, tempHW5struct.l_name, tempHW5struct.age, tempHW5struct.height );
        printf("test\n");
        structArray[ i / sizeof( HW5_struct ) ].f_name = tempHW5struct.f_name;

        //strcpy( structArray[ i / sizeof( HW5_struct ) ].f_name, tempHW5struct.f_name );

        printf("%s\n", structArray[ i / sizeof( HW5_struct ) ].f_name);
        //structArray[ i / sizeof( HW5_struct ) ].l_name = tempHW5struct.l_name;
        structArray[ i / sizeof( HW5_struct ) ].age = tempHW5struct.age;
        structArray[ i / sizeof( HW5_struct ) ].height = tempHW5struct.height;
        structArray[ i / sizeof( HW5_struct ) ].birthday = 1;
        //structArray[ i / sizeof( HW5_struct ) ].address = "8008 Mulberry Ln";
        printf("structure #%d\n was read", ( i / sizeof( HW5_struct ) ) );
    }   

    FILE *newData = fopen( "my_temp.tmp", "wb");

    for( i = 0 ; i < originalDataFileSize ; i += sizeof( HW5_struct ) ) { 
        fseek( originalData, i, SEEK_SET );
        fread( &tempHW5struct, sizeof( HW5_struct ), 1, originalData );
        printf( "f_name: %s\nl_name: %s\nage: %d\nheight: %f\n\n", tempHW5struct.f_name, tempHW5struct.l_name, tempHW5struct.age, tempHW5struct.height );
    }

    HW5_struct_updated tempHW5structUpdated;

    for( i = 0; i < originalDataFileSize ; i += sizeof( HW5_struct ) ) {
        fseek(newData, i * sizeof( HW5_struct_updated ), SEEK_SET);
        fwrite( &tempHW5structUpdated, sizeof( HW5_struct_updated ), 1, newData);
    }

    return 0;
}

void printThisFile( FILE *file, int fileSize) {
    int i = 0;
    HW5_struct tempStruct;

    for( i = 0 ; i < fileSize ; i += sizeof( HW5_struct ) ) {
        fseek( file, i, SEEK_SET );
        fread( &tempStruct, sizeof( HW5_struct ), 1, file);
        printf( "f_name: %s\nl_name: %s\nage: %d\nheight: %f\n\n", tempStruct.f_name, tempStruct.l_name, tempStruct.age, tempStruct.height );
    }
}

this is on a server so each person connects to the server, has the same .c file, compiles and runs it.

when I run it I get

The size of the original file is: 320
test
f_name: Fred
l_name: Hutcheson
age: 32
height: 6.000000

test
Segmentation fault

when I run it on my phone or when my friend runs it we get:

The size of the original file is: 320
test
f_name: Fred
l_name: cheson
age: 1920098636
height: 0.000000

test
Segmentation fault

It is like this with other programs that do simpler things as well, like a program that just prints("Hello World\n") will literally do nothing when I run it and will print normally on every other computer, no idea what's going on.

Answer 1

You never allocate memory for structArray, or make it point anywhere valid. It is uninitialized and will point to an indeterminate (and seemingly random) location.

Then when you dereference it to write to some seemingly random location you will have undefined behavior.

Since you program in C you could use variable-length arrays like

HW5_struct_updated structArray[originalDataFileSize  / sizeof(HW5_struct)];