Question

I’m using Dr. Memory for memory debugging of a C program I wrote today. This program, compiled with MinGW’s gcc only works when I run it from the debugger gdb, so I’m assuming its a memory error. The results.txt file Dr. Memory returned to me is something like:

Dr. Memory version 1.5.0 build 5 builton Aug 31 2012 16:19:51
Application cmdline: ""c:\Users\Lincoln\Desktop\USACO\gift1.exe""
Recorded 63 suppression(s) from default c:\Program Files\Dr. Memory/bin/suppress-default.txt

Error #1: UNINITIALIZED READ: reading register eflags
# 0 replace_memcmp               [d:\derek\drmemory\withwiki\trunk\drmemory\replace.c:557]
# 1 parseInputs                  [c:\Users\Lincoln\Desktop\USACO/gift1.c:55]
# 2 main                         [c:\Users\Lincoln\Desktop\USACO/gift1.c:126]
Note: @0:00:00.473 in thread 3824
Note: instruction: jnz    $0x7388a607

Error #2: UNINITIALIZED READ: reading register eax
# 0 parseInputs               [c:\Users\Lincoln\Desktop\USACO/gift1.c:55]
# 1 main                      [c:\Users\Lincoln\Desktop\USACO/gift1.c:126]
Note: @0:00:00.473 in thread 3824
Note: instruction: test   %eax %eax

Error #3: UNINITIALIZED READ: reading register esi
# 0 replace_memcmp               [d:\derek\drmemory\withwiki\trunk\drmemory\replace.c:556]
# 1 parseInputs                  [c:\Users\Lincoln\Desktop\USACO/gift1.c:55]
# 2 main                         [c:\Users\Lincoln\Desktop\USACO/gift1.c:126]
Note: @0:00:00.474 in thread 3824
Note: instruction: movzx  (%esi,%ecx,1) -> %edi

Error #4: UNADDRESSABLE ACCESS: reading 0x00000004-0x00000005 1 byte(s)
# 0 replace_memcmp               [d:\derek\drmemory\withwiki\trunk\drmemory\replace.c:556]
# 1 parseInputs                  [c:\Users\Lincoln\Desktop\USACO/gift1.c:55]
# 2 main                         [c:\Users\Lincoln\Desktop\USACO/gift1.c:126]
Note: @0:00:00.474 in thread 3824
Note: instruction: movzx  (%esi,%ecx,1) -> %edi

I have no idea how to read this, or where to start trying to fix my program. What do these error messages mean, and how do I fix them?

EDIT: This is my code:

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

struct personsData {
    char * name;
    int accountMoney, receivedMoney, numAquaintances;
};

void parseInputs( FILE * fin, int NP, struct personsData * person ) {

    int i;
    for ( i = 0; i < NP; i ++ )
        fscanf( fin, "%s", person[i].name );

    char * tempName_1;
    while ( !feof( fin ) ) {

        tempName_1 = malloc ( sizeof( char ) * 15 );
        fscanf( fin, "%s", tempName_1 );

        int index = 0;
        while ( memcmp( tempName_1, person[index].name, 15 ) != 0 )
            index ++;
        free( tempName_1 );

        fscanf( fin, "%d %d", &person[index].accountMoney, &person[index].numAquaintances );

        int b;
        char * tempName_2;
        for ( b = 0; b < person[index].numAquaintances; b ++) {
            tempName_2 = malloc( sizeof( char ) * 15 );
            fscanf( fin, "%s", tempName_2 );
            i = 0;
            while ( memcmp( tempName_2, person[i].name, 15 ) != 0 )
                i ++;

            free( tempName_2 );

            person[i].receivedMoney += ( int ) floor( person[index].accountMoney / person[index].numAquaintances );
            person[index].accountMoney = floor( person[index].accountMoney / person[index].numAquaintances ) * person[index].numAquaintances;

        }
    }

}

int main() {

    FILE * fin  = fopen ("gift1.in",  "r");
    FILE * fout = fopen ("gift1.out", "w");

    int NP;
    fscanf( fin, "%d", &NP);

    struct personsData person[NP];
    int i;
    for ( i = 0; i < NP; i ++ ) {
        person[i].accountMoney = 0;
        person[i].receivedMoney = 0;
        person[i].numAquaintances = 0;
        person[i].name = ( char * ) malloc ( sizeof( char ) * 15 );
    }

    parseInputs( fin, NP, person );

    for ( i = 0; i < NP; i ++ ) {
        // print to output file (and also to console for development purposes
        fprintf( fout,  "%s %d\n", person[i].name, person[i].receivedMoney - person[i].accountMoney );
        printf(         "%s %d\n", person[i].name, person[i].receivedMoney - person[i].accountMoney );
    }

    for ( i = 0; i < NP; i ++ ) {
        free( person[i].name );
    }

    exit(0);
}

And this is the input file gift1.in:

5
dave
laura
owen
vick
amr
dave
200 3
laura
owen
vick
owen
500 1
dave
amr
150 2
vick
owen
laura
0 2
amr
vick
vick
0 0

Answer 1

Are you familiar at all with x86 assembly? If you aren’t, some of these errors will be kind of hard to explain. Either way, it’s telling you what lines of your code are involved in the tracebacks, like the first one:

# 0 replace_memcmp               [d:\derek\drmemory\withwiki\trunk\drmemory\replace.c:557]
# 1 parseInputs                  [c:\Users\Lincoln\Desktop\USACO/gift1.c:55]
# 2 main                         [c:\Users\Lincoln\Desktop\USACO/gift1.c:126]

In general, these errors appear to indicate that you’re passing uninitialized memory to memcmp() and dereferencing a NULL pointer. But, as Mitch suggests, we’ll need to see the code to say for sure.