Valgrind: Uninitialised value was created by a heap allocation

问题

I'm getting a few Valgrind errors concerning Unitialised values and Conditional Jumps. Here is my Valgrind output

==28124== Conditional jump or move depends on uninitialised value(s)
==28124==    at 0x4C2E0E9: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==28124==    by 0x400AA7: append_character (in /home/i)
==28124==    by 0x401319: refresh_address (in /home/)
==28124==    by 0x402067: main (in /home/)
==28124==  Uninitialised value was created by a heap allocation
==28124==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==28124==    by 0x4012C0: refresh_address (in /home/)
==28124==    by 0x402067: main (in /home/)
==28124== 
==28124== Conditional jump or move depends on uninitialised value(s)
==28124==    at 0x4C2E0E9: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==28124==    by 0x400AA7: append_character (in /home/)
==28124==    by 0x40134F: refresh_address (in /home/)
==28124==    by 0x402067: main (in /home/)
==28124==  Uninitialised value was created by a heap allocation
==28124==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==28124==    by 0x4012E0: refresh_address (in /home/)
==28124==    by 0x402067: main (in /home/)
==28124== 
==28124== Conditional jump or move depends on uninitialised value(s)
==28124==    at 0x400987: binary_decimal (in /home/)
==28124==    by 0x401377: refresh_address (in /home/)
==28124==    by 0x402067: main (in /home/)
==28124==  Uninitialised value was created by a heap allocation
==28124==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==28124==    by 0x4012E0: refresh_address (in /home/)
==28124==    by 0x402067: main (in /home/)

Here is my 'append_character' function. Pretty simple stuff.

void append_character(char* str, char ch){
    int len = strlen(str) + 1;
    str[len] = ch;
    str[len + 1] = '\0';
}

And here is my 'refresh_address' function. Where the problems seem to arise.

void refresh_address(int memLength, address_info *mem, char *address){
     int j = 0;
     mem -> numSetIndexBits = calculate_set_index_bits();
     mem -> numBlockOffsetBits = calculate_block_offset_bits();
     mem -> numTagBits = calculate_num_tag_bits(memLength, mem);
     mem -> tag = malloc(mem -> numSetIndexBits * sizeof(char) + 1);
     mem -> setIndex = malloc(mem -> numSetIndexBits * sizeof(char) + 1);

     for(j = 0; j < mem -> numTagBits; ++j){
        append_character(mem -> tag, address[j]);
     }

     while (j < (mem -> numSetIndexBits + mem -> numTagBits)) {
        append_character(mem -> setIndex, address[j]);
        j++;
     }    
     mem -> decimalIndex = binary_decimal(mem -> setIndex);
}

I can't think of what I'm doing wrong. Any idea what is causing this? Thanks for the help!

Edit: address_info *mem is initialized in main with the following code where address_info is a struct.

 while(fgets(buffer, 130, stdin)){
    if(sscanf(buffer, "%c:%d:%d", &accessTypes[i], &accessSize[i], &address[i]) != EOF) {
        memory = malloc(sizeof(address_info));
        init_address_info(memory);

        if (accessTypes[i] == 'W') {
                memory -> accessType = "W";
        }
        else {
                memory -> accessType = "R";
        }
        binary_add = binary_address(address[i]);
        mem_length = strlen(binary_add);
        memory -> numSetIndexBits = calculate_set_index_bits();
        refresh_address(mem_length, memory, binary_add); /*Calls malloc for the memory's set index and tag in this function*/
        ++i;
        free(binary_add);
        free(memory);
    }
}

回答1:

One obvious thing that jumps out at me is that you allocate memory for mem->tag and then immediately start appending things to it... but you never ever initialized it (sound familiar?). This means that you are calling strlen() on the char * that you pass in, which refers to mem->tag, but it is uninitialized. That's just asking for trouble and it is very realistic that your strlen call and subsequent append of the character and then a null will exceed your allocation off of the heap.

来源：https://stackoverflow.com/questions/34101845/valgrind-uninitialised-value-was-created-by-a-heap-allocation

标签

malloc

valgrind