Why there has an Invalid read of size 4

Question

there is a Invalid read of size 4 shows up, but i don't know how to avoid it: this is where the valgrind tells me have the invalid read

Queue* recordQueue = newQueue(NULL, NULL);
    
    FILE* file = fopen(fileName, "r");
    char* processName = malloc(8 * sizeof(char));
    assert(processName != NULL);
    unsigned int arriveTime;
    unsigned int serviceTime;
    short memoryRequirement;
    while (fscanf(file, "%d %s %d %hd", &arriveTime, processName, 
        &serviceTime, &memoryRequirement) != EOF) {
        ProcessStatus status = NOT_READY;
        ProcessInfo* processInfo = newProcessInfo(processName, arriveTime, 
            serviceTime, memoryRequirement, status);
        
        processName = malloc(8 * sizeof(char));
        assert(processName != NULL);
        inQueue(recordQueue, newNode(processInfo, NULL, NULL));
    }
    return recordQueue;

and the valgrind shows that:

==237688== Invalid read of size 4
==237688==    at 0x48C3AF2: __vfscanf_internal (vfscanf-internal.c:345)
==237688==    by 0x48C329C: __isoc99_fscanf (isoc99_fscanf.c:30)
==237688==    by 0x10A3AE: readProcessesFronFile (in /home/haozhec/project1/comp30023-2023-project-1/allocate)
==237688==    by 0x10A91D: main (in /home/haozhec/project1/comp30023-2023-project-1/allocate)
==237688==  Address 0xc0 is not stack'd, malloc'd or (recently) free'd
==237688== 
==237688== 
==237688== Process terminating with default action of signal 11 (SIGSEGV)
==237688==  Access not within mapped region at address 0xC0
==237688==    at 0x48C3AF2: __vfscanf_internal (vfscanf-internal.c:345)
==237688==    by 0x48C329C: __isoc99_fscanf (isoc99_fscanf.c:30)
==237688==    by 0x10A3AE: readProcessesFronFile (in /home/haozhec/project1/comp30023-2023-project-1/allocate)
==237688==    by 0x10A91D: main (in /home/haozhec/project1/comp30023-2023-project-1/allocate)

anybody know how to fix it? can anybody tells me how to avoid such problem when using fscanf?

Edit the code to provide a [mre]. In particular, show the input string that is scanned for `processName`. — Eric Postpischil, Apr 09 '23 at 23:33
`processName` can only hold strings of max. length of 7. Are you sure that all your strings in the second column of your file are shorter than 8 characters long? — Pablo, Apr 09 '23 at 23:35
My guess: You have a newline at the end of the input file, so the the `fscanf` will not return an EOF on the last record and thus try next a read on an empty/partial record then "over" reading file buffer . (if your process names are all less than 8 chars) — Zilog80, Apr 10 '23 at 00:12
You're reading an unbound string (`%s`) into an 8 char array. There's probably some padding, but you should put a size parameter on the string to make sure. Also remember to count the '\0'. Also, you're reading signed integers (`%d`) into unsigned integers, use `%u` instead. — John Bayko, Apr 10 '23 at 00:19
@Howard Cui, try `while (fscanf(file, "%d %s %d %hd", &arriveTime, processName, &serviceTime, &memoryRequirement) != EOF)` --> `while (fscanf(file, "%u %7s %u %hd", &arriveTime, processName, &serviceTime, &memoryRequirement) == 4)` — chux - Reinstate Monica, Apr 10 '23 at 00:34

TheEagle · Answer 1 · 2023-04-10T10:21:28.767

If the file doesn't exist for example, fopen will return a null pointer - fscanf will then try to read from the memory pointed to by file, but since it's a null pointer there's no memory there - that's an invalid read. And this is the cause for the SIGSEGV (actually the name of the signal emitted when a segmentation fault occurs).

To prevent such crashes (and give the user feedback as to why your program isn't doing what the user expects), you should check for a null pointer first before doing anything with the file pointer - so, after the fopen line add

if (!file) {
    fprintf(stderr, "Failed opening file '%s': %s\n",
            fileName, strerror(errno));
    return 0;
}

Since file is a pointer (an unsigned integer), it will evaluate to true in a boolean operator if the pointer address is non-zero. Inversely, if the pointer equals 0 (thus is a null pointer) it evaluates to false - ! inverts the boolean result and thus whenever the pointer file equals 0, the if statement will be executed, alerting the user of the situation.

Does this actually fix the problem? This reads as more a comment than an answer as currently written. — Chris, Apr 10 '23 at 01:01
@Chris: If `fopen` returns a null pointer and `fscanf` attempts to access offset 0xc0 in the `FILE` structure, that would explain Valgrind complaining about the access to address 0xc0. — Eric Postpischil, Apr 10 '23 at 01:24
@EricPostpischil, yes. I felt elaboration would make this a better answer. — Chris, Apr 10 '23 at 05:27

Why there has an Invalid read of size 4

1 Answers1