I have a trouble with pointers i think

1

So im trying to make an excersice for my university in C language! My code compiles fine but it crashes in runtime by the exception: "Exception thrown at 0x00007FFAF517D751 (ucrtbased.dll) in EceProj.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF." Can anyone please help me. Below is my code

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>

typedef struct Log {
    char*   field1;
    char*   field2;
    int     field3;
    float   field4;
    float   field5;
    float   field6;
    float   field7;
    float   field8;
    float   field9;
}log_t, *plog;

FILE* OpenFile(const char* path) 
{
    FILE* file = NULL;
    file = fopen(path, "r");
    if (file == NULL)
        printf("File cant be opened\n");
    else
        printf("File is opened\n");
    return file;
}

log_t CreateLog(FILE* file)
{
    plog log = (plog)malloc(sizeof(plog));
    fscanf(file, "%s", log->field1);
    fscanf(file, "%s", log->field2);
    fscanf(file, "%d", &(log->field3));
    fscanf(file, "%f", &(log->field4));
    fscanf(file, "%f", &(log->field5));
    fscanf(file, "%f", &(log->field6));
    fscanf(file, "%f", &(log->field7));
    fscanf(file, "%f", &(log->field8));
    fscanf(file, "%f", &(log->field9));
    return *log;
}

void PrintLog(log_t log)
{
    printf("%s", log.field1);
}

int main()
{
    FILE* file;
    file = OpenFile("DataMeteoE4.txt");
    log_t log = CreateLog(file);
    PrintLog(log);
    fclose(file);
    return 0;
}

The file that i open is this one with some logs with temperature and preasure etc.. These are the contents of the file..

2015-07-22 09:00:00 1346137 13.03 25.13 6.474 3.805 0.832 25.84
2015-07-22 09:01:00 1346138 13.03 25.15 6.5 3.84 0.834 25.89
2015-07-22 09:02:00 1346139 13.03 25.19 6.477 3.851 0.836 26.02
2015-07-22 09:03:00 1346140 13.03 25.22 6.493 3.879 0.841 26.07
2015-07-22 09:04:00 1346141 13.02 25.25 6.516 3.91 0.846 26.01
c
pointers
asked on Stack Overflow Dec 27, 2019 by TomatoPaste • edited Dec 27, 2019 by alk

1 Answer

1

For starters this function

log_t CreateLog(FILE* file)
{
    plog log = (plog)malloc(sizeof(plog));
    fscanf(file, "%s", log->field1);
    fscanf(file, "%s", log->field2);
    fscanf(file, "%d", &(log->field3));
    fscanf(file, "%f", &(log->field4));
    fscanf(file, "%f", &(log->field5));
    fscanf(file, "%f", &(log->field6));
    fscanf(file, "%f", &(log->field7));
    fscanf(file, "%f", &(log->field8));
    fscanf(file, "%f", &(log->field9));
    return *log;
}

has a memory leak because the address to the allocated memory is lost after exiting the function.

You are reading the pointer field1 from a file. Its value can be invalid for the current state of the program. So this function

void PrintLog(log_t log)
{
    printf("%s", log.field1);
}

invokes undefined behavior.

It seems the data members field1 and field2 shall be character arrays instead of pointers.

For example

typedef struct Log {
    char   field1[11];
    char   field2[9];
    //...
answered on Stack Overflow Dec 27, 2019 by Vlad from Moscow • edited Dec 27, 2019 by Vlad from Moscow

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