Segmentation Fault (core dumped) When Passing Dynamic Struct to Function

Question

I am trying to get this to work for my assignment but I keep getting a segmentation fault error but I can't see a problem.

struct line{
        int source;
        int dest;
        int type;
        int port;
        char data[51];
};


int main(){
    struct line *dataIn;
    dataIn = malloc(sizeof(struct line));
    int nRecords = 0;

    readFile(&nRecords, dataIn);

    int i=0;
    for(i = 0; 1 < 100; i++){
        printf("Source: %d Data: %s\n", dataIn[i].source, dataIn[i].data);
    }

    return 0;
}

void readFile(int *nRecords, struct line *dataIn){
    FILE *fileIn;
    fileIn =fopen("data.txt","r");
    if (!fileIn){
        puts("File Open Error");
        return 1;
    }
    while(fscanf(fileIn, "%d:%d:%d:%d:%[^\n]", &dataIn[*nRecords].source, &dataIn[*nRecords].dest, &dataIn[*nRecords].type, &dataIn[*nRecords].port, dataIn[*nRecords].data) == 5){
        nRecords++;
        dataIn = realloc(dataIn,(*nRecords+1)*sizeof(struct line));
    }
    fclose(fileIn);
}

Also when I add the function prototype at the top:

void readFile(int*, struct line*);

I get the error:

Conflicting Types for 'readFile'

You're updating nRecords but not using that in the for loop. Why? — Sourav Ghosh
– Sourav Ghosh, Commented Nov 18, 2015 at 13:30
Your function readFile return -1 if fileIn is null but is of type void. Also make sure that the function prototype is after the struct declaration. — terence hill
– terence hill, Commented Nov 18, 2015 at 13:31
Why char data[51] for data? You said you where parsing a "dynamic structure". If your input is > 51 bytes, you will scribble over the stack pointer and be unable to return from readFile(). I think you need to do two things. 1. rethink the way you manage memory as this is quite convoluted and will be hard to support. 2. Stop using any form of scanf as it's dangerous. — pedwards
– pedwards, Commented Nov 18, 2015 at 13:52

Sourav Ghosh · Accepted Answer · 2015-11-18 14:01:41Z

1

C uses pass-by-value for function parameter passing. In your code, dataIn itself is passed to readFile() using pass-by-value.

Now, dataIn being a pointer itself, you can change the content of dataIn from the function, but you cannot change dataIn itself (look at the realloc()) from the function, and expect that to get reflected back to main().

So, after returning, your dataIn has just one element, as it was malloc()ed previously. Then, the for loop obviously try to access out of bound memory, creating undefined behavior.

If you want to change dataIn from the function, you need to pass a pointer to it to the function.

That said,

nRecords being a pointer, nRecords++; will not update the corresponding int value.
The void function readFile() should not have a return statement with a value like return 1;

edited Nov 18, 2015 at 14:01

answered Nov 18, 2015 at 13:32

Sourav Ghosh

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2 Comments

Paul Ogilvie Over a year ago

Also, you update nRecords, which is a pointer. You should write *nRecords++

Sourav Ghosh Over a year ago

@PaulOgilvie Thanks, added the same. :)

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