C：使用 strcpy 将字符串分配给结构数组导致的段错误

Question

I am reading in a text file containing 10 rows and 3 columns. Each element needs to be put into an array for further manipulation. The fileToArray method will single out each string and i can successfully print each string individually however I cannot seem to assign the strings to each index correctly.

My understanding is that i have a pointer to 10 dataArrays and in my switch statement what essentially happens is dataArray[0].pName = P1... dataArray[1].pName = P2... etc... and similarly for the other variables in the struct.

The text file has a max of 10 rows hence why 10 arrays are initialized.

Is my understanding flawed or is there some blatant errors in my code?

struct Processes
{
    char *pName;
    char *arvTime;
    char *srvTime;
};

void fileToArray(FILE *fp, struct Processes dataArray[])
{
    // temp[14] because 14 is the max size a line can be
    char temp[14];
    char delim[] = " \n";
    int count = 0;
    int a, b, c = 0;

    while(fgets(temp, 14, fp) != NULL)
    {
        char *ptr = strtok(temp, delim);

        while(ptr != NULL)
        {
            // printf("'%s'\n", ptr);
            // ^^^^ This line will successfully print out each string

            switch(count)
            {
            case 0:
                dataArray[a].pName = malloc(strlen(ptr + 1));
                strcpy(dataArray[a].pName, ptr);
                a++;
                count++;
                free(dataArray[a].pName);
                break;

            case 1:
                dataArray[b].arvTime = malloc(strlen(ptr + 1));
                strcpy(dataArray[b].arvTime, ptr);
                b++;
                count++;
                free(dataArray[b].arvTime);
                break;

            case 2:
                dataArray[c].srvTime = malloc(strlen(ptr + 1));
                strcpy(dataArray[c].srvTime, ptr);
                c++;
                count = 0;
                free(dataArray[c].srvTime);
                break;
            }


            ptr = strtok(NULL, delim);

        }
    }
}

int main(int argc, void *argv[])
{
    struct Processes dataArray[10];

    if(argc == 1)
    {
        FILE *fp;

        fp = fopen("process-data.txt", "r");

        fileToArray(fp, dataArray);

        fclose(fp);
    }
    return 0;
}

// Sample text file being read //
P1 1 5
P2 2 2
P3 11 5
P4 17 9
P5 3 1
P6 10 10
P7 4 3
P8 4 1
P9 7 8
P10 5 4

Running this code results in a segfault and i am unsure as to why. Looking at other similar posts the solution seems to be not using malloc() which makes me think i have implemented the function incorrectly.

Answer 1

This bit of code is doing the exact opposite of what you think

malloc(strlen(ptr + 1))

Instead of allocating enough space for the string plus 1 extra for the NUL at the end, it is allocating one less than the length of the string. You want to move the + 1 to the outside of call to strlen like this

malloc(strlen(ptr)+1)

Also you should initialise the values of a and b to 0 or better yet, combine a , b and c into one variable and only increment it when you've read in the 3rd piece of information.

Answer 2

For starters you should to pass to the function the number of elements in the array. So the function declaration should look like

void fileToArray( struct Processes dataArray[], size_t n, FILE *fp );

And it is better when the function returns the number of the filled elements of the array.

size_t fileToArray( struct Processes dataArray[], size_t n, FILE *fp );

and the function can be called at least like

size_t n = fileToArray( dataArray, 10, fp );

In this declaration

int a, b, c = 0;

only the variable c is initialized. All other variables, a, and b, are not initialized. So for example this statement

dataArray[a].pName = malloc(strlen(ptr + 1));

results in undefined behavior.

This expression

malloc(strlen(ptr + 1))

is invalid. It is equivalent to

malloc(strlen( &ptr[1] ))

And there is no any sense to free memory at once after its allocation.

free(dataArray[a].pName);

The function could be defined the following way

size_t fileToArray( struct Processes dataArray[], size_t n, FILE *fp )
{
    // temp[14] because 14 is the max size a line can be
    char temp[14];
    char delim[] = " \n";

    size_t i = 0;

    for( ; i < n && fgets( temp, sizeof( temp ), fp) != NULL; i++ )
    {
        dataArray[i].pName   = NULL;
        dataArray[i].arvTime = NULL;
        dataArray[i].srvTime = NULL;

        char *ptr = strtok( temp, delim );

        for( size_t j = 0; j < 3 && ptr != NULL; j++ )
        {
            switch ( j )
            {
            case 0:
                dataArray[i].pName = malloc( strlen( ptr ) + 1 );
                strcpy( dataArray[i].pName, ptr );
                break;

            case 1:
                dataArray[i].arvTime = malloc( strlen( ptr ) + 1 );
                strcpy( dataArray[i].arvTime, ptr );
                break;

            case 2:
                dataArray[i].srvTime = malloc( strlen( ptr ) + 1 );
                strcpy( dataArray[i].srvTime, ptr );
                break;
            }

            ptr = strtok( NULL, delim );
        }
    }

    return i;
}