在C中的结构内指向char *的指针

Question

I have a probably stupid question... but I have defined a struct with some char* inside. Now when I try to change the values of that chars. It doesn't give problems when it's compiled, but when I execute it, the program stops.

This is a test function that I made for checking where is the problem:

struct myret {
    int age;
    char *name;
    char *affiliation_number;
};

void obtain_name_affiliation_number(struct myret *r)
{
    int age;
    char *name;
    char *affiliation_number;

    FILE *user_data;
    user_data = fopen("user_data.txt", "r");

    fscanf(user_data, "%d", &age);
    fscanf(user_data, "%s", &name);
    fscanf(user_data, "%s", &affiliation_number);

    fclose(user_data);

    r->age = age;
    r->name = name;
    r->affiliation_number = affiliation_number;

    return 0;
}

int main(void)
{
    struct myret r;
    int rc = obtain_name_affiliation_number(&r);
    if (rc == 0) {
        printf("%d %s %s\n", r.age, r.name, r.affiliation_number);
    }

    getchar();
    return 0;
}

Thanks for everything =)

Answer 1

You don't need to use & to pass the address of a char * , remove thoses & :

fscanf(user_data, "%s", &name);
                        ^
fscanf(user_data, "%s", &affiliation_number);
                        ^

Also, allocate memory for name and affiliation_number before using them by malloc for example:

char *name = malloc(100 * sizeof(char));

Answer 2

Just declaring a char * does not allocate any memory for it. You either need to specify a static size (eg char name[100] ) or need to allocate memory dynamically (eg by using malloc ). Remember to free the memory if you use malloc when you are done.

Another problem is the use of & in fscanf for char * . The variable list in scanf and fscanf after format string is a list of pointers where to place the input values. When we use type like int age we need to pass &age , as that & before age makes it a pointer to age , ie a int * . But char *name is already a pointer, so you don't need & here. By using &name you are creating a pointer to pointer or char ** which you don't want here.

Answer 3

You've allocated space for only the addresses of buffers for name and affiliation_number, but never allocated the buffers themselves for those addresses to point to.

Hence, when you use them in fscanf() , problems occur. The compiler warns you (gcc, anyway) with these notes that the pointers are the wrong type - they should be the address of the first byte in the target buffer you want fscanf to overwrite:

foo.c:19:5: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘char **’ [-Wformat]
foo.c:20:5: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘char **’ [-Wformat]

Here are some things to do instead - if your system doesn't support fscanf's "%ms" spec, try this:

• In obtain_name_affiliation, make a char buffer[1024] to read data into.
• Use buffer or &buffer[0] (for certain purists) as the targets in the string fscanfs
• Use %1023s or the like to prevent the data read from overrunning the length of the buffer - overruns can drive your program insane.
• If the fscanf returns success (for one field expected, fscanf should return the value 1, or the input may be bad), use strdup to clone the data into name or affiliation_number . This will malloc() a new memory piece sized to fit the string you read and copy the data into it.

Whether you use those steps or the "%ms" approach, the buffers need to be free() ed later to avoid memory leaks!

This is a bit simplistic (the limit of 1024 in particular), but should get you started down the right path.

Example:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

/* tested with user_data.txt containing "12 Barney 42" or "12 Barney" */

struct myret {
    int age;
    char *name;
    char *affiliation_number;
};

int obtain_name_affiliation_number(struct myret *r)
{
    int success = 0;
    int age;
    char *name = 0;
    char *affiliation_number = 0;

    FILE *user_data = fopen("user_data.txt", "r");
    if(user_data)
    {
#if 0  /* use if you have "%ms" */
        if((1 == fscanf(user_data, "%d",  &age)) &&
           (1 == fscanf(user_data, "%ms", &name)) &&
           (1 == fscanf(user_data, "%ms", &affiliation_number)))
           {
                success = 1;
           } else {
               /* a small annoyance: if only the first "%ms" succeeded,
                * we need to free it:
                */
               if(name)
                   free(name);
           }
#else
        char buffer[1024];
        /* This if-structure can be used with the "%ms" as well, and
         * would make the "annoyance" look a lot cleaner
         */
        if(1 == fscanf(user_data, "%d", &age))
        {
            if(1 == fscanf(user_data, "%1023s", buffer))
            {
                name = strdup(buffer);
                if(1 == fscanf(user_data, "%1023s", buffer))
                {
                    affiliation_number = strdup(buffer);
                    success = 1;
                }
            }
        }
#endif
        fclose(user_data);
    } else perror("error opening data file");

    if(success)
    {
        r->age = age;
        r->name = name;
        r->affiliation_number = affiliation_number;
    }
    return success;
}

int main(void)
{
    struct myret r;
    int rc = obtain_name_affiliation_number(&r);
    if(rc) {
        printf("%d %s %s\n", r.age, r.name, r.affiliation_number);
        free(r.name);
        free(r.affiliation_number);
    }
    else
        fputs("an error occurred reading data\n", stderr);
    getchar();
    return 0;
}

There are other approaches. The name and affiliation_number could be declared in the struct as char name[512]; for example, but that number is almost impossible to choose in advance with any hope of correctness. Instead, this is common:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

struct myret *myret_alloc(int age, char *name, char *affiliation_number)
{
    struct myret *r = 0;
    if(r = (struct myret*)calloc(1, sizeof(struct myret))) {
        r->age = age;
        r->name = name;  /* or use r->name = strdup(name); */
        r->affiliation_number = affiliation_number;
        /* note that using strdup would mean the calling function should
         *  do its own cleanup of its own name and affiliation_number vars
         */
    }
    return r;
}

void myret_free(struct myret *r)
{
    /* this can be called on partially-allocated myret objects */
    if(r->affiliation_number)
        free(r->affiliation_number);
    if(r->name)
        free(r->name);
    free(r);
}

Then the other two functions become (assuming fscanf can "%ms" ):

struct myret *obtain_name_affiliation_number(void)
{
    struct myret *r = (struct myret*)0;
    FILE *user_data = fopen("user_data.txt", "r");
    if(user_data)
    {
        int age;
        char *name = 0;  /* the 0 allows us to see if it was used later */
        char *affiliation_number = 0;

        if((1 == fscanf(user_data, "%d",  &age)) &&
           (1 == fscanf(user_data, "%ms", &name)) &&
           (1 == fscanf(user_data, "%ms", &affiliation_number)))
        {
            /* The name and affiliation_number were malloc()ed by "%ms"
             * so there's nothing to clean up in this function, and
             * we can let myret_free() just free those memory areas.
             * This also means myret_alloc doesn't need strdup().
             */
            r = myret_alloc(age, name, affiliation_number);
        } else {
            if(name) /* clean up name if it got allocated */
                free(name);
        }
        fclose(user_data);
    } else perror("error opening data file");
    return r;
}

int main(void)
{
    struct myret *r = obtain_name_affiliation_number();
    if(r) {
        printf("%d %s %s\n", r->age, r->name, r->affiliation_number);
        myret_free(r);
    }
    else
        fputs("an error occurred reading data\n", stderr);
    getchar();
    return 0;
}

You could also use fscanf for all three at once:

if(3 == fscanf(user_data, "%d %ms %ms", &age, &name, &affiliation_number))
      ...

Good luck!

Answer 4

First, you need to malloc memory for your data, otherwise there will be nowhere to store it:

char *affiliation_number = (char*) malloc(STRING_LENGTH * sizeof (char));

If malloc fails, it will return NULL , you have to check for this error condition:

if (!affiliation_number) { 
     // report error
}

Then, affiliation_number is already a pointer, you don't need & in:

fscanf(user_data, "%s", affiliation_number);

Same for name .

Answer 5

First at all you must use malloc(); and free(); when you're using char* a="always use malloc";

compiler uses it as const char* a

1. You must assign memory to your char* a; when a will be used as a variable string
2. Use free(); after it to avoid memory leaks