C语言在另一个函数中接收struct

Question

when I try to read the value of t[1].name inside equalname function, it won't work. How can I send that value over to another function?

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

struct test {
    char name[100];
    char num[100];
};

int equalname(struct test *t, char string[100]) {
    int i;
    t = malloc(sizeof(struct test)*100);
    printf("t[1].name == %s\n", t[1].name); //prints garbage(t[1].name != name)
    for (i = 0; i < 100; i++) //also, is this even right?
    {
        if (t[i].name == string) //should I use 2 for's and set t[i].name[j] == string[j]?
        {
            printf("t[i].name == name!");
            return i;
            break;
        }
    }
    printf("WRONG");
    return 0;
}

int main() {
    struct test *t;
    t = malloc(sizeof(struct test)*100);
    char name[100];
    printf("Name:\n");
    scanf("%s", name);
    strcpy(t[1].name, name);
    printf("t[1].name == %s\n", t[1].name); //this works (t[1].name == name)
    equalname(t[1].name, name);
}

Answer 1

The good news is you are on the right track -- the bad news is there are a couple of rails missing from your track...

To begin, when you allocate memory with malloc , the block of memory is reserved, but all bytes within the block remain uninitialized . Attempting to access anything within an uninitialized memory location is Undefined Behavior -- the operation of your code is no longer defined from that point forward -- it could appear to work or SegFault or anything in between.

You have two choices, (1) either loop through each struct and explicitly initialize the values, or (2) use calloc that will allocate and initialize all bytes to zero, eg

#define MAXN 100    /* if you need a constrant #define 1 (or more) */

struct test {
    char name[MAXN];
    char num[MAXN];
};
...    

int main() {

    int index;
    struct test *t;
    char name[MAXN] = "";
    t = calloc(MAXN, sizeof *t);    /* use calloc, or initialize values */

    if (t == NULL) {                /* validate every allocation */
        perror ("calloc-t");
        return 1;
    }

Do not allocate t again in equalname . While that does not overwrite your original address due to C using pass by value, and t being a copy of t from main -- it also does nothing for you accept create another uninitialized block of memory. Just pass t from main and use it, eg

int equalname (struct test *t, char *string) 
{
    int i;

    printf("t[1].name == %s\n", t[1].name);

    for (i = 0; i < MAXN; i++)
    {   /* you must use strcmp to compare strings, not == */
        if (strcmp (t[i].name, string) == 0)
        {
            printf ("t[i].name == name!\n");
            return i;
            break;
        }
    }

    printf("WRONG");
    return -1;
}

Next, you cannot compare strings with == . You either have to loop over each character and compare -- or just use strcmp (that's what it was written to do), eg

        if (strcmp (t[i].name, string) == 0)
        {
            printf ("t[i].name == name!\n");
            return i;
            break;  /* break does nothing here */
        }

Next, you must validate every allocation and every user input -- otherwise you are inviting Undefined Behavior . If you fail to validate allocations and fail to validate input -- you can have no confidence you are actually processing valid data in valid memory within your code.

Putting it altogether, you could do something like the following:

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

#define MAXN 100    /* if you need a constrant #define 1 (or more) */

struct test {
    char name[MAXN];
    char num[MAXN];
};

int equalname (struct test *t, char *string) 
{
    int i;

    printf("t[1].name == %s\n", t[1].name);

    for (i = 0; i < MAXN; i++)
    {   /* you must use strcmp to compare strings, not == */
        if (strcmp (t[i].name, string) == 0)
        {
            printf ("t[i].name == name!\n");
            return i;
            break;
        }
    }

    printf("WRONG");
    return -1;       /* on error return a value that cannot be a valid index */
}

int main() {

    int index;
    struct test *t;
    char name[MAXN] = "";
    t = calloc(MAXN, sizeof *t);    /* use calloc, or initialize values */

    if (t == NULL) {                /* validate every allocation */
        perror ("calloc-t");
        return 1;
    }

    printf ("Name: ");
    if (scanf ("%s", name) != 1) {  /* validate ALL user input */
        fprintf (stderr, "error: invalid input - name.\n");
        return 1;
    }
    strcpy (t[1].name, name);

    printf ("t[1].name == %s\n", t[1].name);
    index = equalname (t, name);    /* save the return! */

    if (index == -1) {  /* validate the operation of your function */
        fprintf (stderr, "string '%s' not found.\n", name);
        return 1;
    }

    printf ("string found at index '%d'.\n", index);

    return 0;
}

Example Use/Output

$ ./bin/struct_find_str
Name: Alfred
t[1].name == Alfred
t[1].name == Alfred
t[i].name == name!
string found at index '1'.

Let me know if you have further questions.