Array of Structure

[manavi18]

This code runs, but doesnt prints the second string in the structure properly. Moreover if the size is increased from 2 to 3, it gives segmentation fault. Any help will be greatly appreciated. :)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
typedef struct {
  char *name;
  int   number;
} MyStr;
 
typedef MyStr* MyStrPointer;
 
int GetNumberOfMyStrs(void);
MyStr * GetNextMyStr(void);
char * GetBuffer(MyStrPointer *MyStrPtr, int *size);
 
 
int main()
{
  char *str;
  MyStrPointer MyStrPtr;
  int numseq;
  int i;
 
  str=GetBuffer(&MyStrPtr, &numseq);
  printf("String b: %s", str);
  printf("\nPrint strings (in main).\n");
  for (i=0; i<numseq; i++) {
    printf("String %d : %s", i, MyStrPtr[i].name);
    printf("\nSize : %d\n", MyStrPtr[i].number);
  }
 
    printf("Free memory\n");
    for (i=0; i<numseq; i++) {
    printf("%d ", i);
    free(MyStrPtr[i].name);
  }
 
  free(MyStrPtr);
 
  return 0;
}
 
int GetNumberOfMyStrs(void) {
  int value;
 
  /* Consider a simple case */
  value = 2;
  return value;
}
 
MyStr * GetNextMyStr(void) {
  char string[100];
  int length;
 
 static MyStr seq; 
 
  printf("Enter string\n");
  fgets(string, sizeof(string), stdin);
  length = strlen(string);
 length++; 
  seq.name=(char*)malloc((length)*sizeof(char));
 
  strncpy(seq.name, string, (length-1));
 
  seq.name[length] = '\0';
 
  seq.number = length;
 
    printf("\nseq name: %s", seq.name);
    printf("\nseq number: %d", seq.number);
 
  return &seq;
}
 
char * GetBuffer(MyStrPointer *MyStrPtr, int *size) {
  MyStrPointer SSPtr;
 
  int i;
 
  *size = GetNumberOfMyStrs();
 
  *MyStrPtr = (MyStr*)malloc((*size) * sizeof(MyStr));
 
    for (i=0; i<*size; i++) {
 
    SSPtr = GetNextMyStr();
    (*MyStrPtr)[i].name = (char*)malloc(((*SSPtr).number)*sizeof(char));
 
 
        (*MyStrPtr)[i].number = (*SSPtr).number;
 
    strncpy((*MyStrPtr[i]).name, (*SSPtr).name, ((*SSPtr).number-1));
 
    (*MyStrPtr)[i].name[(*SSPtr).number]='\0';
 
    printf("\ni=%d size=%d number=%d name= %s", i, *size, (*SSPtr).number, (*SSPtr).name);
    printf("\nMyStrPtr[%d].name: %s", i, (*MyStrPtr)[i].name);    
  }
 
  printf("\nOut loop");
  free((*SSPtr).name);
  printf("\n before return");
  return ((*MyStrPtr)[0].name);
}

[weaknessforcats]

printf("String %d : %s", i, MyStrPtr[i].name);

This says you are using an array of MyStrPtr but you have not defined any array at all. All you have is one pointer.

[manavi18]

Thanks ... I got it! :D
Happy Holidays!

[gsivaprakashreddy]

Hi! check the output of below mentioned program.

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

typedef struct
{
char name[20];
int number;
} MyStr;

MyStr MyStrPointer[3];

int GetNumberOfMyStrs(void);
MyStr *GetNextMyStr(void);
//int GetBuffer(MyStrPointer *, int *);

int main()
{
char *str;
int numseq;
int i;
GetBuffer(&MyStrPointer, &numseq);
printf("\nPrint strings (in main).\n");
for (i=0; i<numseq; i++)
{
printf("String %d : %s", i, MyStrPointer[i].name);
printf("\nSize : %d\n", MyStrPointer[i].number);
}
return 0;
}

int GetNumberOfMyStrs(void)
{
int value;
/* Consider a simple case */
value = 3;
return value;
}

MyStr * GetNextMyStr(void)
{
char string[100];
int length;
static MyStr seq;
printf("Enter string\n");
fgets(string, sizeof(string), stdin);
length = strlen(string);
length++;
strncpy(seq.name, string, (length-1));
seq.name[length] = '\0';
seq.number = length;
printf("\nseq name: %s", seq.name);
printf("\nseq number: %d", seq.number);
return &seq;
}

int GetBuffer(MyStr *SSPtr, int *size)
{
int i;
*size = GetNumberOfMyStrs();
for (i=0; i<*size; i++)
{
SSPtr = GetNextMyStr();
printf("\ni=%d size=%d number=%d name= %s", i, *size, (*SSPtr).number, (*SSPtr).name);
strncpy(MyStrPointer[i].name, (*SSPtr).name, ((*SSPtr).number-1));
MyStrPointer[i].name[(*SSPtr).number]='\0';
MyStrPointer[i].number = ( *SSPtr).number ;
printf("String %d : %s", i, MyStrPointer[i].name);
printf("\nSize : %d\n", MyStrPointer[i].number);
}
printf("\nOut loop");
printf("\n before return");
return 0;
}

with regards,
G. Siva Prakash Reddy.

[primeSo]

I have some doubts after reading through your program, i would like to point them out here to gain some clarification from you all.

1) Line 21 & 24
You declared a integer variable, numseq but didn't initilialize them, is it alright in the context of C programming? i thought we should always initialize any variable in C before we use them, in this case you pass the variable by reference to a function.

2) Line 75 &80
Is it good to pass the variable with undefined value to the function, namely GetBuffer() and get the value from user in that function itself? I think it is better to get the value from the user in the main() first before we proceed with the process, am i right ?

3)Line 35 & 38
According to what i have learnt, we should always malloc-ed and free-ed a variable in the same function, in other words, we should always free the same variable in the function where we malloc the variable, am i correct? If this is the case, the function MyStr * GetNextMyStr(void) and char * GetBuffer(MyStrPointer *MyStrPtr, int *size) will generate memory leak, am i right? because both of the function malloc-ed a variable but the variables were being freed in the main() instead of the called function.

4) Line 55
what is the effect of the storage type "static" in this statement ? Preserve the value in the variable in each call to the function it is declared ? can we return a static variable to a calling function ?

5) Line 103
return ((*MyStrPtr)[0].name); <---- means you always return the first element in the array to the calling function. in your case, the main()? (no matter how element in your array, always return the first one only).

6) Lastly, as what weaknessforcats mentioned, you haven't declare any array of MyStr type.

[primeSo]

MyStr MyStrPointer[3];

int GetNumberOfMyStrs(void);
MyStr *GetNextMyStr(void);
//int GetBuffer(MyStrPointer *, int *);

int main()
{
char *str;
int numseq;
int i;
GetBuffer(&MyStrPointer, &numseq);
printf("\nPrint strings (in main).\n");
for (i=0; i<numseq; i++)
{
printf("String %d : %s", i, MyStrPointer[i].name);
printf("\nSize : %d\n", MyStrPointer[i].number);
}
return 0;
}

You declare a global array of type MyStr which consists of 3 elements. But you pass them as below

GetBuffer(&MyStrPointer, &numseq);

is it correct? this mean "pass the address of the memory location which point to the first element of the array? hence, you need to use two * (
int GetBuffer(MyStr **SSPtr, int *size)) to receive them in the called function ? I thought we need to pass the name of the array only when we want to pass them as parameter to a function, in this case suppose to be

GetBuffer(MyStrPointer, &numseq);