C-创建字符串“ from”结构参数

Question

Have a

typedef struct person {
    char name[20]
    char surname[20]
} person_t;

I need to create a string like XXXXXX:YYYYYY with the function like char* personToString(person_t *p) . I tried to make it:

char* personToString(person_t* p) {

  int n1,n2;
  n1=strlen(p->name);
  n2=strlen(p->surname);
  char *p = (char*) malloc((n1+n2+2)*sizeof(char));
  strcat(p,puser->name);
  strcat(p,":");
  strcat(p,puser->surname);

  return p;
}

This give me a reasonable output but I have some errors testing with valgrind! I also think that there is a way more classy to write the function!

Answer 1

When you malloc memory for p the memory will hold garbage values. Strcat will append a string after the null character, but in an uninitialized string will hold random values.

Replace the first strcat with strcpy.

Answer 2

You need to

strcpy(p,puser->name);

not

strcat(p,puser->name);

malloc does not initialize the buffer to zero, so strcat is searching for a null byte in p first and probably not finding one, reading past the end of the buffer and thus crashing.

Instead of one strcpy plus two strcat you can also write one call to sprintf:

sprintf(p, "%s:%s", puser->name, puser->surname);

Answer 3

First you should call string copy, then strcat:

strcat(p,puser->name);

should be:

strcpy(p,puser->name);

because memory allocated with malloc function keeps values garbage, by doing strcat for first you are concatenating after garbage -- it also brings Undefined behaviour in your code.

You can use void* calloc (size_t num, size_t size); instead of malloc(), calloc function initialized allocated memory with 0 (then strcat() no problem). Also dynamically allocated memory you should deallocate memory block using void free (void* ptr);) explicitly.

Answer 4

This looks good to me,

char* personToString( struct person_t *p )
{
  int len = strlen(p->name) + strlen(p->surname) + 2; // holds ':' + NULL 
  char *str = malloc( len ); // Never cast malloc's return value in C

  // Check str for NULL
  if( str == NULL )
  {
      // we are out of memory
      // handle errors
      return NULL;
  }

  snprintf( str, len, "%s:%s", p->name, p->surname);

  return str;
}

NOTE:

1. Never cast malloc 's return value in C.
2. Use snprintf when multiple strcat is needed, its elegant.
3. free the return value str here in caller.

Fixed struct and char variables .