[c] strdup() - what does it do in C?

What is the purpose of the strdup() function in C?

It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in. The malloc'ed buffer is returned to the caller, hence the need to run free on the return value.

char * strdup(const char * s)
{
  size_t len = 1+strlen(s);
  char *p = malloc(len);

  return p ? memcpy(p, s, len) : NULL;
}

Maybe the code is a bit faster than with strcpy() as the \0 char doesn't need to be searched again (It already was with strlen()).

The strdup() function is a shorthand for string duplicate, it takes in a parameter as a string constant or a string literal and allocates just enough space for the string and writes the corresponding characters in the space allocated and finally returns the address of the allocated space to the calling routine.

strdup and strndup are defined in POSIX compliant systems as:

char *strdup(const char *str);
char *strndup(const char *str, size_t len);

The strdup() function allocates sufficient memory for a copy of the string str, does the copy, and returns a pointer to it.

The pointer may subsequently be used as an argument to the function free.

If insufficient memory is available, NULL is returned and errno is set to ENOMEM.

The strndup() function copies at most len characters from the string str always null terminating the copied string.

char * strdup(const char * s)
{
  size_t len = 1+strlen(s);
  char *p = malloc(len);

  return p ? memcpy(p, s, len) : NULL;
}

Maybe the code is a bit faster than with strcpy() as the \0 char doesn't need to be searched again (It already was with strlen()).

The most valuable thing it does is give you another string identical to the first, without requiring you to allocate memory (location and size) yourself. But, as noted, you still need to free it (but which doesn't require a quantity calculation, either.)

From strdup man:

The strdup() function shall return a pointer to a new string, which is a duplicate of the string pointed to by s1. The returned pointer can be passed to free(). A null pointer is returned if the new string cannot be created.

strdup() does dynamic memory allocation for the character array including the end character '\0' and returns the address of the heap memory:

char *strdup (const char *s)
{
    char *p = malloc (strlen (s) + 1);   // allocate memory
    if (p != NULL)
        strcpy (p,s);                    // copy string
    return p;                            // return the memory
}

So, what it does is give us another string identical to the string given by its argument, without requiring us to allocate memory. But we still need to free it, later.

It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in. The malloc'ed buffer is returned to the caller, hence the need to run free on the return value.

strdup() does dynamic memory allocation for the character array including the end character '\0' and returns the address of the heap memory:

char *strdup (const char *s)
{
    char *p = malloc (strlen (s) + 1);   // allocate memory
    if (p != NULL)
        strcpy (p,s);                    // copy string
    return p;                            // return the memory
}

So, what it does is give us another string identical to the string given by its argument, without requiring us to allocate memory. But we still need to free it, later.

It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in. The malloc'ed buffer is returned to the caller, hence the need to run free on the return value.

No point repeating the other answers, but please note that strdup() can do anything it wants from a C perspective, since it is not part of any C standard. It is however defined by POSIX.1-2001.

The most valuable thing it does is give you another string identical to the first, without requiring you to allocate memory (location and size) yourself. But, as noted, you still need to free it (but which doesn't require a quantity calculation, either.)

The statement:

strcpy(ptr2, ptr1);

is equivalent to (other than the fact this changes the pointers):

while(*ptr2++ = *ptr1++);

Whereas:

ptr2 = strdup(ptr1);

is equivalent to:

ptr2 = malloc(strlen(ptr1) + 1);
if (ptr2 != NULL) strcpy(ptr2, ptr1);

So, if you want the string which you have copied to be used in another function (as it is created in heap section), you can use strdup, else strcpy is enough,

No point repeating the other answers, but please note that strdup() can do anything it wants from a C perspective, since it is not part of any C standard. It is however defined by POSIX.1-2001.

From strdup man:

The strdup() function shall return a pointer to a new string, which is a duplicate of the string pointed to by s1. The returned pointer can be passed to free(). A null pointer is returned if the new string cannot be created.

The strdup() function is a shorthand for string duplicate, it takes in a parameter as a string constant or a string literal and allocates just enough space for the string and writes the corresponding characters in the space allocated and finally returns the address of the allocated space to the calling routine.

No point repeating the other answers, but please note that strdup() can do anything it wants from a C perspective, since it is not part of any C standard. It is however defined by POSIX.1-2001.

From strdup man:

The strdup() function shall return a pointer to a new string, which is a duplicate of the string pointed to by s1. The returned pointer can be passed to free(). A null pointer is returned if the new string cannot be created.

The statement:

strcpy(ptr2, ptr1);

is equivalent to (other than the fact this changes the pointers):

while(*ptr2++ = *ptr1++);

Whereas:

ptr2 = strdup(ptr1);

is equivalent to:

ptr2 = malloc(strlen(ptr1) + 1);
if (ptr2 != NULL) strcpy(ptr2, ptr1);

So, if you want the string which you have copied to be used in another function (as it is created in heap section), you can use strdup, else strcpy is enough,

The most valuable thing it does is give you another string identical to the first, without requiring you to allocate memory (location and size) yourself. But, as noted, you still need to free it (but which doesn't require a quantity calculation, either.)

It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in. The malloc'ed buffer is returned to the caller, hence the need to run free on the return value.

The most valuable thing it does is give you another string identical to the first, without requiring you to allocate memory (location and size) yourself. But, as noted, you still need to free it (but which doesn't require a quantity calculation, either.)

strdup and strndup are defined in POSIX compliant systems as:

char *strdup(const char *str);
char *strndup(const char *str, size_t len);

The strdup() function allocates sufficient memory for a copy of the string str, does the copy, and returns a pointer to it.

The pointer may subsequently be used as an argument to the function free.

If insufficient memory is available, NULL is returned and errno is set to ENOMEM.

The strndup() function copies at most len characters from the string str always null terminating the copied string.