How do I use strdup?

Question

I am calling strdup and have to allocate space for the variable before calling strdup.

char *variable;
variable = (char*) malloc(sizeof(char*));
variable = strdup(word);

Am I doing this right? Or is there something wrong here?

Answer 1

If you're using the POSIX standard strdup(), it calculates the space needed and allocates it and copies the source string into the newly allocated space. You don't need to do a malloc() yourself; indeed, it immediately leaks if you do it since you overwrite the only pointer to the space you allocated with the pointer to the space that strdup() allocated.

Hence:

char *variable = strdup(word);
if (variable == 0) …process out of memory error; do not continue…
…use variable…
free(variable);

If you do need to do the memory allocation, then you need to allocate strlen(word)+1 bytes in variable and you can then copy word into that newly allocated space.

char *variable = malloc(strlen(word)+1);
if (variable == 0) …process out of memory error; do not continue…
strcpy(variable, word);
…use variable…
free(variable);

Or calculate the length once and use memmove() or perhaps memcpy():

size_t len = strlen(word) + 1;
char *variable = malloc(len);
if (variable == 0) …process out of memory error; do not continue…
memmove(variable, word, len);
…use variable…
free(variable);

Don't forget to ensure you know where the free() is for every malloc().