在C中使用strcpy函数，不知道长度

Question

Im trying to write a simple function that get name and return it after adding an extension to it. for example, if i have the char pointer to "abcd" the function should return "abcd.as"

I tried to write this function that get char pointer and return a pointer to a new char after adding the extension. But is not working does someone know why?

char* AddFileExtension(char* FileName)
{
    char* FixFileName=NULL;
    char* Extension = ".as";
    strcpy(FixFileName, FileName);
    strcat(FixFileName, Extension);
    return FixFileName;
}

Answer 1

You have to allocate memory for FixFileName :

char* Extension = ".as";
char* FixFileName = malloc(strlen(FileName) + strlen(Extension) + 1);

Don't forget to free() the memory when you're done with it. For obvious reasons, this has to be done outside the function.

Answer 2

you have to allocate memory for FixFileName and the size of allocate memory should be the lenght of FileName + sizeof(".as")

Note: the sizeof will count the null character of ".sa" string so no need to add 1 for the null charachter of string FixFileName

char* AddFileExtension(char* FileName)
{
#define EXTENSION_AS ".as"    
    char* FixFileName= malloc(strlen(FileName) + sizeof(EXTENSION_AS));

    sprintf(FixFileName, "%s%s", FileName, EXTENSION_AS);
    return FixFileName;
}

do not forget to free the allocated memory when it became useless in your program with free()

Answer 3

char* AddFileExtension(char* FileName)
{
    char* FixFileName=NULL;
    char* Extension = ".as";
    FixFileName=(char *) malloc(strlen(FileName)+strlen(Extension)+1);
    strcpy(FixFileName, FileName);
    strcat(FixFileName, Extension);
    return FixFileName;
}

Try the above code. You have to free the memory once you are done