地址没有从main传递到函数的指针。 C

Question

I am trying to read elements in a function and store them in an array in main. I use a pointer to a dynamic array, but its not working. The address i get when i deference the pointer is NULL and the program crashes.

Here is the array :

char* first_name = malloc(first_name_length*sizeof(char));

Function call from main:

get_first_name(fp, first_name_length, first_name);

Function declaration:

void get_first_name(FILE* fp, int length,char* first);

Function definition:

void get_first_name(FILE* fp, int length, char* first){

    char c;
    int i = 0;
    int number_of_conversions;

    number_of_conversions = fscanf(fp, "%c", &c);

    while (!isspace(c)){

        first[i] = c;

        number_of_conversions = fscanf(fp, "%c", &c);

        i++;
    }
}

Answer 1

something like the following will do the job

void get_first_name(FILE* fp, int length, char* first)
{

    int c;
    int i = 0;

    while ( EOF != (c = fgetc(fp) ) && (' ' != c) && (i < (length -1)) )
    {
        first[i] = c;
        i++;
    }
    // terminate the string
    first[i] = '\0';
}

Answer 2

1. Open File in read mode

fp = fopen ( "file.txt", "r"));

2. Make sure file is not Empty. using fgetc() will help keeping it simple.

   void get_first_name(FILE* fp, int length, char* first)
   {
   char c;
   int i = 0;
   c=fgetc(fp);

   while (c!=EOF && !isspace(c) ){
   first[i] = c;
   c=fgetc(fp);
   i++;
   }}

3 . If you are using fscanf() for some purpose , you can use fseek() for checking if the file is Empty or not.