Malloc with structs.

This is a discussion on Malloc with structs. within the C Programming forums, part of the General Programming Boards category; I have a problem where I need to take a number of students, and then enter their names and a ...

  1. #1
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    Malloc with structs.

    I have a problem where I need to take a number of students, and then enter their names and a grade for each. I have it all working except for the malloc. I'm also using structs.
    Code:
    #define MaxNameLength 512
    #define MinStudents 1
    #define MaxStudents 10
    
    struct Student
    {
    	char*	names;
    	float	pGrades;
    };
    typedef  struct Student  Student;
     
    int main( int argc, char** argv )
    {
    	int		     numStudents = 0;
    	int		     i;
    	char		   longName[MaxNameLength];	
    	int		     nameLength;
    	Student*        students;
    
            Student.names =  malloc( numStudents * sizeof( Student.names* ) );
    	if( !Student.names )	
    	{
    		printf( "Insufficient memory to allocate names array of length %u\n",
    				numStudents * sizeof( char* ) );
    		exit( 1 );
    	}
    That is the first malloc for the students names.

    Code:
    	
            Student.pGrades =  malloc( numStudents * sizeof( Student.pGrades ) );
            if( !Student.pGrades )
            {
                    printf( "Insufficient memory to allocate names array of length %u\n",
                                    numStudents * sizeof( float ) );
                    exit( 1 );
            }
    This is the malloc for their grades.
    Any help would be appreciated, if this doesnt really make sense, I can post all of the code.

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    Consider the idiom:
    Code:
    p = malloc ( n * sizeof *p );
    http://faq.cprogramming.com/cgi-bin/...&id=1043284351
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  3. #3
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    Yes well by that the code would look like this.
    Code:
    struct Student
    {
    	char*	names;
    	float	pGrades;
    };
    typedef  struct Student  Student;
    
    int main( int argc, char** argv )
    {
        int                  numStudents;
        Student*        students;
    
        students = malloc( numStudents * sizeof( Student* )
    }
    This bit of code allocates for the whole of the struct, I want to allocate specifically for names and pGrades.

    Im not trying to allocate for type struct, I want to allocate names and pGrades which are char* and float. Im just not sure on the syntax of allocating something inside the struct. Showing me
    Code:
    p = malloc ( n * sizeof *p );
    doesnt help much.

  4. #4
    Just Lurking Dave_Sinkula's Avatar
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    So do the same type of thing form the member name. In this case, n would be MaxNameLength?

    And no, it would be
    Code:
    students = malloc( numStudents * sizeof *students)
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  5. #5
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    Code:
    struct Student
    {
    	char*	names;
    	float	pGrades;
    };
    typedef  struct Student  Student;
    Code:
    Student.pGrades =  malloc( numStudents * sizeof( Student.pGrades ) );
    I think your a little mixed up as to what malloc does. When you allocate room for that structure, it will give you room for a pointer, and room for a float. you need need to allocate space for the pointer to point at (if you plan on using it). But you don't allocate space and try to set the float value to its starting address

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