Manage data inside a buffer

Question

I'm using this code to read a file into a buffer. The file is full of structs of evaluacion type (including some char and int variables). Now I have the whole file in a buffer, how can I find the values of one variable in the buffer now? For example buf.notamedia < 4. There are supposed to be many of them inside the file.

#include <unistd.h>
#include <sys/stat.h>
int revisanotas(int fd)
{
    int nbytes = 1;
    int nbytese = 0;
    evaluacion buf;
    struct stat datos;
    fstat(fd, &datos);

    printf("Size of file =  %d \n", datos.st_size);
    char *buffer = (char *)malloc(datos.st_size);
    int actual = read(fd, buffer, datos.st_size);

    printf("actual = %d\n", actual);

    if (buf.notamedia >= 4.5 && buf.notamedia < 5)
    {
        /* ... */
    }
}

Any idea is very welcome

Answer 1

Easiest to define the buffer as a pointer to the data structure and use that to dereference the data (although you should ensure the file size is a multiple of the structure size).

i.e.

evaluacion* buffer = (evaluation*)malloc(datos.st_size);
if(buffer[0].notamedia >= 4.5)

You can then increment the index to access other structures you loaded.

Answer 2

I'm doing as you said, but I'm only getting one iteration, I don't know what I'm doing wrong :(

evaluacion* buffer=(evaluacion*)malloc(datos.st_size);
int actual = read(fd,buffer,datos.st_size);

printf("Number of structs = %d", (datos.st_size/(sizeof(evaluacion))));

for (i=0;i<(datos.st_size/(sizeof(evaluacion)));i++);
{
printf("Notamedia = %f\n",buffer[i].notamedia);
if (buffer[i].notamedia >= 4.5 && buffer[i].notamedia < 5)
{
printf("Notamedia = %f\n",buffer[i].notamedia);
}
{
}
}
}

Answer 3

Thanks for the comments, I think I solved the problem, I modified the code:

#include <unistd.h>
#include <sys/stat.h>
int revisanotas(int fd)
{
int nbytes=1;
int nbytese=0;
int i=0;
int n=0;
struct stat datos;
fstat(fd, &datos);
evaluacion buf;
printf("File size =  %d \n", datos.st_size);
evaluacion* buffer=(evaluacion*)malloc(datos.st_size);
int actual = read(fd,buffer,datos.st_size);

do 
{
i++;
if (buffer[i].notamedia >= 4.5 && buffer[i].notamedia < 5)
{
n=n+1;
/*printf("Notamedia = %f\n",buffer[i].notamedia);
*/
buffer[i].notamedia=5;
}

}while (i<(datos.st_size/(sizeof(evaluacion)))); 
nbytese=write(fd,buffer,datos.st_size);
printf("Written bytes = %d\n",nbytese);
return(n);
}

Now, If the condition is matched, I'm modifying the buffer. Once I read all the structs I write the file in the disk again, but I still have a problem, every time, instead of write the file in the same position, seems like I'm adding the same information after the old one, so if I read the file once I get 3.5Mb, two times 7MB and so on :S. Any idea what can I do? Thanks