Thread: how to malloc memory?

To process the file as follows:

Code:

AATTTTAATATA
CGGCCCCCGC
...

I define a struct as follows:

Code:

struct SNPData
{
       char** SNPData;
       //char** MajOrMin;
};
int main()
{
    struct SNPData *s;
    s = readSNPData("1.txt");

     return 0;
}

but I don't know how to malloc memory to SNPData.

part code

Code:

/*  read into memory from input file */
struct SNPData* readSNPData(char *input)
{
       struct SNPData *SNPs;
       FILE *fp;
       fp = fopen(input,"r");
     
       int i = 0;
       int isKnownLen = 0;
       while(! feof(fp))
       {
               if ( isKnownLen = 0 )
               {
                    char *buf = (char *) malloc( 1*sizeof(char) );
                    char c;
                    while( (c = fgetc(fp)) != '\n' )
                    {
                           *(buf+i) = c;
                           realloc(buf,i+2);
                           ++i;
                    }
                    isKnownLen = 1;
               }else
               {
                    //fscanf(fp,"%s",tmp);
                    char *buf = (char *) malloc( i*sizeof(char) );
                    fgets(buf,i,fp);
               }                    
       }
       fclose(fp);
}

how to malloc memory to SNPData and where is the codes mallocing memory added?

First off, you need an instance of your structure. Since all you have is a pointer, you need something like:

Code:

SNPs = malloc( sizeof *SNPs );

That gives you one instance of SNPS to play with. Now, inside that, assuming you're trying to do some sort of rows/cols thing here:

Code:

SNPs->SNPData = malloc( rows * sizeof( *(SNPs->SNPData) ) );

Then, in a loop, for each one of those rows:

Code:

SNPs->SNPData[ thisrow ] = malloc( ...some size for each row... );

Then call free in the reverse of how you allocated.


Quzah.

You need to read up about "Schlemiel the painter's algorithm". It's what you have in your program and it is very much a bad thing.

> while(! feof(fp))
This is a FAQ entry.

> if ( isKnownLen = 0 )
I'm guessing you mean == here

> char *buf = (char *) malloc( 1*sizeof(char) );
This (the casting) is a FAQ entry as well

> realloc(buf,i+2);
You need to read the manual to find out what realloc returns.

Originally Posted by tabstop

So the point here is that neither of the two lines of code here is correct. The first is incorrect, as the result of malloc should not be cast. The second is incorrect, as you are asking for memory and then throwing it away.

You intend it to do so. However, that is not what it actually does.

How to correct it ?
I only know the second fault. My result corrected is as follows:

Code:

buf = realloc(buf,i+2);

How to dynamic malloc memory ? can anyone give me a example

> is this done like above correct?
No, you shouldn't be using it at all.

Code:

char buff[BUFSIZ];
while ( fgets( buff, BUFSIZ, fp ) != NULL ) {
  // do stuff
}

> I want to malloc memory dynamicly,
Yes, you
- read it into a fixed buffer
- work out what space you need
- allocate it
- copy the buffer
- rinse and repeat until the file is read.

Or if you really want a char at a time, then go with

Code:

int ch;
while ( (ch=fgetc(fp)) != EOF )

Extending a buffer using realloc by 1 character at a time can get extremely expensive if the block keeps moving.

But until you can read a file reliably (ie, not using feof() in the way you're using it), the malloc issue is rather moot.