fgets无法读取C中的完整行

Question

I have a file data.csv which contains float type data:

---

0.22,0.33,0.44

0.222,0.333,0.444

---

I need to read this file into a two dimensional dynamic array. But I am not able to read the full line with fgets . Not sure why?

Here is my C code which I used on Ubuntu:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    FILE *fp;
    float **data;    
    int i,j,rows=2,cols=3;   
    char * token;
    fp=fopen("data.csv","r");
    if(fp==NULL) {
            fprintf(stderr,"Can't open input file");
            exit(1);
    }

    data= malloc(rows * sizeof(float*)); 
    char *rowbuffer=malloc( cols * ( sizeof(float)+sizeof(char) ) );
    i=0;
    while(fgets(rowbuffer,sizeof(rowbuffer),fp) !=NULL) {      
        data[i] = malloc(cols * sizeof(float));      
        j=0;
        printf("\n %s",rowbuffer);
        for (token = strtok(rowbuffer,","); token != NULL; token = strtok(NULL, ",")) {
             data[i][j++] = atof(token);
             /*printf("%s",token);*/
        }
        i++;  
    }
    free(rowbuffer);
    for(i = 0; i < rows; i++)
        free(data[i]);
    free(data);
    fclose(fp);
}

The output is like:

0.22,0.

33,0.44

0.222,0

444

Error in `./test': double free or corruption (out): 0x0000000000adf270

Aborted (core dumped)

Can anyone tell why is this error? :( Or is there a better way to read this kind of data file?

Answer 1

One issue is here:

char *rowbuffer=malloc( cols * ( sizeof(float)+sizeof(char) ) );

sizeof(float) is the size that a float uses in memory, not in its text representation. When reading from files, you should allocate a buffer to contain a whole line in text format . In your case a good bet could be the following:

int bufsize = cols * (3 + DBL_MANT_DIG - DBL_MIN_EXP + 1) + 1;

(See this for why that value and what you need to #include : What is the maximum length in chars needed to represent any double value? . The trailing + 1 is to account for the newline character, which fgets() does read and include in the buffer.)

But that assumes that there are no formatting errors in the input file, so you might want to add some extra slack to that value.

Once you have that value, use it in both the malloc() and fgets() :

char *rowbuffer=malloc(bufsize);
i=0;
while(fgets(rowbuffer,bufsize,fp) !=NULL) {
...

On a side note, your input file looks like it could be better read using scanf() .

Answer 2

Your coding problem is in :

fgets(rowbuffer,sizeof(rowbuffer),fp)

sizeof(rowbuffer) will give you only the size of the pointer, not the size of the memory allocated to the pointer.

To resolve the issue, you need to supply the proper size of the allocated memory [ cols * ( sizeof(float)+sizeof(char) ] to fgets() .

Your logical problem is in :

You assumed that a printed represntation of a float value will take the same amount of memory as it takes for that of a float variable. No, that's not true. In the printed representation , each digit (including the decimal point and any leading or trailing 0 after the decimal) will consume one byte of memory each. You should keep that in mind while allocating memory for the destination buffer.