Question

Hi I am trying to read a binary file and process it but I think I am using fread wrong way, when I try to use fread the no. of bytes read are less then the size of file. can anyone help me, white I am doing wrong

    #include <stdio.h>
    #include <limits.h>

    int main()
    {

    FILE *fin=fopen("./file.pcap","rb");

    char line[LINE_MAX];

    FILE *fout=fopen("out.txt","w");

    while(fread(line,sizeof(line),1,fin)){
        fwrite(&line,sizeof(line),1,fout);
    }

    fclose(fin);
    fclose(fout);

    }

The first file is around 51236 and out.txt is 51200

Answer 1

Fread returns the number of “elements” read. You have said an element is LINE_MAX bytes long, so when you get to the end of the file, there is not a full element for you, so fread() returns 0 and your output ends up truncated.

try flipping to read “a number of bytes” rather than “1 block of LINE_MAX bytes”:

size_t numBlocks;
while((numBlocks = fread(line, 1, sizeof(line), fin)) > 0) {
    fwrite(line, 1, numBlocks, fout);
}