Jack S Jack S - 1 month ago 7
C Question

Displaying size of a file [C]

I'm making a simple sockets program to send a text file or a picture file over to another socket connected to a port. However, I want to also send the size of the file over to the client socket so that it knows how many bytes to receive.

I also want to implement something where I can send a certain number of bytes instead of the file itself. For example, if a file I wanted to send was 14,003 bytes and I felt like sending 400 bytes, then only 400 bytes would be sent.

I am implementing something like this:

#include <stdio.h>

int main(int argc, char* argv[]) {
FILE *fp;
char* file = "text.txt";
int offset = 40;
int sendSize = 5;
int fileSize = 0;

if ((fp = fopen(file, "r")) == NULL) {
printf("Error: Cannot open the file!\n");
return 1;
} else {
/* Seek from offset into the file */
//fseek(fp, 0L, SEEK_END);
fseek(fp, offset, sendSize + offset); // seek to sendSize
fileSize = ftell(fp); // get current file pointer
//fseek(fp, 0, SEEK_SET); // seek back to beginning of file
}

printf("The size is: %d", fileSize);
}


offset
is pretty much going to go 40 bytes into the file and then send whatever
sendSize
bytes over to the other program.

I keep getting an output of
0
instead of
5
. Any reason behind this?

Answer

I think your Seek does not work due to the 3rd parameter: try to seek with
(fp, offset, SEEK_SET);

as he will try to use the number sendSize+Offset as the "origin" constant, it will be compared to the 3 constant values as below (it is 0, 1 or 2) and as nothing compares it seem to return 0 all time.

http://www.cplusplus.com/reference/cstdio/fseek/

Parameters

stream, offset, origin

Position used as reference for the offset. It is specified by one of the following constants defined in exclusively to be used as arguments for this function:

Constant Reference position
SEEK_SET Beginning of file
SEEK_CUR Current position of the file pointer
SEEK_END End of file

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