FTP protocol STOR cmd receiving file correctly in c - c

Question

I'm working at a simple FTP-Server (just for fun) and the last step is to store a file with the "STOR" cmd.
rec_file method:
int rec_file(char *filename, int sockfd){
int bytes_read;
char buffer[4096];
FILE *f = fopen(filename, "wb");
while (bytes_read=read(sockfd,buffer,sizeof(buffer))){
fwrite(buffer , 1 , sizeof(buffer) , f);
}
fclose(f);
return 0;
}
Test File a.txt:
TEST123
456
789
TEST!!
sent file a.txt:
TEST123
456
789
TEST!! AÛD Ìk P ™sB ÃB p Àa ðÿÿÿÿÿÿ P # €Ìk Ìk # dB % 0 % Øa P 0 | w n [ Ìk 8Y
ý % 8Y
ý # Àk W.B  G
ý 8Y
ý QH /home/felix/ftp ¨A PQ
ý Ãk R
ý `Q
ý =F ) -rA ÿÿÿÿÿÿÿÿ Çk ¨ * 0k 1„A Çk -rA ßJI Çk ¨ 0k 1„A Çk ¨
So what am I doing wrong?

Answer 1

Your code may write more bytes to the file than you have received from the socket.
The return value of read which gets stored in bytes_read will tell you how many bytes have actually been read. Even if the amount of data is large, read may not always read a full buffer, it may return a smaller number. You should also handle negative values as errors.
The code below has a very simple error handling. In a real application you might have to retry the read call on certain errno values instead of stopping to read data.
while ((bytes_read=read(sockfd,buffer,sizeof(buffer))) > 0){
fwrite(buffer , 1 , bytes_read, f);
}
if(bytes_read < 0) {
perror("read failed");
/* maybe other error handling */
}