Issue read write file using pipe fork shared memory - c

Question

I am trying to use pipes in C. I have two create two pipes between parent and child process.I have to read a file in chunks of 4096 bytes (or smaller if there is less) and I have to send through the pipes the amount of data that was read and how many times there have been readings. For example, to copy a 6KB file, the parent writes the first 4KB data of the file to the shared memory and send two integers, 1 and 4096, to the child via the pipe. The child receives these two numbers, copies 4096 bytes from the shared memory to the output file, and sends back 1 to the parent via the other pipe. After receiving 1, the parent copies the left 2KB data to the shared memory and send 2 and 2048 to the child. The child receives them from the pipe, copies 2048 bytes to the output file, and replies with 2 to the parent. The parent then send 0, 0 to the child. The child receives 0 and replies with a 0 and then exit. The parent receives 0 and exits too.
Currently my program works for file less than one block but not for file greater then one block (4096 bytes)
697, thank you for pointing out, I have modified my program as following but still has issues, basically how to control the flow as parent-child-parent-child ...
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#define SIZE 4096
int file_exist (char *filename)
{
struct stat buffer;
return (stat (filename, &buffer) == 0);
}
int main(int argv, char *argc[]) {
/*Check if program is called correctly*/
if(argv != 3) {
printf("Please call program appropriately\n");
exit(EXIT_FAILURE);
}
FILE *r, *w;
void *sharedMem;
int datapipe[2];
int blockpipe[2];
int shm;
char userInput[5];
char *name = "fsuid_cop4610";
if (file_exist (argc[2]))
{
printf("Would you like to overwrite file (yes/no): ");
scanf("%s", userInput);
if(!strcmp(userInput, "yes")) {
printf("Overwriting file...\n");
//fclose(w);
w = fopen(argc[2], "wb");
if(w == NULL) {
perror("Error with write file");
exit(EXIT_FAILURE);
}
}
else if(!strcmp(userInput, "no")) {
printf("Will not overwrite\n");
exit(EXIT_FAILURE);
}
else {
printf("User input not accepted\n");
exit(EXIT_FAILURE);
}
}
/*Check if read file can open*/
r = fopen(argc[1], "rb");
if(r == NULL) {
perror("Error opening read file");
exit(EXIT_FAILURE);
}
fseek(r, 0, SEEK_END); // seek to end of file
int inputlength = ftell(r); // get current file pointer
printf("inputlength is %d\n",inputlength);
int numofblock = inputlength/SIZE + 1;
fseek(r, 0, SEEK_SET); // seek back to beginning of file
/*Check if write file can open*/
if (pipe(datapipe) < 0) {
perror("Pipe");
exit(EXIT_FAILURE);
}
if (pipe(blockpipe) < 0) {
perror("Pipe");
exit(EXIT_FAILURE);
}
/*Check if forking process is successful*/
pid_t pid = fork();
if(pid < 0) {
perror("Fork");
exit(EXIT_FAILURE);
}
shm = shm_open(name, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
if(shm == -1) {
perror("Shared memory");
exit(EXIT_FAILURE);
}
if(ftruncate(shm, SIZE) == -1) {
perror("Shared Memory");
exit(EXIT_FAILURE);
}
sharedMem = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, shm, 0);
if(sharedMem == MAP_FAILED) {
perror("Mapping shared memory");
exit(EXIT_FAILURE);
}
if (pid>0) { // parent
printf(" parent before close data0,block1\n");
close(datapipe[0]); // close read, will write data
close(blockpipe[1]); // close write, will read block number
printf(" parent close data0,block1\n");
for (int i=1; i<=numofblock; i++)
{
printf("... parent process\n");
int blocknumber=i;
printf("parent read from input file into shared memory\n");
int P2SHM = fread(sharedMem, 1, SIZE, r);
if(P2SHM < 0) {
perror("Could not store to shared memory");
exit(EXIT_FAILURE);
}
//printf("parent shared memory conent: %s\n",(char *)sharedMem);
printf("parent data read %d\n",P2SHM);
if (i==1)
{
printf("i=%d parent write to data pipe\n",i);
if(write(datapipe[1], &P2SHM, sizeof(int)) < 0) {
perror("parent failed to write to pipe bytes");
exit(EXIT_FAILURE);
}
if(write(datapipe[1], &blocknumber, sizeof(int)) < 0) {
perror("parent failed to write to pipe block number");
exit(EXIT_FAILURE);
}
}
printf("parent read from block pipe\n");
int C2P = read(blockpipe[0], &blocknumber, sizeof(int));
if(C2P < 0) {
perror("parent failed to read value from blockpipe");
exit(EXIT_FAILURE);
}
/*else if(C2P == 0) {
printf("End of file reached\n");
}*/
else {
printf("parent block %d Received succesfully\n", blocknumber);
if (i>= 2)
{
printf("i=%d parent write to data pipe\n",i);
//close(datapipe[0]); // close read, will write data
if(write(datapipe[1], &P2SHM, sizeof(int)) < 0) {
perror("parent failed to write to pipe bytes");
exit(EXIT_FAILURE);
}
if(write(datapipe[1], &blocknumber, sizeof(int)) < 0) {
perror("parent failed to write to pipe block number");
exit(EXIT_FAILURE);
}
//close(datapipe[1]);
}
if(C2P == 0) {
printf("parent End of file reached\n");
}
}
} // end for
printf(" parent before close data1,block0\n");
close(datapipe[1]);
close(blockpipe[0]);
printf(" parent close data1,block0\n");
printf(" ... existing parent process\n");
}
else { // pid=0 child
printf(" child before close data1,block0\n");
close(datapipe[1]); // close write, will read data
close(blockpipe[0]); // close read, will write block number
printf(" child close data1,block0\n");
for (int j=1; j<=numofblock; j++)
{
printf(".... child process\n");
int cBytes, len, len2;
int blocknumber = 1;
printf("child read from datapipe\n");
len = read(datapipe[0], &cBytes, sizeof(cBytes));
len2 = read(datapipe[0], &blocknumber, sizeof(blocknumber));
printf("child wrote to blockpipe blocknumber=%d\n", blocknumber);
write(blockpipe[1], &blocknumber, sizeof(blocknumber));
printf("child There are %i bytes\n", cBytes);
if(len >= 0)
{
printf("child writing to file\n");
//fwrite(sharedMem, 1, sizeof(sharedMem), w);
//printf("child shared memory conent: %s\n",(char *)sharedMem);
char* res = (char *)sharedMem;
//printf("res = %s\n",res);
//printf("errno before write=%d",errno);
shm_unlink(name);
//int writtenbyte = fwrite(res, sizeof(char), strlen(res), w);
int writtenbyte = fwrite(res, sizeof(char), cBytes, w);
if(errno == EINTR) {
printf("somewhting wrong");
}
//printf("errno after write=%d",errno);
printf("writtenbyte = %d\n",writtenbyte);
//fclose(w);
//fputs((char *)sharedMem, w);
//fwrite(s->name, sizeof(char), strlen(s->name) + 1, fp);
}
/*else if (len == 0) {
printf("End of fle reached\n");
}*/
else { // len < 0
perror("Failed to read from pipe");
exit(EXIT_FAILURE);
}
} // after for
printf(" child before close data0,block1\n");
close(datapipe[0]);
close(blockpipe[1]);
printf(" child close data0,block1\n");
printf("... exiting Child process\n");
}
//shm_unlink(name);
//fclose(r);
fclose(w);
return 0;
}
Result:
inputlength is 4177
parent before close data0,block1
parent close data0,block1
... parent process
parent read from input file into shared memory
parent data read 4096
i=1 parent write to data pipe
parent read from block pipe
child before close data1,block0
child close data1,block0
.... child process
child read from datapipe
child wrote to blockpipe blocknumber=1
child There are 4096 bytes
child writing to file
parent block 1 Received succesfully
... parent process
parent read from input file into shared memory
parent data read 81
parent read from block pipe
parent block 2 Received succesfully
i=2 parent write to data pipe
Note child write block number 1, but parent received block number 1, then received block number 2, and after the 2nd write from parent it stopped, without writing output file. Any thoughts for that?