File pointer goes to end after read by child - c

Question

Parent has opened a file to read, I fork two children to read from file and write on different files.
child 1 reads the first line, and child 2,reads nothing. When I do an ftell, it reaches the end.
Can anyone please explain this behaviour?
f[0] = fopen("input", "r");
for ( i = 1; i <= 2; i++ ){
if ((pid = fork()) != 0){
waitpid(pid);
}
else
{
snprintf ( buffer, 10, "output%d", i );
printf("opening file %s \n",buffer);
f[i] = fopen( buffer, "w");
fgets(buff2, 10, f[0]);
fprintf(f[i], "%s", buff2);
fclose(f[i]);
_exit(0);
}
}
fclose(f[0]);

Answer 1

Your problem is buffering. stdio reads files on fully buffered mode by default, which means a call to fgets(3) will actually read a huge block of characters from the file, buffer everything, and then return the first line, while leaving the rest in the buffer, in the perspective of being called again in the future (remember that stdio strives for minimizing the number of read(2) and write(2) syscalls). Note that stdio buffering is a user-space thing; all the kernel sees is a single process reading a huge block on that file, and so the cursor is updated accordingly.
Common block sizes are 4096 and 8192; your input file is probably smaller than that and so the first process that calls fgets(3) ends up reading the whole file, leaving the cursor in the end. Buffering is tricky.
What can you do? One solution I can think of is to disable buffering (since this is an input stream we're talking about, we can't use line buffered mode, because line buffering is meaningless for input streams). So if you disable buffering on the input stream before forking, everything will work. This is done with setvbuf(3).
Here's a working example:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>
static FILE *f[3];
static char buffer[128];
static char buff2[128];
int main(void) {
pid_t pid;
int i;
if ((f[0] = fopen("input", "r")) == NULL) {
perror("Error opening input file");
exit(EXIT_FAILURE);
}
if (setvbuf(f[0], NULL, _IONBF, 0) < 0) {
perror("setvbuf(3) failed");
exit(EXIT_FAILURE);
}
for (i = 1; i <= 2; i++) {
if ((pid = fork()) < 0) {
perror("fork(2) failed");
exit(EXIT_FAILURE);
}
if (pid != 0) {
if (waitpid(pid, NULL, 0) < 0) {
perror("waitpid(2) failed");
exit(EXIT_FAILURE);
}
} else {
snprintf(buffer, sizeof(buffer), "output%d", i);
printf("opening file %s\n", buffer);
if ((f[i] = fopen(buffer, "w")) == NULL) {
perror("fopen(2) failed");
exit(EXIT_FAILURE);
}
errno = 0;
if (fgets(buff2, sizeof(buff2), f[0]) == NULL) {
if (errno != 0) {
perror("fgets(3) error");
exit(EXIT_FAILURE);
}
}
fprintf(f[i], "%s", buff2);
fclose(f[i]);
exit(EXIT_SUCCESS);
}
}
fclose(f[0]);
return 0;
}
Note that this may incur a significant performance hit. Your code will be making a lot more syscalls, and it might be too expensive for huge files, but it doesn't seem to be a problem since apparently you're dealing with relatively small input files.

Answer 2

Here's an extract of my fork() man page:
The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that, for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the child process can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish standard input and output for newly created processes as well as to set up pipes.
So your behaviour is completely normal. If you want your child to have its own file descriptor, it should open its own file.
For example, you could do the following:
for ( i = 1; i <= 2; i++ )
{
if ((pid = fork()) != 0)
{
waitpid(pid);
}
else
{
f[0] = fopen("input", "r"); // New
snprintf ( buffer, 10, "output%d", i );
printf("opening file %s \n",buffer);
f[i] = fopen( buffer, "w");
fgets(buff2, 10, f[0]);
fprintf(f[i], "%s", buff2);
fclose(f[i]);
fclose(f[0]); //New
_exit(0);
}
}
Also, you should check for errors (almost all the functions in your else could fail with error).