fread() blocks when reading pipe with closed write end

[bill]

I am confused by the behavior of the following code. The function
copy() takes two FILE *'s and copies the data from one to the other.
In the main routine, I create a pipe and copy stdin to the write end
while copying the read end to stdout. If the child handles copying the
read end of the pipe to stdout, the main program returns. If the child
handles copying stdin to the write end of the pipe, the parent hangs in
copy(), blocking on fget() (or fread()). I've executed in gdb and
watched the child close the write end of the pipe before the parent
ever gets to the read, read the data off the pipe, and then block
indefinitely. The write end is closed, why doesn't fget() or fread()
return EOF? And why does the behavior change when the role of
parent/child is reversed? (replace "if(pid)" with "if(!pid)")

I've removed most of the error checking from this for clarity, but
point out that the fclose() succeeds. (I've tried replacing fclose
with close(filedes[1]), to no affect.)

#include<stdio.h>
#include<inttypes.h>

void
copy(FILE *in, FILE *out)
{
int x;
while( (x = fgetc(in)) != EOF)
if (fwrite( &x, sizeof(x),1,out) != 1)
break;
/*
while( fread(&x, sizeof(x),1,in) == 1)
if (fwrite( &x, sizeof(x),1,out) != 1)
break;
*/
return;
}

int
main()
{
int filedes[2];
FILE *a, *b;
int pid;

pipe(filedes);
a = fdopen(filedes[0],"r");
b = fdopen(filedes[1],"w");
pid = fork();
if (!pid) {
copy(a, stdout);
}
else {
int err;
copy (stdin, b);
err = fclose(b);
fprintf(stderr,"fclose returned: %d\n",err);
}
}
~

 

[Gordon Burditt]

I am confused by the behavior of the following code. The function

copy() takes two FILE *'s and copies the data from one to the other.
In the main routine, I create a pipe and copy stdin to the write end
while copying the read end to stdout. If the child handles copying the
read end of the pipe to stdout, the main program returns. If the child
handles copying stdin to the write end of the pipe, the parent hangs in
copy(), blocking on fget() (or fread()). I've executed in gdb and
watched the child close the write end of the pipe before the parent

Did the *parent* close the write end of the pipe?

ever gets to the read, read the data off the pipe, and then block
indefinitely. The write end is closed,

Are *ALL* of the write ends closed?

why doesn't fget() or fread()
return EOF? And why does the behavior change when the role of
parent/child is reversed? (replace "if(pid)" with "if(!pid)")

Gordon L. Burditt

 

[bill]

No, and no. Thank you! That was quite frustrating.

 

[Default User]

No, and no. Thank you! That was quite frustrating.

What and what? Thank who? What was?



Brian

 

[Barry Schwarz]

I am confused by the behavior of the following code. The function
copy() takes two FILE *'s and copies the data from one to the other.
In the main routine, I create a pipe and copy stdin to the write end
while copying the read end to stdout. If the child handles copying the
read end of the pipe to stdout, the main program returns. If the child
handles copying stdin to the write end of the pipe, the parent hangs in
copy(), blocking on fget() (or fread()). I've executed in gdb and
watched the child close the write end of the pipe before the parent
ever gets to the read, read the data off the pipe, and then block
indefinitely. The write end is closed, why doesn't fget() or fread()
return EOF? And why does the behavior change when the role of
parent/child is reversed? (replace "if(pid)" with "if(!pid)")

I've removed most of the error checking from this for clarity, but
point out that the fclose() succeeds. (I've tried replacing fclose
with close(filedes[1]), to no affect.)

#include<stdio.h>
#include<inttypes.h>

void
copy(FILE *in, FILE *out)
{
int x;
while( (x = fgetc(in)) != EOF)
if (fwrite( &x, sizeof(x),1,out) != 1)

In addition to any other problems, you read one byte but write more
than one (probably four).

break;
/*
while( fread(&x, sizeof(x),1,in) == 1)
if (fwrite( &x, sizeof(x),1,out) != 1)
break;
*/
return;
}

int
main()
{
int filedes[2];
FILE *a, *b;
int pid;

pipe(filedes);
a = fdopen(filedes[0],"r");
b = fdopen(filedes[1],"w");
pid = fork();
if (!pid) {
copy(a, stdout);
}
else {
int err;
copy (stdin, b);
err = fclose(b);
fprintf(stderr,"fclose returned: %d\n",err);
}
}
~

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