unable to fseek in /dev/zero and /dev/null #19570
Description
Description
The following invocation:
php -n -r 'fseek(fopen("/dev/zero", "rb"), 1*1024*1024, SEEK_SET);'Resulted in this output:
Warning: fseek(): Stream does not support seeking in Command line code on line 1
But I expected this output instead:
- there's nothing illegal/wrong about seeking in a /dev/zero (or /dev/null for that matter) handle.
If you try doing it in C, you'll see there is no complaint from the OS whatsoever, for example if you run
#include <stdio.h>
#include <stdlib.h>
int main(void) {
FILE *fp = fopen("/dev/zero", "rb");
if (fp == NULL) {
perror("Failed to open /dev/zero");
return EXIT_FAILURE;
}
if (fseek(fp, 1024 * 1024, SEEK_CUR) != 0) {
perror("Failed to fseek to 1MB");
fclose(fp);
return EXIT_FAILURE;
}
int c = fgetc(fp);
if (c == EOF) {
if (ferror(fp)) {
perror("Error reading from /dev/zero");
} else {
fprintf(stderr, "Unexpected EOF on /dev/zero\n");
}
fclose(fp);
return EXIT_FAILURE;
}
if (putchar(c) == EOF) {
perror("Failed to write to stdout");
fclose(fp);
return EXIT_FAILURE;
}
if (fclose(fp) != 0) {
perror("Failed to close /dev/zero");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}or
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
int main(void) {
int fd = open("/dev/zero", O_RDONLY);
if (fd == -1) {
perror("Failed to open /dev/zero");
return EXIT_FAILURE;
}
// Seek forward 1MB
if (lseek(fd, 1024 * 1024, SEEK_CUR) == (off_t)-1) {
perror("Failed to lseek to 1MB");
close(fd);
return EXIT_FAILURE;
}
unsigned char c;
ssize_t n = read(fd, &c, 1);
if (n == -1) {
perror("Error reading from /dev/zero");
close(fd);
return EXIT_FAILURE;
} else if (n == 0) {
fprintf(stderr, "Unexpected EOF on /dev/zero\n");
close(fd);
return EXIT_FAILURE;
}
if (write(STDOUT_FILENO, &c, 1) == -1) {
perror("Failed to write to stdout");
close(fd);
return EXIT_FAILURE;
}
if (close(fd) == -1) {
perror("Failed to close /dev/zero");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}you will get no complains.
PHP Version
PHP 8.3.6 (cli) (built: Jul 14 2025 18:30:55) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.3.6, Copyright (c) Zend Technologies
with Zend OPcache v8.3.6, Copyright (c), by Zend Technologies
Operating System
Ubuntu 24.04 on WSL2 on Windows 10