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#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int head(const int fd, int lines)
{
static const size_t SIZE = 1024 * 4;
char buffer[SIZE];
while (0 < lines) {
const ssize_t available = read(fd, buffer, SIZE);
if (-1 == available)
return 1;
if (0 == available)
return 0;
size_t offset = 0;
for (ssize_t n = 0; n < available; ++n) {
if (buffer[n] == '\n') {
lines--;
if (0 <= lines) {
const ssize_t result = write(1, buffer + offset, 1 + n - offset);
if (-1 == result)
return 1;
offset = 1 + n;
}
if (0 >= lines)
return 0;
}
}
if (0 < lines) {
const ssize_t result = write(1, buffer + offset, available - offset);
if (-1 == result)
return 1;
}
}
return 0;
}
int main(int argc, char * argv[])
{
int opt;
int lines = 10;
while (-1 != (opt=getopt(argc, argv, "n:"))) {
switch (opt) {
case 'n':
errno = 0;
lines = strtol(optarg, (char **) NULL, 10);
if (lines < 0)
errno = EINVAL;
if (0 != errno) {
perror(optarg);
return 1;
}
break;
default:
dprintf(2, "Usage: %s [-n lines] [file...]\n", argv[0]);
return 1;
}
}
const int files_count = argc - optind;
int i = optind;
int overall = 0;
do {
const char * const path = argc == i ? "-" : argv[i];
const unsigned int is_stdin = '-' == path[0] && 0 == path[1];
const int fd = is_stdin ? 0 : open(path, O_RDONLY);
if (1 < files_count)
printf(optind == i ? "==> %s <==\n" : "\n==> %s <==\n", path);
if (0 <= fd) {
const int result = head(fd, lines) | is_stdin ? 0 : close(fd);
if (0 != result)
perror(path);
overall |= result;
}
else {
perror(path);
overall = 1;
}
}
while (++i < argc);
return overall;
}
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