Related
What is wrong with using feof() to control a read loop? For example:
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
char *path = "stdin";
FILE *fp = argc > 1 ? fopen(path=argv[1], "r") : stdin;
if( fp == NULL ){
perror(path);
return EXIT_FAILURE;
}
while( !feof(fp) ){ /* THIS IS WRONG */
/* Read and process data from file… */
}
if( fclose(fp) != 0 ){
perror(path);
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
What is wrong with this loop?
TL;DR
while(!feof) is wrong because it tests for something that is irrelevant and fails to test for something that you need to know. The result is that you are erroneously executing code that assumes that it is accessing data that was read successfully, when in fact this never happened.
I'd like to provide an abstract, high-level perspective. So continue reading if you're interested in what while(!feof) actually does.
Concurrency and simultaneity
I/O operations interact with the environment. The environment is not part of your program, and not under your control. The environment truly exists "concurrently" with your program. As with all things concurrent, questions about the "current state" don't make sense: There is no concept of "simultaneity" across concurrent events. Many properties of state simply don't exist concurrently.
Let me make this more precise: Suppose you want to ask, "do you have more data". You could ask this of a concurrent container, or of your I/O system. But the answer is generally unactionable, and thus meaningless. So what if the container says "yes" – by the time you try reading, it may no longer have data. Similarly, if the answer is "no", by the time you try reading, data may have arrived. The conclusion is that there simply is no property like "I have data", since you cannot act meaningfully in response to any possible answer. (The situation is slightly better with buffered input, where you might conceivably get a "yes, I have data" that constitutes some kind of guarantee, but you would still have to be able to deal with the opposite case. And with output the situation is certainly just as bad as I described: you never know if that disk or that network buffer is full.)
So we conclude that it is impossible, and in fact unreasonable, to ask an I/O system whether it will be able to perform an I/O operation. The only possible way we can interact with it (just as with a concurrent container) is to attempt the operation and check whether it succeeded or failed. At that moment where you interact with the environment, then and only then can you know whether the interaction was actually possible, and at that point you must commit to performing the interaction. (This is a "synchronisation point", if you will.)
EOF
Now we get to EOF. EOF is the response you get from an attempted I/O operation. It means that you were trying to read or write something, but when doing so you failed to read or write any data, and instead the end of the input or output was encountered. This is true for essentially all the I/O APIs, whether it be the C standard library, C++ iostreams, or other libraries. As long as the I/O operations succeed, you simply cannot know whether further, future operations will succeed. You must always first try the operation and then respond to success or failure.
Examples
In each of the examples, note carefully that we first attempt the I/O operation and then consume the result if it is valid. Note further that we always must use the result of the I/O operation, though the result takes different shapes and forms in each example.
C stdio, read from a file:
for (;;) {
size_t n = fread(buf, 1, bufsize, infile);
consume(buf, n);
if (n == 0) { break; }
}
The result we must use is n, the number of elements that were read (which may be as little as zero).
C stdio, scanf:
for (int a, b, c; scanf("%d %d %d", &a, &b, &c) == 3; ) {
consume(a, b, c);
}
The result we must use is the return value of scanf, the number of elements converted.
C++, iostreams formatted extraction:
for (int n; std::cin >> n; ) {
consume(n);
}
The result we must use is std::cin itself, which can be evaluated in a boolean context and tells us whether the stream is still in the good() state.
C++, iostreams getline:
for (std::string line; std::getline(std::cin, line); ) {
consume(line);
}
The result we must use is again std::cin, just as before.
POSIX, write(2) to flush a buffer:
char const * p = buf;
ssize_t n = bufsize;
for (ssize_t k = bufsize; (k = write(fd, p, n)) > 0; p += k, n -= k) {}
if (n != 0) { /* error, failed to write complete buffer */ }
The result we use here is k, the number of bytes written. The point here is that we can only know how many bytes were written after the write operation.
POSIX getline()
char *buffer = NULL;
size_t bufsiz = 0;
ssize_t nbytes;
while ((nbytes = getline(&buffer, &bufsiz, fp)) != -1)
{
/* Use nbytes of data in buffer */
}
free(buffer);
The result we must use is nbytes, the number of bytes up to and including the newline (or EOF if the file did not end with a newline).
Note that the function explicitly returns -1 (and not EOF!) when an error occurs or it reaches EOF.
You may notice that we very rarely spell out the actual word "EOF". We usually detect the error condition in some other way that is more immediately interesting to us (e.g. failure to perform as much I/O as we had desired). In every example there is some API feature that could tell us explicitly that the EOF state has been encountered, but this is in fact not a terribly useful piece of information. It is much more of a detail than we often care about. What matters is whether the I/O succeeded, more-so than how it failed.
A final example that actually queries the EOF state: Suppose you have a string and want to test that it represents an integer in its entirety, with no extra bits at the end except whitespace. Using C++ iostreams, it goes like this:
std::string input = " 123 "; // example
std::istringstream iss(input);
int value;
if (iss >> value >> std::ws && iss.get() == EOF) {
consume(value);
} else {
// error, "input" is not parsable as an integer
}
We use two results here. The first is iss, the stream object itself, to check that the formatted extraction to value succeeded. But then, after also consuming whitespace, we perform another I/O/ operation, iss.get(), and expect it to fail as EOF, which is the case if the entire string has already been consumed by the formatted extraction.
In the C standard library you can achieve something similar with the strto*l functions by checking that the end pointer has reached the end of the input string.
It's wrong because (in the absence of a read error) it enters the loop one more time than the author expects. If there is a read error, the loop never terminates.
Consider the following code:
/* WARNING: demonstration of bad coding technique!! */
#include <stdio.h>
#include <stdlib.h>
FILE *Fopen(const char *path, const char *mode);
int main(int argc, char **argv)
{
FILE *in;
unsigned count;
in = argc > 1 ? Fopen(argv[1], "r") : stdin;
count = 0;
/* WARNING: this is a bug */
while( !feof(in) ) { /* This is WRONG! */
fgetc(in);
count++;
}
printf("Number of characters read: %u\n", count);
return EXIT_SUCCESS;
}
FILE * Fopen(const char *path, const char *mode)
{
FILE *f = fopen(path, mode);
if( f == NULL ) {
perror(path);
exit(EXIT_FAILURE);
}
return f;
}
This program will consistently print one greater than the number of characters in the input stream (assuming no read errors). Consider the case where the input stream is empty:
$ ./a.out < /dev/null
Number of characters read: 1
In this case, feof() is called before any data has been read, so it returns false. The loop is entered, fgetc() is called (and returns EOF), and count is incremented. Then feof() is called and returns true, causing the loop to abort.
This happens in all such cases. feof() does not return true until after a read on the stream encounters the end of file. The purpose of feof() is NOT to check if the next read will reach the end of file. The purpose of feof() is to determine the status of a previous read function
and distinguish between an error condition and the end of the data stream. If fread() returns 0, you must use feof/ferror to decide whether an error occurred or if all of the data was consumed. Similarly if fgetc returns EOF. feof() is only useful after fread has returned zero or fgetc has returned EOF. Before that happens, feof() will always return 0.
It is always necessary to check the return value of a read (either an fread(), or an fscanf(), or an fgetc()) before calling feof().
Even worse, consider the case where a read error occurs. In that case, fgetc() returns EOF, feof() returns false, and the loop never terminates. In all cases where while(!feof(p)) is used, there must be at least a check inside the loop for ferror(), or at the very least the while condition should be replaced with while(!feof(p) && !ferror(p)) or there is a very real possibility of an infinite loop, probably spewing all sorts of garbage as invalid data is being processed.
So, in summary, although I cannot state with certainty that there is never a situation in which it may be semantically correct to write "while(!feof(f))" (although there must be another check inside the loop with a break to avoid a infinite loop on a read error), it is the case that it is almost certainly always wrong. And even if a case ever arose where it would be correct, it is so idiomatically wrong that it would not be the right way to write the code. Anyone seeing that code should immediately hesitate and say, "that's a bug". And possibly slap the author (unless the author is your boss in which case discretion is advised.)
No it's not always wrong. If your loop condition is "while we haven't tried to read past end of file" then you use while (!feof(f)). This is however not a common loop condition - usually you want to test for something else (such as "can I read more"). while (!feof(f)) isn't wrong, it's just used wrong.
feof() indicates if one has tried to read past the end of file. That means it has little predictive effect: if it is true, you are sure that the next input operation will fail (you aren't sure the previous one failed BTW), but if it is false, you aren't sure the next input operation will succeed. More over, input operations may fail for other reasons than the end of file (a format error for formatted input, a pure IO failure -- disk failure, network timeout -- for all input kinds), so even if you could be predictive about the end of file (and anybody who has tried to implement Ada one, which is predictive, will tell you it can complex if you need to skip spaces, and that it has undesirable effects on interactive devices -- sometimes forcing the input of the next line before starting the handling of the previous one), you would have to be able to handle a failure.
So the correct idiom in C is to loop with the IO operation success as loop condition, and then test the cause of the failure. For instance:
while (fgets(line, sizeof(line), file)) {
/* note that fgets don't strip the terminating \n, checking its
presence allow to handle lines longer that sizeof(line), not showed here */
...
}
if (ferror(file)) {
/* IO failure */
} else if (feof(file)) {
/* format error (not possible with fgets, but would be with fscanf) or end of file */
} else {
/* format error (not possible with fgets, but would be with fscanf) */
}
feof() is not very intuitive. In my very humble opinion, the FILE's end-of-file state should be set to true if any read operation results in the end of file being reached. Instead, you have to manually check if the end of file has been reached after each read operation. For example, something like this will work if reading from a text file using fgetc():
#include <stdio.h>
int main(int argc, char *argv[])
{
FILE *in = fopen("testfile.txt", "r");
while(1) {
char c = fgetc(in);
if (feof(in)) break;
printf("%c", c);
}
fclose(in);
return 0;
}
It would be great if something like this would work instead:
#include <stdio.h>
int main(int argc, char *argv[])
{
FILE *in = fopen("testfile.txt", "r");
while(!feof(in)) {
printf("%c", fgetc(in));
}
fclose(in);
return 0;
}
Is there any method to call getline() and, if there is no input given, not to block and waiting?
I have the following code:
while(true){
if(recv(sd, tBuffer, sizeof(tBuffer), MSG_PEEK | MSG_DONTWAIT) > 0) break;
getline(cin,send);
}
I want to wait for an input, but if I receive some data at sd socket, I want stop waiting for the data and exit the while. My code right now, just stucks on first iteration at getline(). I want to evaluate getline(), and if is no input available, go back at if.
Is this possible?
PS: I tried with cin.peek(), but that blocks for input too.
You should be able to do this by setting non-blocking mode on standard input, file descriptor 0:
int flags = fcntl(0, F_GETFL, 0);
fcntl(0, F_SETFL, flags | O_NONBLOCK);
Now, if there's no input available, the underlying read() system call will return 0, and std::cin will think that this is end of file, and set eof() on std::cin.
When you wish to read from standard input again, just clear() the stream's state.
The only complicating factor here is that this makes it difficult to detect a real end-of-file condition on std::cin. Not much a problem when standard input is an interactive terminal; but if standard input can be a file this is going to be an issue.
In that case, your only realistic option is to forego std::cin completely, put non-blocking mode on file descriptor 0, poll() or select() it, to determine when there's something to read, then read() it.
Although you could also use poll() or select() with std::cin, this is going to get complicated, because you will need to explicitly check if there's anything already buffered in std::cin's streambuf, because that would, obviously, preempt any kind of poll() or select() checking; but by attempting to read something from std::cin, you still run the risk of reading the buffered data, then attempting to read() from the underlying file descriptor that's now in non-blocking mode, this resulting in a fake end-of-file condition.
To summarize: you need invest some additional time reading and understanding how file streams, and stream buffers work; and how file descriptors actually work, and how non-blocking mode works; in order to figure out the correct logic you will need to use.
Oh, and if you insist on going the non-blocking route with std::cin, and getline(), you will have no easy way to determine if the string returned by getline() ends because getline() actually read a newline from standard input, or it reached a premature fake-end of file condition and not the entire line of input has actually been read.
So, with non-blocking mode, and std::cin, you'll be pretty much forced to use read(), instead of getline().
The istream::getsome() method can be used to perform non-blocking reads. You can use it to build a non-blocking equivalent of std::getline.
bool getline_async(std::istream& is, std::string& str, char delim = '\n') {
static std::string lineSoFar;
char inChar;
int charsRead = 0;
bool lineRead = false;
str = "";
do {
charsRead = is.readsome(&inChar, 1);
if (charsRead == 1) {
// if the delimiter is read then return the string so far
if (inChar == delim) {
str = lineSoFar;
lineSoFar = "";
lineRead = true;
} else { // otherwise add it to the string so far
lineSoFar.append(1, inChar);
}
}
} while (charsRead != 0 && !lineRead);
return lineRead;
}
This functions works identically to the original std::getline() function except it always returns instantly. I've got it on my gists because it comes in handy occasionally.
I used select() to retrieve the status of the stdin file descriptor. This worked on Ubuntu and on an embedded Linux board. If stdin still has not received an enter keystroke from the user, select will wait some time and report that stdin is not ready. stopReading can stop monitoring the stdin and continue on other stuff. You can edit it as needed. It may not work on special input tty's.
#include <sys/select.h>
static constexpr int STD_INPUT = 0;
static constexpr __suseconds_t WAIT_BETWEEN_SELECT_US = 250000L;
// Private variable in my class, but define as needed in your project
std::atomic<bool> stopReading;
...
std::string userInput = "";
while (false == stopReading)
{
struct timeval tv = { 0L, WAIT_BETWEEN_SELECT_US };
fd_set fds;
FD_ZERO(&fds);
FD_SET(STD_INPUT, &fds);
int ready = select(STD_INPUT + 1, &fds, NULL, NULL, &tv);
if (ready > 0)
{
std::getline(std::cin, userInput);
break;
}
}
What is wrong with using feof() to control a read loop? For example:
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
char *path = "stdin";
FILE *fp = argc > 1 ? fopen(path=argv[1], "r") : stdin;
if( fp == NULL ){
perror(path);
return EXIT_FAILURE;
}
while( !feof(fp) ){ /* THIS IS WRONG */
/* Read and process data from file… */
}
if( fclose(fp) != 0 ){
perror(path);
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
What is wrong with this loop?
TL;DR
while(!feof) is wrong because it tests for something that is irrelevant and fails to test for something that you need to know. The result is that you are erroneously executing code that assumes that it is accessing data that was read successfully, when in fact this never happened.
I'd like to provide an abstract, high-level perspective. So continue reading if you're interested in what while(!feof) actually does.
Concurrency and simultaneity
I/O operations interact with the environment. The environment is not part of your program, and not under your control. The environment truly exists "concurrently" with your program. As with all things concurrent, questions about the "current state" don't make sense: There is no concept of "simultaneity" across concurrent events. Many properties of state simply don't exist concurrently.
Let me make this more precise: Suppose you want to ask, "do you have more data". You could ask this of a concurrent container, or of your I/O system. But the answer is generally unactionable, and thus meaningless. So what if the container says "yes" – by the time you try reading, it may no longer have data. Similarly, if the answer is "no", by the time you try reading, data may have arrived. The conclusion is that there simply is no property like "I have data", since you cannot act meaningfully in response to any possible answer. (The situation is slightly better with buffered input, where you might conceivably get a "yes, I have data" that constitutes some kind of guarantee, but you would still have to be able to deal with the opposite case. And with output the situation is certainly just as bad as I described: you never know if that disk or that network buffer is full.)
So we conclude that it is impossible, and in fact unreasonable, to ask an I/O system whether it will be able to perform an I/O operation. The only possible way we can interact with it (just as with a concurrent container) is to attempt the operation and check whether it succeeded or failed. At that moment where you interact with the environment, then and only then can you know whether the interaction was actually possible, and at that point you must commit to performing the interaction. (This is a "synchronisation point", if you will.)
EOF
Now we get to EOF. EOF is the response you get from an attempted I/O operation. It means that you were trying to read or write something, but when doing so you failed to read or write any data, and instead the end of the input or output was encountered. This is true for essentially all the I/O APIs, whether it be the C standard library, C++ iostreams, or other libraries. As long as the I/O operations succeed, you simply cannot know whether further, future operations will succeed. You must always first try the operation and then respond to success or failure.
Examples
In each of the examples, note carefully that we first attempt the I/O operation and then consume the result if it is valid. Note further that we always must use the result of the I/O operation, though the result takes different shapes and forms in each example.
C stdio, read from a file:
for (;;) {
size_t n = fread(buf, 1, bufsize, infile);
consume(buf, n);
if (n == 0) { break; }
}
The result we must use is n, the number of elements that were read (which may be as little as zero).
C stdio, scanf:
for (int a, b, c; scanf("%d %d %d", &a, &b, &c) == 3; ) {
consume(a, b, c);
}
The result we must use is the return value of scanf, the number of elements converted.
C++, iostreams formatted extraction:
for (int n; std::cin >> n; ) {
consume(n);
}
The result we must use is std::cin itself, which can be evaluated in a boolean context and tells us whether the stream is still in the good() state.
C++, iostreams getline:
for (std::string line; std::getline(std::cin, line); ) {
consume(line);
}
The result we must use is again std::cin, just as before.
POSIX, write(2) to flush a buffer:
char const * p = buf;
ssize_t n = bufsize;
for (ssize_t k = bufsize; (k = write(fd, p, n)) > 0; p += k, n -= k) {}
if (n != 0) { /* error, failed to write complete buffer */ }
The result we use here is k, the number of bytes written. The point here is that we can only know how many bytes were written after the write operation.
POSIX getline()
char *buffer = NULL;
size_t bufsiz = 0;
ssize_t nbytes;
while ((nbytes = getline(&buffer, &bufsiz, fp)) != -1)
{
/* Use nbytes of data in buffer */
}
free(buffer);
The result we must use is nbytes, the number of bytes up to and including the newline (or EOF if the file did not end with a newline).
Note that the function explicitly returns -1 (and not EOF!) when an error occurs or it reaches EOF.
You may notice that we very rarely spell out the actual word "EOF". We usually detect the error condition in some other way that is more immediately interesting to us (e.g. failure to perform as much I/O as we had desired). In every example there is some API feature that could tell us explicitly that the EOF state has been encountered, but this is in fact not a terribly useful piece of information. It is much more of a detail than we often care about. What matters is whether the I/O succeeded, more-so than how it failed.
A final example that actually queries the EOF state: Suppose you have a string and want to test that it represents an integer in its entirety, with no extra bits at the end except whitespace. Using C++ iostreams, it goes like this:
std::string input = " 123 "; // example
std::istringstream iss(input);
int value;
if (iss >> value >> std::ws && iss.get() == EOF) {
consume(value);
} else {
// error, "input" is not parsable as an integer
}
We use two results here. The first is iss, the stream object itself, to check that the formatted extraction to value succeeded. But then, after also consuming whitespace, we perform another I/O/ operation, iss.get(), and expect it to fail as EOF, which is the case if the entire string has already been consumed by the formatted extraction.
In the C standard library you can achieve something similar with the strto*l functions by checking that the end pointer has reached the end of the input string.
It's wrong because (in the absence of a read error) it enters the loop one more time than the author expects. If there is a read error, the loop never terminates.
Consider the following code:
/* WARNING: demonstration of bad coding technique!! */
#include <stdio.h>
#include <stdlib.h>
FILE *Fopen(const char *path, const char *mode);
int main(int argc, char **argv)
{
FILE *in;
unsigned count;
in = argc > 1 ? Fopen(argv[1], "r") : stdin;
count = 0;
/* WARNING: this is a bug */
while( !feof(in) ) { /* This is WRONG! */
fgetc(in);
count++;
}
printf("Number of characters read: %u\n", count);
return EXIT_SUCCESS;
}
FILE * Fopen(const char *path, const char *mode)
{
FILE *f = fopen(path, mode);
if( f == NULL ) {
perror(path);
exit(EXIT_FAILURE);
}
return f;
}
This program will consistently print one greater than the number of characters in the input stream (assuming no read errors). Consider the case where the input stream is empty:
$ ./a.out < /dev/null
Number of characters read: 1
In this case, feof() is called before any data has been read, so it returns false. The loop is entered, fgetc() is called (and returns EOF), and count is incremented. Then feof() is called and returns true, causing the loop to abort.
This happens in all such cases. feof() does not return true until after a read on the stream encounters the end of file. The purpose of feof() is NOT to check if the next read will reach the end of file. The purpose of feof() is to determine the status of a previous read function
and distinguish between an error condition and the end of the data stream. If fread() returns 0, you must use feof/ferror to decide whether an error occurred or if all of the data was consumed. Similarly if fgetc returns EOF. feof() is only useful after fread has returned zero or fgetc has returned EOF. Before that happens, feof() will always return 0.
It is always necessary to check the return value of a read (either an fread(), or an fscanf(), or an fgetc()) before calling feof().
Even worse, consider the case where a read error occurs. In that case, fgetc() returns EOF, feof() returns false, and the loop never terminates. In all cases where while(!feof(p)) is used, there must be at least a check inside the loop for ferror(), or at the very least the while condition should be replaced with while(!feof(p) && !ferror(p)) or there is a very real possibility of an infinite loop, probably spewing all sorts of garbage as invalid data is being processed.
So, in summary, although I cannot state with certainty that there is never a situation in which it may be semantically correct to write "while(!feof(f))" (although there must be another check inside the loop with a break to avoid a infinite loop on a read error), it is the case that it is almost certainly always wrong. And even if a case ever arose where it would be correct, it is so idiomatically wrong that it would not be the right way to write the code. Anyone seeing that code should immediately hesitate and say, "that's a bug". And possibly slap the author (unless the author is your boss in which case discretion is advised.)
No it's not always wrong. If your loop condition is "while we haven't tried to read past end of file" then you use while (!feof(f)). This is however not a common loop condition - usually you want to test for something else (such as "can I read more"). while (!feof(f)) isn't wrong, it's just used wrong.
feof() indicates if one has tried to read past the end of file. That means it has little predictive effect: if it is true, you are sure that the next input operation will fail (you aren't sure the previous one failed BTW), but if it is false, you aren't sure the next input operation will succeed. More over, input operations may fail for other reasons than the end of file (a format error for formatted input, a pure IO failure -- disk failure, network timeout -- for all input kinds), so even if you could be predictive about the end of file (and anybody who has tried to implement Ada one, which is predictive, will tell you it can complex if you need to skip spaces, and that it has undesirable effects on interactive devices -- sometimes forcing the input of the next line before starting the handling of the previous one), you would have to be able to handle a failure.
So the correct idiom in C is to loop with the IO operation success as loop condition, and then test the cause of the failure. For instance:
while (fgets(line, sizeof(line), file)) {
/* note that fgets don't strip the terminating \n, checking its
presence allow to handle lines longer that sizeof(line), not showed here */
...
}
if (ferror(file)) {
/* IO failure */
} else if (feof(file)) {
/* format error (not possible with fgets, but would be with fscanf) or end of file */
} else {
/* format error (not possible with fgets, but would be with fscanf) */
}
feof() is not very intuitive. In my very humble opinion, the FILE's end-of-file state should be set to true if any read operation results in the end of file being reached. Instead, you have to manually check if the end of file has been reached after each read operation. For example, something like this will work if reading from a text file using fgetc():
#include <stdio.h>
int main(int argc, char *argv[])
{
FILE *in = fopen("testfile.txt", "r");
while(1) {
char c = fgetc(in);
if (feof(in)) break;
printf("%c", c);
}
fclose(in);
return 0;
}
It would be great if something like this would work instead:
#include <stdio.h>
int main(int argc, char *argv[])
{
FILE *in = fopen("testfile.txt", "r");
while(!feof(in)) {
printf("%c", fgetc(in));
}
fclose(in);
return 0;
}
I'm tring to do a simple exercise here, but i need to understand how EOF works first.
void main()
{
char s1[1000];
while (scanf("%s", s1)!=EOF)
;
printf("%s",s1);
}
The idea is to have multiple lines in input, and display them.
The problem I have is that if I put
Hello World
This is stackoverflow
When printf is called, it only prints
stackoverflow
Why isn't it printing everything and how do I make it print?
Regards
Remove the semicolon ;:
while (scanf("%s", s1)!=EOF)
printf("%s",s1);
Note that this will still exhibit odd behavior at end of file depending on how it ends exactly. Furthermore, it splits the input into words, which are separated by spaces or new lines. You may want to simply split into lines.
So you may be better served with for instance:
while (gets(s1)!=NULL)
puts(s1);
This code fragments reads your input line by line until end-of-file.
To read everything (or as much as your buffer can hold), you can use:
char s1[1000] = "";
fread(s1, sizeof(s1) - 1, 1, stdin);
puts(s1);
However, my preferred method of reading a text file is:
using namespace std;
string line;
while (getline(cin, line))
{
cout << line << endl;
}
That is because usually I want to process a file line by line, and getline with a string ensures the line buffer is always big enough.
You probably want this:
char s1[1000][20];
int i = 0 ;
while (!feof(stdin))
fgets(s1[i++], 20, stdin) ;
int j ;
for (j = 0; j < i; j++)
printf("%s\n", s1[j]);
Here you can enter at most 1000 lines that are maximum 19 characters long.
What you have is a loop that reads words into a buffer until it reaches EOF (and does nothing with those words), followed by a printf to print the contents of the buffer. The printf is after the loop (not in it), so executes once after the loop completes. At that time, the buffer will contain the last word read, so that is what gets printed.
The EOF return test means "nothing more to be read", which isn't necessarily an end of file (might be an error condition of some kind), but in practice that distinction can be ignored. Looping until your reading function returns EOF or NULL (depends on function) is good practice.
If you want to print each word as it is read, you need to put a printf in the loop.
If you want to store the words for later processing, you need to store them somewhere. That means declaring some storage space, or allocating space on the heap, and some bookkeeping to track how much space you've used/allocated.
If you want lines rather than words, you should use fgets instead of scanf("%s". Note that fgets returns NULL rather than EOF when there's nothing more to be read.
Because it only prints the last thing that is read from the file ("stackoverflow"). This is caused by the semicolon after the end of your while(...); - this means that you are doing while(...) { /* do nothing */} - which is probably not what you wanted
Also, printf("%s",s1)!='\0'; makes no sense at all. For one thing, printf returns the number of characters printed - '\0' is the value zero written as a character constant. And of course, doing != 0 of the result without some sort of use of the comparison is pretty much pointless too.
Use fgets instead of scanf if you want to read one line at at time. scanf will stop reading when it finds a whitespace. fgets will read till the end of the line.
Use fgets(). Simple and sweet
char buf[1000];
while (fgets(buf, sizeof buf, stdin) != NULL) {
fputs(buf, stdout);
}
Here is how end-of-file works in C. The input channels are called input streams; disk files and stdin are both input streams. The "end-of-file" state is a flag that a stream has, and that flag is triggered when you try to read from a stream, but it turns out there are no more characters in the stream, and there never will be any more. (If the stream is still active but just waiting for user input for example, it is not considered to be end-of-file; read operations will block).
Streams can have other error states, so looping until "end-of-file" is set is usually wrong. If the stream does go into an error state then your loop will never exit (aka. "infinite loop").
The end-of-file state can be checked by feof. However, some input operations also can signal an error as well as, or instead of, returning the actual data they were intended to read. These functions can return the value EOF. Usually these functions return EOF in both cases: end-of-file, and stream error. This is different to feof which only returns true in the case of end-of-file.
For example, getchar() and scanf will return EOF if it was end-of-file, but also if the stream is in an error state.
So it is OK to use getchar()'s result as a loop condition, but not feof on its own.
Also, it is sometimes not OK to use scanf() != EOF as a loop condition. It's possible that there is no stream error, but just that the data you requested wasn't there. For example, if you scan for "%d" but there are letters in the stream. Instead, it's better to check for successful conversion (scanf returns the number of successful conversions it performed). Then when you exit your loop, you can go on to call feof and ferror to see whether it was due to end-of-file, or error, or just unexpected input.
I have to modify a simple shell I wrote for a previous homework assignment to handle I/O redirection and I'm having trouble getting the pipes to work. It seems that when I write and read to stdout and from stdin after duplicating the file descriptors in the separates processes, the pipe works, but if I use anything like printf, fprintf, gets, fgets, etc to try and see if the output is showing up in the pipe, it goes to the console even though the file descriptor for stdin and stdout clearly is a copy of the pipe (I don't know if that's the correct way to phrase that, but the point is clear I think).
I am 99.9% sure that I am doing everything as it should be at least in plain C -- such as closing all the file descriptors appropriately after the dup() -- and file I/O works fine, so this seems like an issue of a detail that I am not aware of and cannot find any information on. I've spent most of the day trying different things and the past few hours googling trying to figure out if I could redirect cin and cout to the pipe to see if that would fix it, but it seems like it's more trouble than it's worth at this point.
Should this work just by redirecting stdin and stdout since cin and cout are supposed to be sync'd with stdio? I thought it should, especially since the commands are probably written in C so they would use stdio, I would think. However, if I try a command like "cat [file1] [file2] | sort", it prints the result of cat [file1] [file2] to the command line, and the sort doesn't get any input so it has no output. It's also clear that cout and cin are not affected by the dup() either, so I put two and two together and came to this conclusion
Here is a somewhat shortened version of my code minus all the error checking and things like that, which I am confident I am handling well. I can post the full code if it come to it, but it's a lot so I'll start with this.
I rewrote the function so that the parent forks off a child for each command and connects them with pipes as necessary and then waits for the child processes to die. Again, write and read on the file descriptors 0 and 1 work (i.e. write to and reads from the pipe), stdio on the FILE pointers stdin and stdout do not work (do not write to pipe).
Thanks a lot, this has been killing me...
UPDATE: I wasn't changing the string cmd for each of the different commands so it didn't appear to work because the pipe just went to the same command so the final output was the same... Sorry for the dumbness, but thanks because I found the problem with strace.
int call_execv( string cmd, vector<string> &argv, int argc,
vector<int> &redirect)
{
int result = 0, pid, /* some other declarations */;
bool file_in, file_out, pipe_in, pipe_out;
queue<int*> pipes; // never has more than 2 pipes
// parse, fork, exec, & loop if there's a pipe until no more pipes
do
{
/* some declarations for variables used in parsing */
file_in = file_out = pipe_in = pipe_out = false;
// parse the next command and set some flags
while( /* there's more redirection */ )
{
string symbol = /* next redirection symbol */
if( symbol == ">" )
{
/* set flags, get filename, etc */
}
else if( symbol == "<" )
{
/* set flags, get filename, etc */
}
else if( pipe_out = (symbol == "|") )
{
/* set flags, and... */
int tempPipes[2];
pipes.push( pipe(tempPipes) );
break;
}
}
/* ... set some more flags ... */
// fork child
pid = fork();
if( pid == 0 ) // child
{
/* if pipe_in and pipe_out set, there are two pipes in queue.
the old pipes read is dup'd to stdin, and the new pipes
write is dup'd to stdout, other two FD's are closed */
/* if only pipe_in or pipe_out, there is one pipe in queue.
the unused end is closed in whichever if statement evaluates */
/* if neither pipe_in or pipe_out is set, no pipe in queue */
// redirect stdout
if( pipe_out ){
// close newest pipes read end
close( pipes.back()[P_READ] );
// dup the newest pipes write end
dup2( pipes.back()[P_WRITE], STDOUT_FILENO );
// close newest pipes write end
close( pipes.back()[P_WRITE] );
}
else if( file_out )
freopen(outfile.c_str(), "w", stdout);
// redirect stdin
if( pipe_in ){
close( pipes.front()[P_WRITE] );
dup2( pipes.front()[P_READ], STDIN_FILENO );
close( pipes.front()[P_READ] );
}
else if ( file_in )
freopen(infile.c_str(), "r", stdin);
// create argument list and exec
char **arglist = make_arglist( argv, start, end );
execv( cmd.c_str(), arglist );
cout << "Execution failed." << endl;
exit(-1); // this only executes is execv fails
} // end child
/* close the newest pipes write end because child is writing to it.
the older pipes write end is closed already */
if( pipe_out )
close( pipes.back()[P_WRITE] );
// remove pipes that have been read from front of queue
if( init_count > 0 )
{
close( pipes.front()[P_READ] ); // close FD first
pipes.pop(); // pop from queue
}
} while ( pipe_out );
// wait for each child process to die
return result;
}
Whatever the problem, you are not checking any return values. How do you know if the pipe() or the dup2() command succeeded? Have you verified that stdout and stdin really point to the pipe right before execv? Does execv keep the filedescriptors you give it? Not sure, here is the corresponding paragraph from the execve documentation:
By default, file descriptors remain open across an execve(). File descriptors that are marked close-on-exec are closed; see the description of FD_CLOEXEC in fcntl(2). (If a
file descriptor is closed, this will cause the release of all record locks obtained on the underlying file by this process. See fcntl(2) for details.) POSIX.1-2001 says
that if file descriptors 0, 1, and 2 would otherwise be closed after a successful execve(), and the process would gain privilege because the set-user_ID or set-group_ID per‐
mission bit was set on the executed file, then the system may open an unspecified file for each of these file descriptors. As a general principle, no portable program,
whether privileged or not, can assume that these three file descriptors will remain closed across an execve().
You should add more debug output and see what really happens. Did you use strace -f (to follow children) on your program?
The following:
queue<int*> pipes; // never has more than 2 pipes
// ...
int tempPipes[2];
pipes.push( pipe(tempPipes) );
Is not supposed to work. Not sure how it compiles since the result of pipe() is int. Note only that, tempPipes goes out of scope and its contents get lost.
Should be something like that:
struct PipeFds
{
int fds[2];
};
std::queue<PipeFds> pipes;
PipeFds p;
pipe(p.fds); // check the return value
pipes.push(p);