When reading K&R, I became interested in how the file position is determined. By file position, I mean where in the file the stream is currently reading or writing. I think it must have something to do with the file pointer, or the piece of data it's pointing to. So I checked stack overflow, and find the following answer:
Does fread move the file pointer?
The answer indicates that file pointer will change with the change of file position. This makes me very confused, because in my understanding, a file pointer for a certain file should always point to the same address, where information about this file is stored. So I wrote a small piece of code, trying to find the answer:
#include<stdio.h>
int main(void)
{
char s[1000];
FILE *fp,*fp1,*fp2;
fp = fopen("input","r");
fp1 = fp; /* File poiter before fread */
fread(s,sizeof(char),100,fp);
fp2 = fp; /* File pointer after fread */
printf("%d\n",(fp1 == fp2) ? 1 : -1);
}
It gives the output 1, which I believe indicates that the file pointer actually doesn't move and is still pointing to the same address. I have also changed the fread line to be a fseek, which gave the same output. So does file pointer move with the change of file position, or where am I wrong in the verifying process?
Thanks!
I think you are confusing the general concept of pointers in C, vs. the nomenclature of a "file pointer". FILE is just a structure that contains most of the "housekeeping" attributes that the C stdio runtime library needs to interact with when using the stdio functions such as, fopen(), fread(), etc. Here is an example of the structure:
typedef struct {
char *fpos; /* Current position of file pointer (absolute address) */
void *base; /* Pointer to the base of the file */
unsigned short handle; /* File handle */
short flags; /* Flags (see FileFlags) */
short unget; /* 1-byte buffer for ungetc (b15=1 if non-empty) */
unsigned long alloc; /* Number of currently allocated bytes for the file */
unsigned short buffincrement; /* Number of bytes allocated at once */
} FILE;
Note that this may be somewhat platform-dependent, so don't take it as gospel. So when you call fopen(), the underlying library function interacts with the O/S's file system APIs and caches relevant information about the file, buffer allocation, etc, in this structure. The fopen() function allocates memory for this structure, and then returns the address of that memory back to the caller in the form of a C Pointer.
Assigning the pointers values to another pointer has no effect on the attributes inside the FILE structure. However, the FILE structure, internally, may have indexes or "pointers" to the underlying O/S file. Hence, the confusion in terminology. Hope that helps.
You are right fp is never changed by fread, fseekor other f... functions. Except, of course, if you do fp = fopen(...), but then you are assigning the return value of fopen to fp and then fp changes of course.
Remember, in C parameters are passed by value, so fread cannot change it's value.
But fread does change the internal structure fp points to.
You made some confusion between a file pointer, under common definition, and the pointer in the file.
Normally with the term file pointer we refer to a pointer to a FILE structure. That structure contains all variables necessary to manage file access. This structure is created upon a successful opening of a file, and remains the same (same address) for all the time until you fclose() the file (when became undefined).
Inside the FILE structure there are many pointers that points to the file block on disk and to the position inside the current record. These pointers, managed by file I/O routines, changes when file is accessed (read or write).
And these pointers are that to which the answer you cited refers.
Related
So I have declared a pointer *f of type FILE and now I say that that pointer is equal to fopen("text.txt", "r"). So since a pointer stores an address, is fopen giving back the address of a file?
FILE *f;
f = fopen("text.txt","r");
is fopen giving back the adress of a file?
No. There is no such thing as "address of a file".
What fopen returns is a pointer to dynamically allocated opaque structure FILE, which describes how to get the contents of the file. This description is opaque in a sense that it provides no useful info to you. But routines such as fgets(), fread(), etc. know how to use that info to get the actual file contents.
fclose deallocates this structure, so if you have matching fopen and fclose there are no memory leaks (from these functions).
The function fopen is returning the address of an object of type FILE. According to §7.21.1 ¶2 of the ISO C11 standard, this object must be capable of
recording all the information needed to control a stream, including its file position indicator, a pointer to its associated buffer (if any), an error indicator that records whether a read/write error has occurred, and an end-of-file indicator that records whether the end of the file has been reached;
The exact size and contents of this object may differ from compiler to compiler and is of no interest to the average programmer. All that the average programmer must know is that they must pass the pointer returned by fopen to other I/O functions provided by the C standard library.
The standard does not specify how FILE should be defined. The only thing it says is what result you will get when you pass an object of that type to various functions. This means that this type may be different in various implementations. This is ONE way that is used:
typedef struct _iobuf
{
char* _ptr;
int _cnt;
char* _base;
int _flag;
int _file;
int _charbuf;
int _bufsiz;
char* _tmpfname;
} FILE;
It comes from MinGW32.
Yes, it's giving back the address of a FILE object. Now that type is opaque. That is, you are meant to not know the actual contents of the data structure being referenced and hence must never dereference the pointer (happily, the compiler will not let you get away with this).
As far as I understand passing a pointer to a function essentially passes the copy of the pointer to the function in C. I have a FILE pointer that I pass to a function func(), func() reads a line from a file and then when we return to main(). I read another line from the file using the same FILE pointer.
However, while I would imagine that I'd read the line exactly from before func() was called, I actually read the next line after what func() had read. Can you please explain why FILE pointer behaves this way?
This is my code:
#include <stdio.h>
#define STR_LEN 22
void func(FILE *fd);
int main() {
FILE *fd;
char mainString[STR_LEN];
if (!(fd = fopen("inpuFile", "r"))) {
printf("Couldn't open file\n");
fprintf(stderr, "Couldn't open file\n");
}
func(fd);
fgets(mainString, STR_LEN, fd);
printf("mainString = %s\n", mainString);
fclose(fd);
return 0;
}
void func(FILE *fd) {
char funcString[STR_LEN];
fgets(funcString,STR_LEN, fd);
printf("funcString = %s\n", funcString);
}
However, while I would imagine that I'd read the line exactly from before func was called ...
I can't imagine why you would imagine that. What if the FILE* references a network connection that has no replay capability at all where reading is consumption. Where would the line be stored such that you could read it again? There would be absolutely no place to put it.
Not only would I not imagine that, it's kind of crazy.
As far as I understand passing a pointer to a function essentially passes the copy of the pointer to the function in C.
Correct. But a copy of a pointer points to the very same object. If I point to a car and you copy me, you're pointing to the very same one and only car that I'm pointing to.
Because FILE pointer points on some data that gets changed when the file is read/written.
So the pointer doesn't change (still points to the handler structure of the file) but the data pointed by the structure does.
Try passing pointer as const FILE * you'll see that you cannot because fread operation (and others) alter the pointed data.
One way would be to duplicate the file descriptor, which dup does, but doesn't work on buffered FILE object, only raw file descriptors.
The problem is in your initial statement:
As far as I understand passing a pointer to a function essentially passes the copy of the pointer to the function in C.
This does not change much, as whatever you are accessing as a pointer, still holds the location of the FILE you are accessing, the whole point of using pointers as arguments for a function in C, is so that you can modify a certain value outside the scope of a function.
For example, common usage of an integer pointer as a function argument:
void DoSomethingCool(int *error);
Now using this code to catch the error would work like this:
int error = 0;
DoSomethingCool(&error);
if(error != 0)
printf("Something really bad happened!");
In other words, the pointer will actually modify the integer error, by accessing it's location and writing to it.
An important thing to keep in mind to avoid these kinds of misunderstandings is to recognize that all a pointer is, is essentially the address of something.
So you could (in theory, by simplifying everything a lot) think of an int * as simply an int, the value of which happens to be an address of some variable, for a FILE *, you can think of it as an int, where the value of the int is the location of the FILE variable.
FILE *fd is a pointer only in the sense that its implementation uses C construct called a "pointer". It is not a pointer in the sense of representing a file position.
FILE *fd represents a handle to a file object inside the I/O library, a struct that includes the actual position of the file. In a grossly simplified way, you can think of fd as a C pointer to a file pointer.
When you pass fd around your program, I/O routines make modifications to the file position. This position is shared among all users of fd. If a func() makes a change to that position by reading some data or by calling fseek, all users of the same fd will see the updated position.
I'm writing down a function that should save 3 structures (2 of them are arrays of structs) in a binary file. Here's my function:
void saveFile(Struct1 *s1, Struct2 *s2, Struct3 s3) {
FILE *fp = NULL;
fp = fopen("save.bin", "w+b");
if (fp == NULL) {
printf("Save failed.\n");
}
fwrite(s1, sizeof(Struct1), struct3.nElements, fp);
fwrite(s2, sizeof(Struct2), NELEMENTS, fp);
fwrite(&s3, sizeof(Struct3), 1, fp);
printf("Save done.\n");
}
s1 have struct3.nElements, s2 have NELEMENTS (that's a constant) and s3 is just one struct and not an array. When I try to open the save.bin with HexEditor it gives very different results from the ones I was expecting, I'm wondering if I used correctly the fwrite function, especially for array of structs.
There are small issues with you function that might cause problems:
you define the function as taking s3 by value. Why not pass a pointer to the third struct? Is the saveFile function properly declared before the calling code? Are you sure the calling code passes the struct by value?
You forget to close the stream. The handle gets lost, and the contents is not flushed to disk until the program exits.
You open the file in "w+b" mode: write with read. It is correct to use binary mode, but unnecessary to add the + for read. Just use "wb".
If fopen fails, you output a diagnostic message, but you do not return from the function. You will invoke undefined behavior when trying to write to a NULL stream pointer.
Regarding your question, the dump of the file does not correspond to what you expect... give us more information, such as the definitions of the different structures and the hex dump. Here are some ideas:
Some of the fields in the structures might need a specific aligned and thus be separated from the previous field by padding bytes. The values of those padding bytes is not necessarily 0: if the structures are in automatic storage or allocated with malloc, their initial state is undefined and can change as a side effect of storing other fields.
Integers can have different sizes and be stored in little endian or big endian order in the file, depending on the specific architecture your program is compiled for. For this reason, values stored by your program should only be read back with the appropriate, but reasonably similar code, running on the same architecture and OS.
If your structures contain pointers, you cannot really make sense from the values stored in the output file.
I am trying to understand what does file pointer increment means .
I have declared file pointer fp and assuming that when I use fopen for any file (say test99.txt) and try to read it then
compiler dynamically allocate memory ( in heap ofcourse because I think internally fopen make use of malloc in order to put file in main memory)
and once file stream/data of file is put in memory then I am assuming fp contains the start address of that file stream.
Now incrementing fp by 1 (++fp) which is pointer of type FILE will increase/hop the position of fp by total size of data/stream inside that file
test99.txt. If not, and let's say incrementing fp will move fp pointer to the next character (1 byte) within the file stream then why is the below output ? ( why fp moved by 16 bytes:see the diff)
Where am I misunderstanding .
Information : size of file is 66 bytes(8KB) . Using program on unix platform on 64 bit machine (ia64 hp server) . Compiler is of HP HP-ACC-Link. Program in C
#include<stdio.h>
#include<stdlib.h>
int main()
{
FILE *fp;
char ch;
fp=fopen("/home/akhils/file_dir/test99.txt","r");
printf("Address of file is %d", fp);
printf("\nLet's see value of ++fp %d\n", ++fp);
fclose(fp);
}
Output : Address of file is 2130569600
Let's see value of ++fp 2130569616
You should not be incrementing FILE* pointers, it only points to one FILE structure allocated by fopen for you.
fp is a pointer to object of type FILE. It doesn't refer to any data of the actual file, it contains some internal data like handles and stuff.
When you increase this pointer, it is increased by the size of the FILE object (it depends on implementation, 16 bytes in your example). So if it was 2130569600, it will become 2130569616. But if you try to access something by this incremented address, you will probably get an error.
The incremental function adds a number which is the weight of the type you are using.
For pointers, it permits you to go to the next pointer if you have a table of pointer. Otherwise, it's a bad use.
Here, it adds 16 bytes to the original address because a FILE pointer weight 16 bytes, like all pointers. Like Maxim Egorushkin said, it only point to the the structure allocated.
Sorry for my bad english, I hope I'll be helpfull for you.
inline void readSymColValUpdRow(int *row, const int nmat,
int **col, double **val, const int nnz,
FILE *fpcol)
{
*col = (int*)_mm_malloc(sizeof(int)*nnz, 64);
*val = (double*)_mm_malloc(sizeof(double)*nnz, 64);
FILE *fpval;
*fpval = *fpcol;
const int BUF_LEN = nnz*10;
char buf[BUF_LEN];
fgets(buf, BUF_LEN, fpval);
fgets(buf, BUF_LEN, fpval);
...//other code
}
Here's what I'm trying to do. I need two file pointers referring to different places in a file within the function "readSymColValUpdRow()". Thus I've declared a file pointer *fpval and assign the content of fpcol to it. "fpcol" is a valid file pointer passed by function parameter. I try to make fpval pointing two lines ahead.
However, I always get "Segmentation fault" by doing so. Once I comment
*fpval = *fpcol;
and other relevant codes everything's just fine. I don't really understand what's going wrong here. Thank you for your help.
Note that FILE * does not mean that the pointer value represents a position in a file, and that you can treat it as a regular pointer!
It means "pointer to FILE", where FILE is an "object", i.e. a bunch of data fields that are used to represent an open file, which in turn has a current position.
When you do I/O on a file, those fields (or fields even further down) change, but the FILE * pointer value itself doesn't. So you can't copy the FILE * to "save" the position in a file. You must use ftell() and fseek() to jump around in a file.
You are deferencing fpval and the variable is unitialized, this causes the segmentation fault. Also you cannot make a copy of the file pointer that way, you need to make a copy of the underlying file descriptor, for example:
FILE *fpval = fdopen(dup(fileno(fpcol)), "rb");
fileno will obtain the file descriptor, dup will make a duplicate of it and fdopen will open that file descriptor and create the new FILE pointer.
Then close it after you are done with it.
fpval is just an uninitialised pointer, hence the crash. But what you're trying to do will not work even if you correctly duplicate the FILE *. Instead you should just maintain two seek positions within the file and use ftell/fseek to switch between the two areas from which you're reading.