I am trying to exit a program without using exit(). I have come up with a very convoluted and dirty solution (I am a Beginner).
I would like to use if statements and if it is true, then I would like to use goto to go the main function and then return 3; and end the program.
Here is a bit of code:
FILE *filepointer;
char * line = NULL;
size_t len = 0;
size_t read;
int linecount = 0;
filepointer = fopen(filename, "r");
if (filepointer == NULL)
{
printf("[ERR] Could not read file %s.\n",filename );
goto FILE_ERROR;
}
...
int main(){
...
FILE_ERROR: return 3;
}
This however does not work as I cannot get jump between functions because I get undeclared Label as an error. Is there any way I can exclude exit() from my program and still end it returning a certain value. If there is a better solution, please let me know
The only good answer to this question is: don't do it. gotos used in this way make your code very hard to reason about.
Refactor your code so that you have a clear structure and hierarchy of calls. Use return values to propagate success/failure throughout the call stack.
goto can't be used to jump across different functions; it can only be used within the same function. To jump between functions, you can look at setjmp() and longjmp() functions.
Having said, since you claim to be a beginner, I am not convinced you really need to use either of the above. You can simply modify your function to return an "error" value. And in main(), check its value and return from main() with the error value.
By design, a goto cannot jump from one function to another. It can only be used to jump within a given function.
There are ways to jump between functions, but doing so is not only very poor design but also dangerous as it is very easy to put your program in an invalid state.
The proper way to handle this is to have your function return a specific value (or set of values) to indicate an error. Then the calling function would check for one of those error values and act accordingly.
For example:
int readFile(char *filename)
{
FILE *filepointer;
char * line = NULL;
size_t len = 0;
size_t read;
int linecount = 0;
filepointer = fopen(filename, "r");
if (filepointer == NULL)
{
// add strerror(error) to the error message to know why fopen failed
printf("[ERR] Could not read file %s: %s.\n",filename, strerror(errno) );
// error completion
return 0;
}
...
// successful completion
return 1;
}
int main(){
...
if (readFile("myfile") == 0) {
return 3;
}
...
}
If you wanted to use a go-to , and insisted on doing that, you could I guess try to expand your 1st function so it includes / encapsulates the 2nd function, and get rid of the 2nd function conpletely, so your able to do go tos and subroutines within this much larger function.
Is that an option you could try ? (If you were dead cert on using Goto's, ) ?
I would give that a go.
Related
Ok, here is the deal...i remember creating a program past week that required me to open a file in binary mode and write data to it. At first, i tried using the fopen function, checking if the result is ok, then try to write data. I remember that at first try, the fwrite operation wasn't working. Then, after moving declaration of variables from a place to another, i was finally able to make the fwrite to insert data to the file.
Now, i need to create another similar program to do some other stuff, so i wanted to use the same allocation code (actually, i wanted to create a specific function to do the same), and here is what i was able to come up with:
#include <stdio.h>
int openfile(FILE *main, char *name, int option);
int main(void)
{
FILE *main;
int header_init;
int result;
switch (openfile(main,"main_file.bin",1)) {
case 1:
header_init = -1;
//fseek(main,0,SEEK_SET); --> useless
fwrite(&header_init,sizeof(int),1,main);
printf("ftell = %d\n",ftell(main)); break;
case 2:
fread(&header_init,sizeof(int),1,main);
printf("%d\n",header_init); break;
default:
printf("Error trying to open file\n");
}
printf("header_init is %d\n",header_init);
fclose(main); exit(0);
}
int openfile(FILE *main, char *name, int option)
{
int result_alloc;
int F_OK;
if (result_alloc = access (name, F_OK) != 0) {
printf("File not found, allocating a new one\n");
if ((main= fopen(name,"w+b")) != NULL) return 1;
}
else {
printf("File exist, allocating as r+b\n");
if ((main= fopen(name,"r+b")) != NULL) return 2;
}
printf("Failure trying to open");
return 0;
}
For some unfortunate reason, the fwrite operation is not writing -1 to the allocated file. My intention with this program is so that it will always check for existence of that specific file: if there is one in place, simply open it with r+b to allow update functions without overwriting the actual file contents. Otherwise, allocate a new one with a header value of -1 (i will use this file as a record file with chained list structure).
Seriously, i cannot understand why this is not working. The idea is the same of my previous program. The only thing that changed is that i created a function, because this is going to happen me later (because of the 3rd parameter that will allow me to reduce my code and make it more "readable" - at least this is the intention!). I HAVE to admit that i have some attention to details problem, but i am working hard to get better at it, i am probably missing something stupid in this code, but after hours looking at it i really wanted to ask here for some help. Thanks
Edit: I am running it under z/Linux. What i am trying to understand is, why the code above doesn't write -1 to the file, but the one below writes ok?
#include <stdio.h>
int main(void)
{
FILE *main;
int result_alloc;
int header_init;
int F_OK;
if (result_alloc = access ("test.bin", F_OK) != 0) {
printf("File not found, allocating a new one\n");
if ((main = fopen("test.bin","w+b")) == NULL) {
printf("Failure trying to open file");
return 1;
}
else {
header_init = -1;
printf("current pos is: w+b %d\n",ftell(main));
fwrite(&header_init,sizeof(int),1,main);
printf("current pos is: write header_init %d\n",ftell(main));
}
}
else {
if ((main = fopen("test.bin","r+b")) == NULL) {
printf("Failure trying to open file");
return 2;
}
else {
printf("current pos is: r+b %d\n",ftell(main));
fread(&header_init,sizeof(int),1,main);
printf("current pos is: read header_init %d\n",ftell(main));
}
}
}
The main issue that the assignment to the main variable inside of the openfile function is not seen by the calling function. Because C is pass by value, you're only changing the value of the function parameter, not the value of the variable that was passed in. So when openfile returns, the main variable inside of the main function is unchanged.
What you need to do is pass the address of that variable to the function, then within the function you dereference the local variable (which is a pointer) and assign a value to the dereferenced variable.
Also, it's not a good idea to have a variable with the same name as a function as it hides the function in that scope and can cause confusion.
So you would define your function as follows:
int openfile(FILE **fp, char *name, int option);
You would then call it like this (changing the name of the main variable to fp):
FILE *fp;
...
openfile(&fp,"main_file.bin",1)
Then inside of openfile, you dereference fp to change the value in the calling function:
*fp = fopen(name,"w+b")
The reason why the second code sample is working is that you're assigning directly to a local variable and then using that same variable later on in the function.
Also, you're "lucky" that the second piece of code is working because you did this:
int F_OK;
F_OK is already defined in unistd.h, where access() is defined. So by doing this you're redeclaring it and not giving it a value, causing undefined behavior. Get rid of this definition, and #include <unistd.h>, and the call to access() is guaranteed to work as expected.
For my assignment I have to create a program similar to the -wc unix command which counts words, lines, etc.
I have to read in flags and read in a text file.
I've set up all the flags and now I'm trying to read in a text file. I don't think I'm doing this right.
void readInFile(char** argv, int arg)
{
FILE *myFile;
char c;
myFile = fopen(argv[arg], "r");
if(!myfile)
{
printf("%s not found!", argv[arg]);
exit(EXIT_FAILURE);
}
}
in my main I call the function readInFile() and pass 2 arguments. Argv and the element where the file should be. So assume this to be correct.
I need help with actually opening up the file. I feel like my fopen() is wrong. I'm new to reading/writing files in C. Thanks alot!
I'm going to give you some general advice here.
Usually functions should do a single job. In this case, you are writing a function to read in a single file. So, don't pass a pointer to all the command-line arguments; pass in a single read-only pointer to the name of the file to open. Then in main() select the correct argument and pass that as the argument.
void readInFile(char const *filename)
Now, if this function will be reading in the file and doing nothing else, it needs to return the data somehow. But if this function will be doing the equivalent of wc, maybe it will read the file and print stuff, not return any data to the main() function. Then maybe the name should be improved:
void wordcount(char const *filename)
The actual call to fopen() looks fine to me.
You check for error, and then call exit() immediately. That's one way to do it. Another way to do it is to return an error code from your function, and have the caller (the main() function) check for failure, and handle the error there.
int wordcount(char const *filename)
{
// ... do stuff
if (failed)
return 1; // return nonzero error code on failure
// ... do more stuff
return 0; // success code
}
int main(int argc, char const **argv)
{
char const *filename;
int result;
filename = argv[1];
result = wordcount(filename);
if (result)
{
fprintf(stderr, "unable to open file '%s'\n", filename, result);
exit(result);
}
return 0;
}
For a program this simple, it doesn't matter much. But once you start building larger systems in software, you will be happier if your functions work well together, and part of that is making functions that return error codes rather than terminating your whole program on any error.
Why am I using 0 for the success code, and non-zero for failure? It's a common way to do it. It's easy to test for non-zero, like if (result) and there are many non-zero codes but only one zero, so you can return many different kinds of errors, but there is only one value needed for "success".
Note that instead of calling exit() from main(), you can just use the return statement. When you return 0 from main(), that signals success, and a non-zero value indicates an error. So you could just use return result; from main() if you like.
In my dummy code, I'm just returning 1 as the error code. But actually, when you call fopen() it returns an error code to you, in a global variable called errno. Probably a better option is to make your function return the actual error code as specified in errno. You could even modify the print statement in the main() function print the errno code, or use the strerror() function to turn that error code into a human-readable message.
Your call to fopen is correct, assuming that argv[arg] is a valid string which refers to a file that exists on the filesystem.
There is a small typo in the program snippet. if(!myfile) should prpbably be if(!myFile). With this change, I presume the code should work. Can you please elaborate the error faced by you?
P.S: I tried your program and it seems to work!
What I'm trying to do is to write a function that creates a file, fills it with data and returns the file to main(). The question is - what is the right way to "return file". Should I pass pointer / file descriptor or just disregard all this and use filename?
EDIT: doing the right way (I hope):
int mkrandfile(const char *name, int range, int qnt)
{
FILE *afile;
int i = 0;
if (afile = fopen(name, "w+"))
{
while((i <= qnt) && fprintf(afile, "%d ", rand() % range - range/2))
i++;
fclose(afile);
if (i != qnt + 1)
return -2;
}
else
return -1;
}
You are correct that returning afile from that function is wrong. Once the file is closed, that file handle is no longer valid. Use of it by the caller would result in undefined behavior.
While it is probably not the best idea from a modularity standpoint, you could leave the file open and then return the handle. But I think one of the following might be better:
Pass in an opened file handle to the function (let the caller open and close it).
Make the caller open the file again on its own.
The first option may be more efficient when file caching by the OS (if applicable here) is considered.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Valid use of goto for error management in C?
Recently I've encountered C code like this:
if( !condition1){
goto failure;
}
some_stuff_that_needs_manual_undoing();
if( !condition2){
goto failure;
}
// And many more lines such as the ones above
return 0;
failure:
// Do a lot of stuff here, free, file closing and so on
return -1;
To sum up the situation:
I have a long function that do several things in a row (let's say open a file, then allocate memory based on file contents, then connect do database and so so on). Of course I want to free resources correctly, but there are lot of places which can cause function to end prematurely (and all of them need clean up).
The Question: How to do this correctly?
Although goto doesn't seem to be that bad practice it also doesn't seem to be good solution.
I though of following:
Use a macro which would do the job, e.g.:
#define CLEANUP if( memory != NULL)free(memory); \
if( fp != NULL) fclose(fp);\
// ...
if( !condition1){
CLEANUP
return -1;
}
if( !condition2){
CLEANUP
return -2;
}
// ...
This will result in duplicate assembly, but the cleanup code would be in one place.
Encapsulate function into another function
int _some_stuff_do_work(void **memory, FILE **file, ...){
// Would just return on error
}
int some_stuff() {
void *memory = NULL;
FILE *file = NULL;
_some_stuff_do_work( &memory, &file, ...);
if( fp) fclose(fp);
}
This could get really ugly if there will be more than 3-5 things that will need cleaning up (the function would then take many arguments and that always calls for a problems).
OOP - Destructor
typedef struct {
void *memory;
FILE *fp;
} LOCAL_DATA;
// Destructor
void local_data_destroy( LOCAL_DATA *data)
{
if( data->fp){
free(data->fp);
data->fp = NULL;
}
}
But this could lead to many functions (and structures) that would be used only once in whole application and it seems like it could create a hell of a mass.
Loop and break statements
while(1){
if( !condition1){
break;
}
// ...
break;
}
if( fp) fclose(fp);
I found this on may places but using one iteration loop? I don't know, it seems to be completely un-intuitive.
Goto is the way to go. First understand why exactly "goto is bad" and then you will see that in situation you're describing goto is not really bad. Avoiding goto at all costs is just wrong and comes from shallow understanding of good programming principles.
This is way to go (Linux kernel source):
http://goo.gl/uSgp5
I'm making a load balancer (a very simple one). It looks at how long the user has been idle, and the load on the system to determine if a process can run, and it goes through processes in a round-robin fashion.
All of the data needed to control the processes are stored in a text file.
The file might look like this:
PID=4390 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4397 IDLE=3.000000 BUSY=1.500000 USER=4.000000
PID=4405 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4412 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4420 IDLE=3.000000 BUSY=1.500000 USER=4.000000
This is a university assignment, however parsing the text file isn't supposed to be a big part of it, which means I can use whatever way is the quickest for me to implement.
Entries in this file will be added and removed as processes finish or are added under control.
Any ideas on how to parse this?
Thanks.
Here is a code that will parse your file, and also account for the fact that your file might be unavailable (that is, fopen might fail), or being written while you read it (that is, fscanf might fail). Note that infinite loop, which you might not want to use (that's more pseudo-code than actual code to be copy-pasted in your project, I didn't try to run it). Note also that it might be quite slow given the duration of the sleep there: you might want to use a more advanced approach, that's more sort of a hack.
int pid;
float idle, busy, user;
FILE* fid;
fpos_t pos;
int pos_init = 0;
while (1)
{
// try to open the file
if ((fid = fopen("myfile.txt","rw+")) == NULL)
{
sleep(1); // sleep for a little while, and try again
continue;
}
// reset position in file (if initialized)
if (pos_init)
fsetpos (pFile,&pos);
// read as many line as you can
while (!feof(fid))
{
if (fscanf(fid,"PID=%d IDLE=%f BUSY=%f USER=%f",&pid, &idle, &busy, &user))
{
// found a line that does match this pattern: try again later, the file might be currently written
break;
}
// add here your code processing data
fgetpos (pFile,&pos); // remember current position
pos_init = 1; // position has been initialized
}
fclose(fid);
}
As far as just parsing is concerned, something like this in a loop:
int pid;
float idle, busy, user;
if(fscanf(inputStream, "PID=%d IDLE=%f BUSY=%f USER=%f", %pid, &idle, &busy, &user)!=4)
{
/* handle the error */
}
But as #Blrfl pointed out, the big problem is to avoid mixups when your application is reading the file and the others are writing to it. To solve this problem you should use a lock or something like that; see e.g. the flock syscall.
Use fscanf in a loop. Here's a GNU C tutorial on using fscanf.
/* fscanf example */
#include <stdio.h>
typedef struct lbCfgData {
int pid;
double idle;
double busy;
double user;
} lbCfgData_t ;
int main ()
{
// PID=4390 IDLE=0.000000 BUSY=2.000000 USER=2.000000
lbCfgData_t cfgData[128];
FILE *f;
f = fopen ("myfile.txt","rw+");
for ( int i = 0;
i != 128 // Make sure we don't overflow the array
&& fscanf(f, "PID=%u IDLE=%f BUSY=%f USER=%f", &cfgData[i].pid,
&cfgData[i].idle, &cfgData[i].busy, cfgData[i].user ) != EOF;
i++
);
fclose (f);
return 0;
}