Programming Forums > Application Development > C++

FarAway · Feb 16th, 2005, 9:46 AM

Hi all,

I am a beginner to C. I got this problem about malloc() and free().

How can i read a line from a file, put it into an array by using malloc, do something to it, free the array, and then read next line?

e.g. sample.txt:

hello how are you
nice to meet you
bye bye

read and put "hello how are you" into an array by using malloc, print it out, and then free the memory, then read "nice to meet you", and so on...

Thanks.... =)

Dizzutch · Feb 16th, 2005, 10:09 AM

whenever you create something (except for a primitive type, int, char, float, double) you need to allocate space for it in RAM. malloc takes as argument the size of what you're going to allocate, and returns a pointer that needs to be typecast to what you allocated for.
ex:

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char *word; //we're gonna store a 5 letter word
word = (char *)malloc(sizeof(char) * 5);

the (char *) tells the program that whatever space malloc returns is going to be used by a char, and sizeof(char) * 5 tells malloc how many bytes to allocate in memory. In your case you might just want to allocate an arbitrary amount, examply 255 bytes. So you won't run out.
free(word) will just tell the memory that the space allocated for word can be reused. So after you call free(word) you won't be able to access the word variable again.
reading from the file can be done using fopen, fscanf, and fclose. There are many examples of that online, but i'll give you a quick example

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FILE *fp; //file pointer
char *line = (char *)malloc(255); a char is 1 byte so for a char you can just allocate 255 bytes
fp = fopen("file.txt", "r"); open file for reading
fscanf(fp, "%s\n", &word); read a line from the file untill a newline character comes along
fclose(fp); //close the file
printf("%s\n", word); //print the word read
free(word); //free the space

hope this helps, there's a lof ot examples on google if you're stuck, and feel free to post your code here if you have trouble.

good luck
Dizz

FarAway · Feb 16th, 2005, 10:27 AM

Hi Dizzutch,

Thank you for replying...

Actually i am writing codes to read a line from a file, then use fork() and execv() to create a new process, waitpid for the process to die, then read next line.

e.g. sample.txt
/usr/bin/w -s
/bin/ls -al

so i need to do

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char *line[] = {"/usr/bin/w", "-s", 0};
execv("/usr/bin/w", line);

there are up to 50 parameters need to be passed, and each 50 characters.

so how can i read each line and put the parameters into a allocated array, execute it, then free the array?

Thank you.. =)

Dizzutch · Feb 16th, 2005, 10:39 AM

ah, i thought you had no idea how to use malloc and free..

you can allocate 50*50 bytes, and just access it as an array

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char *array = (char *)malloc(sizeof(char) * (50*50));

since this is dynamically allocated you can access it as

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array[0] = foo;
array[1] = bar;

I'm assuming that you're not making up the 50*50 and do have limits. if you don't than i'd just allocate a large amount per line, and read in per line. determine the size, and realocate a new bit of memory for the size of the string you just read in.
Good luck, hope you catch my drift.

Dizz

FarAway · Feb 16th, 2005, 10:50 AM

Thanks Dizzutch...

Dizzutch · Feb 16th, 2005, 11:08 AM

np, hope it helps.

Feb 16th, 2005, 9:46 AM	#1
FarAway Newbie Join Date: Feb 2005 Posts: 4 Rep Power: 0	how to use malloc() and free() Hi all, I am a beginner to C. I got this problem about malloc() and free(). How can i read a line from a file, put it into an array by using malloc, do something to it, free the array, and then read next line? e.g. sample.txt: hello how are you nice to meet you bye bye read and put "hello how are you" into an array by using malloc, print it out, and then free the memory, then read "nice to meet you", and so on... Thanks.... =)

Feb 16th, 2005, 10:09 AM	#2
Dizzutch Professional Programmer Join Date: Dec 2004 Location: Worcester, MA Posts: 441 Rep Power: 4	whenever you create something (except for a primitive type, int, char, float, double) you need to allocate space for it in RAM. malloc takes as argument the size of what you're going to allocate, and returns a pointer that needs to be typecast to what you allocated for. ex: (Toggle Plain Text) char word; //we're gonna store a 5 letter word word = (char )malloc(sizeof(char) * 5); char word; //we're gonna store a 5 letter word word = (char )malloc(sizeof(char) * 5); the (char ) tells the program that whatever space malloc returns is going to be used by a char, and sizeof(char) 5 tells malloc how many bytes to allocate in memory. In your case you might just want to allocate an arbitrary amount, examply 255 bytes. So you won't run out. free(word) will just tell the memory that the space allocated for word can be reused. So after you call free(word) you won't be able to access the word variable again. reading from the file can be done using fopen, fscanf, and fclose. There are many examples of that online, but i'll give you a quick example (Toggle Plain Text) FILE fp; //file pointer char line = (char )malloc(255); a char is 1 byte so for a char you can just allocate 255 bytes fp = fopen("file.txt", "r"); open file for reading fscanf(fp, "%s\n", &word); read a line from the file untill a newline character comes along fclose(fp); //close the file printf("%s\n", word); //print the word read free(word); //free the space FILE fp; //file pointer char line = (char )malloc(255); a char is 1 byte so for a char you can just allocate 255 bytes fp = fopen("file.txt", "r"); open file for reading fscanf(fp, "%s\n", &word); read a line from the file untill a newline character comes along fclose(fp); //close the file printf("%s\n", word); //print the word read free(word); //free the space hope this helps, there's a lof ot examples on google if you're stuck, and feel free to post your code here if you have trouble. good luck Dizz __________________ naked pictures of you \| PFO F@H stats

Feb 16th, 2005, 10:27 AM	#3
FarAway Newbie Join Date: Feb 2005 Posts: 4 Rep Power: 0	Hi Dizzutch, Thank you for replying... Actually i am writing codes to read a line from a file, then use fork() and execv() to create a new process, waitpid for the process to die, then read next line. e.g. sample.txt /usr/bin/w -s /bin/ls -al so i need to do (Toggle Plain Text) char line[] = {"/usr/bin/w", "-s", 0}; execv("/usr/bin/w", line); char line[] = {"/usr/bin/w", "-s", 0}; execv("/usr/bin/w", line); there are up to 50 parameters need to be passed, and each 50 characters. so how can i read each line and put the parameters into a allocated array, execute it, then free the array? Thank you.. =)

Feb 16th, 2005, 10:39 AM	#4
Dizzutch Professional Programmer Join Date: Dec 2004 Location: Worcester, MA Posts: 441 Rep Power: 4	ah, i thought you had no idea how to use malloc and free.. you can allocate 5050 bytes, and just access it as an array (Toggle Plain Text) char array = (char )malloc(sizeof(char) (5050)); char array = (char )malloc(sizeof(char) (5050)); since this is dynamically allocated you can access it as (Toggle Plain Text) array[0] = foo; array[1] = bar; array[0] = foo; array[1] = bar; I'm assuming that you're not making up the 5050 and do have limits. if you don't than i'd just allocate a large amount per line, and read in per line. determine the size, and realocate a new bit of memory for the size of the string you just read in. Good luck, hope you catch my drift. Dizz __________________ naked pictures of you \| PFO F@H stats

Feb 16th, 2005, 11:08 AM	#6
Dizzutch Professional Programmer Join Date: Dec 2004 Location: Worcester, MA Posts: 441 Rep Power: 4	np, hope it helps. __________________ naked pictures of you \| PFO F@H stats

Feb 16th, 2005, 10:50 AM	#5
FarAway Newbie Join Date: Feb 2005 Posts: 4 Rep Power: 0	Thanks Dizzutch...

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Programming Forums > Application Development > C++

how to use malloc() and free()