c,memory,mapping,mmap , C - Memory Mapping with Multiple Processes


C - Memory Mapping with Multiple Processes

Question:

Tag: c,memory,mapping,mmap

I have an assignment requiring me to write a multi-processed program that works with a memory-mapped file containing a string of characters. After the parent process maps the file to memory, it spawns 2 children processes to modify the file. Child 1 outputs the contents of the file, converts the file's contents to their upper case equivalent, then outputs the file's new contents. Child 2 waits 1 second to let child 1 finish, outputs the file's contents, removes any hyphen " - " characters, then outputs the file's new contents. My problem with both child processes is that after first displaying the file's contents, the processes attempt to modify the contents of the file, but neither child outputs the file's new contents. I get no errors when running or compiling so I can't find out what the problem is. And of course, I'm new to memory mapping so feel free to let me know what I'm doing wrong. Here is my source code:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <signal.h>
#include <string.h>

int main (int argc, char *argv[]) { 
    struct stat buf;
    int fd, length, status, i, j, k;
    char *mm_file;
    char *string = "this is a lowercase-sentence.";
    length = strlen(string);

    fd = open(argv[1], O_CREAT | O_RDWR, 0666); //Creates file with name given at command line
    write(fd, string, strlen(string)); //Writes the string to be modified to the file
    fstat(fd, &buf); //used to determine the size of the file

    //Establishes the mapping
    if ((mm_file = mmap(0, (size_t) buf.st_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) == (caddr_t) - 1) {
       fprintf(stderr, "mmap call fails\n");
    }

    //Initializes child processes
    pid_t MC0;
    pid_t MC1;

    //Creates series of child processes which share the same parent ID
    if((MC0 = fork()) == 0) {
        printf("Child 1 %d reads: \n %s\n", getpid(), mm_file);
        //{convert file content to uppercase string};
        for (i = 0; i < length; i++) {
            string[i] = toupper(string[i]);
        }
        //sync the new contents to the file
        msync(0, (size_t) buf.st_size, MS_SYNC);
        printf("Child 1 %d reads again: \n %s\n", getpid(), mm_file);
        exit(EXIT_SUCCESS); //Exits process
    } else if ((MC1 = fork()) == 0) {
        sleep(1); //so that child 2 will perform its task after child 1 finishes
        ("Child 2 %d reads: \n %s\n", getpid(), mm_file);
        //{remove hyphens}
        for (j = 0; j < length; i++) {
            if (string[i] == '-') {
                string[i] = ' ';
            }
        }
        //sync the new contents to the file
        msync(0, (size_t) buf.st_size, MS_SYNC);
        printf("Child 2 %d reads again: \n %s\n", getpid(), mm_file);
        exit(EXIT_SUCCESS); //Exits process
   } 

   // Waits for all child processes to finish before continuing.
   waitpid(MC0, &status, 0);
   waitpid(MC1, &status, 0);

   return 0;
}

Then my output is as follows:

**virtual-machine:~$** ./testt file

Child 1 3404 reads: 

this is a lowercase-sentence.

Child 2 3405 reads: 

this is a lowercase-sentence.

All child processes have finished. Now exiting program.

**virtual-machine:~$**

But my desired result would be:

**virtual-machine:~$** ./testt file

Child 1 3404 reads: 

this is a lowercase-sentence.

Child 1 3404 reads again: 

THIS IS A LOWERCASE-SENTENCE.

Child 2 3405 reads: 

THIS IS A LOWERCASE-SENTENCE.

Child 2 3405 reads: 

THIS IS A LOWERCASE SENTENCE.

All child processes have finished. Now exiting program.

**virtual-machine:~$**

Any help is greatly appreciated.


Answer:

There are a few errors here. Firstly, you write into the file and then map it into the memory. The mapping is correct, but the writing not. If the string has n characters, you have to write n+1 characters, since strings in C are null-terminated. Now you only have n, so all C string functions will try to access at least one more byte, which is not good. And if that one extra byte is not null (zero), the functions will go even further. In debug more they might be zeroed, but in optimized code usually not. So you have to use

write(fd, string, strlen(string)+1); //Writes the string to be modified to the file

Then you do this:

    for (i = 0; i < length; i++) {
        string[i] = toupper(string[i]);
    }

This only changes the data that is referred by the pointer string, which has nothing to do with the memory mapped file. You should have:

    for (i = 0; i < length; i++) {
        mm_file[i] = toupper(mm_file[i]);
    }

The same is with the second child process.

Also your msync() call is a bit suspect. You give the memory address as 0, which is not within your memory mapped file, so it will not sync the contents. You need to call msync(mm_file, (size_t) buf.st_size, MS_SYNC);

Also, many compilers will put the constant strings into read-only memory, so you might not even be allowed to change the data referred to by string. In this case it seems you are allowed.

Do also remember, that the length of the file is one byte larger than the length of the string, so use the variables correctly. Currently you do, since you sync the file with file length and handle the string with string length.


Related:


C programming - Confusion regarding curly braces


c,scope
The following code is for replacing multiple consecutive spaces into 1 space. Although I manage to do it, I am confused in the use of curly braces. This one is actually running fine: #include <stdio.h> #include <stdlib.h> int main() { int ch, lastch; lastch = 'a'; while((ch = getchar())!= EOF)...

How to read string until two consecutive spaces?


c,format,sscanf,c-strings
A well known function of the scanf() functions is that you can pass a format to scan input according to this format. For my case, I cannot seem to find a solution searching this and this documentation. I have a string (sInput) as the following: #something VAR1 this is a...

C++11 Allocation Requirement on Strings


c++,string,c++11,memory,standards
I had heard that C++11 was going to require strings to be allocated in contiguous memory. I even thought I saw a stack overflow question on it, but I can't seem to find it. I know that in practice both gcc and Visual Studio do allocate strings contiguously, I'm just...

C binary tree sort - extending it


c,binary-tree,binary-search-tree
I need some help in C Help me to extend the binary tree sort on C. I need to return a sorted array in sort function. here it is: #include <stdio.h> #include <stdlib.h> struct btreenode { struct btreenode *leftchild ; int data ; struct btreenode *rightchild ; } ; void...

Infinite loop with fread


c,arrays,loops,malloc,fread
I'm trying to allocate an array 64 bytes in size and then loop over the array indexes to put a read a byte each from the inputfile. but when I don't malloc() the array indexes, the loop stays in index0 (so each time it loops it replaces the content in...

Passing int using char pointer in C


c,exec,ipc
I'm trying to figure out how to pass an int using a char pointer. It fails once the int value is too large for the char. This is what I'm trying to figure out: char *args[5]; int i = 20; /*some other code/assignments*/ args[2] = (char *)&i; execv(path, args); How...

Counting bytes received by posix read()


c,function,serial-port,posix
I get confused with one line of code: temp_uart_count = read(VCOM, temp_uart_data, 4096); I found more about read function at http://linux.die.net/man/3/read, but if everything is okay it returns 0, so how we can get num of bytes received from that? temp_uart_count is used to count how much bytes we received...

How to control C Macro Precedence


c,macros
#define VAL1CHK 20 #define NUM 1 #define JOIN(A,B,C) A##B##C int x = JOIN(VAL,NUM,CHK); With above code my expectation was int x = 20; But i get compilation error as macro expands to int x = VALNUMCHK; // Which is undefined How to make it so that NUM is replaced first...

CallXXXMethod undefined using JNI in C


java,c,jni
So I've tried to use the JNI interface to call Java methods from C. Calling static methods is no problem, but I get stuck when I want to call a method on an object. The code is as follows: #include <stdio.h> #include <string.h> #include <jni.h> int main() { JavaVMOption options[1];...

Is post-increment operator guaranteed to run instantly?


c,c89,post-increment,ansi-c
Let's say I have the following code: int i = 0; func(i++, i++); The increment is happening right after returning the value? Is it guaranteed that the first argument will be 0, and the second argument will be 1?...

Disadvantages of calling realloc in a loop


c,memory-management,out-of-memory,realloc
I'm trying to implement some math algorithms in C on Windows 7, and I need to repeatedly increase size of my array. Sometimes it fails because realloc can't allocate memory. But if I allocate a lot of memory at once in the beginning it works fine. Is it a problem...

How does this code print odd and even?


c,if-statement,macros,logic
#define MACRO(num, str) {\ printf("%d", num);\ printf(" is");\ printf(" %s number", str);\ printf("\n");\ } int main(void) { int num; printf("Enter a number: "); scanf("%d", &num); if (num & 1) { MACRO(num, "Odd"); } else { MACRO(num, "Even"); } return 0; } Please explain the above code (if/else condition and how...

What does `strcpy(x+1, SEQX)` do?


c,strcpy
I'm wondering what this syntax of strcpy() does in line 65 and 66: 24 #define SEQX "TTCATA" 25 #define SEQY "TGCTCGTA" 61 M = strlen(SEQX); 62 N = strlen(SEQY); 63 x = malloc(sizeof(char) * (M+2)); /* +2: leading blank, and trailing \0 */ 64 y = malloc(sizeof(char) * (N+2)); 65...

Multiple definition and file management


c,arrays,compilation,compiler-errors,include
I'm writing a program for vocabulary training, for myself. And the program itself should be available in different languages, atm in German and English. What I want is to have a Main File which manage all and two separate files for the functions in the right language. I compile all...

Reverse ^ operator for decryption


c,algorithm,security,math,encryption
I'm trying to reverse the following code in order to provide a function which takes the buffer and decrypts it. void crypt_buffer(unsigned char *buffer, size_t size, char *key) { size_t i; int j; j = 0; for(i = 0; i < size; i++) { if(j >= KEY_SIZE) j = 0;...

C language, vector of struct, miss something?


c,vector,struct
This is a part of my program that I want to create a vector of struct typedef struct { char nome[501]; int qtd; int linha; int coluna; } tPeca; tPeca* criarPecas(FILE *pFile, int tam) { int i; tPeca *pecaJogo = (tPeca*)malloc(tam*sizeof(tPeca)); if (pecaJogo == NULL) return NULL; for (i =...

How does ((a++,b)) work? [duplicate]


c,function,recursion,comma
This question already has an answer here: What does the comma operator `,` do in C? 8 answers In the below block of code, I am trying to understand how the line return reverse((i++, i)) is working. #include <stdio.h> void reverse(int i); int main() { reverse(1); } void reverse(int...

OpenGL glTexImage2D memory issue


c,opengl
I'm loading a cubemap to create a skybox, everything is fine and the skybox renders properly with a correct texture application. However, I decided to check my program safety with valgrind, Valgrind gives this error: http://pastebin.com/seqmXjyx The line 53 in sky.c is: glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X + i, 0, GL_RGB, texture.width, texture.height, 0,...

How to increment the value of an unsigned char * (C)


c++,c,openssl,byte,sha1
I have a value stored as an unsigned char * (in C). This holds the SHA1 hash of a string. My goal is to cover the SHA1 key space. Since I'm using <openssl/evp.h> to generate the hashes, I end up with an unsigned char* holding the SHA1 value. Now I...

How can I align stack to the end of SRAM?


c,embedded,stm32,gnu-arm,coocox
I have a STM32F103VCT6 microcontroller with 48kb of SRAM, and recently i've got a memory collision: I have some static variable (lets call it A) located in heap with size of 0x7000 and I wrote some simple function to get info about stack and heap: void check(int depth) { char...

Unexpected result when calculating a percentage - even when factoring in integer division rules


c,percentage,integer-overflow,integer-division
I am trying to express a battery voltage as a percentage. My battery level is a (global) uint16 in mV. I have a 16-bit CPU. Here is my code: static uint8 convertBattery(void){ uint16 const fullBattery = 3000; /* 3V = 3000mV */ uint8 charge; charge = ((battery*100)/fullBattery); return charge; }...

getchar() not working in c


c,while-loop,char,scanf,getchar
getchar() is not working in the below program, can anyone help me to solve this out. I tried scanf() function in place of getchar() then also it is not working. I am not able to figure out the root cause of the issue, can anyone please help me. #include<stdio.h> int...

VS2012 Identifer not found when part of static lib


c,visual-studio-2012,linker,static-libraries
Using VS2012 C/C++: I created and linked a static lib called "libtools" to my project. Calls to functions in the libtools lib worked as expected. I created and linked a second static lib called "shunt" to my project. But when I incorporate a call to a function in shunt, I...

Text justification C language


c,text,alignment
I have to solve a problem that involves left justification string length and leading zeros. I have the following table : BEGIN CLOSE CONCATENATE DELETE END INITIALIZE PRINT WRITE This is produced by a simple program. My problem is to find out how to convert it like that : It...

CGO converting Xlib XEvent struct to byte array?


c,go,xlib,cgo
I am creating a simple window manager (code based of the c code in tinywm) in Golang. To use Xlib, I am using cgo, so my header is: // #cgo LDFLAGS: -lX11 // #include <X11/Xlib.h> And I have a variable declaration, like: event := C.XEvent{} And then, I use this...

Does strlen() always correctly report the number of char's in a pointer initialized string?


c,strlen
As long as I use the char and not some wchar_t type to declare a string will strlen() correctly report the number of chars in the string or are there some very specific cases I need to be aware of? Here is an example: char *something = "Report all my...

C++ / C #define macro calculation


c++,c,macros
Suppose I have #define DETUNE1 sqrt(7)-sqrt(5) #define DETUNE2 sqrt(11)-sqrt(7) And I call these multiple times in my program. Are DETUNE1 and DETUNE2 calculated every time it is called? Thanks. Please don't downvote this, I really want to know and a search didn't turn up anything definite. ...

execl() works on one of my code, but doesn't work on another


c,execl
I already used execl() in code, and it worked well. But this time, I really have no idea why it doesn't work. So here's the code that do not work #include <unistd.h> #include <stdio.h> int main() { int i = 896; printf("please\n"); execl("home/ubuntu/server/LC/admin/admin", (char*)i, NULL); printf("i have no idea why\n");...

scanf get multiple values at once


c,char,segmentation-fault,user-input,scanf
I need to get in one single shot different inputs from one single line. In particular I need to get a single char and then, depending on which char value I just read, it can be a string and an int or a string, an int and another string and...

free causing different results from malloc


c,string,malloc,free
Below is a C program i have written to print different combination of characters in a string. This is not an efficient way as this algorithm creats a lot of extra strings. However my question is NOT about how to solve this problem more efficiently. The program works(inefficiently though) and...

Does realloc() invalidate all pointers?


c,pointers,dynamic-memory-allocation,behavior,realloc
Note, this question is not asking if realloc() invalidates pointers within the original block, but if it invalidates all the other pointers. I'm new to C, and am a bit confused about the nature of realloc(), specifically if it moves any other memory. For example: void* ptr1 = malloc(2); void*...

Segmentation fault with generating an RSA and saving in ASN.1/DER?


c,openssl,cryptography,rsa
#include <string.h> #include <openssl/aes.h> #include <openssl/rand.h> #include <openssl/bio.h> #include <openssl/rsa.h> #include <openssl/evp.h> #include <openssl/pem.h> #define RSA_LEN 2048 #define RSA_FACTOR 65537 int genRSA2048(unsigned char **pub,unsigned int *pub_l,unsigned char **priv,unsigned int *priv_l){ RSA *pRSA = NULL; pRSA = RSA_generate_key(RSA_LEN,RSA_FACTOR,NULL,NULL); if (pRSA){ pub_l = malloc(sizeof(pub_l)); *pub_l = i2d_RSAPublicKey(pRSA,pub); priv_l = malloc(sizeof(priv_l));...

Is there any way of protecting a variable for being modified at runtime in C?


c,variables,constants
I was wondering if there is any way of protecting a variable for being modified once initialized (something like "constantize" a variable at runtime ). For example: #include <stdio.h> #include <stdlib.h> int main(void) { int v, op; scanf( "%d", &op ); if( op == 0 ) v = 1; else...

Set precision dynamically using sprintf


c,printf,format-string
Using sprintf and the general syntax "%A.B" I can do this: double a = 0.0000005l; char myNumber[50]; sprintf(myNumber,"%.2lf",a); Can I set A and B dynamically in the format string?...

What all local variables goto Data/BSS segment?


c++,c,nm
The man page of nm here: MAN NM says that The symbol type. At least the following types are used; others are, as well, depending on the object file format. If lowercase, the symbol is usually local; if uppercase, the symbol is global (external) And underneath it has "b" and...

Efficient comparison of small integer vectors


c,integer,compare,bit-manipulation,string-comparison
I have small vectors. Each of them is made of 10 integers which are between 0 and 15. This means that every element in a vector can be written using 4 bits. Hence I can concatenate my vector elements and store the whole vector in a single long type (in...

Program to reverse a string in C without declaring a char[]


c,string,pointers,char
I need to reverse a given string and display it without using the value At[index] notation , I tried the below program using pointers,but it does not print anything for the reverse string, Please help! int main() { char* name=malloc(256); printf("\nEnter string\n"); scanf("%s",name); printf("\nYou entered%s",name); int i,count; count=0; //find the...

Galois LFSR - how to specify the output bit number


c,prng,shift-register
I am trying to understand how change the galois LFSR code to be able to specify the output bit number as a parameter for the function mentioned below. I mean I need to return not the last bit of LFSR as output bit, but any bit of the LFSR (...

Is it safe to read and write on an array of 32 bit data byte by byte?


c,memory,memory-alignment
So I have a void * data of 32 bit unsigned integers which represents the pixels. Is it okay for me to access one of the pixels with a char * and modify the values directly? Or is it better to store my new pixel in a temporary uint32_t variable...

Segmentation Fault if I don't say int i=0


c,arrays,segmentation-fault,initialization,int
void removeVowels(char* array){ int i,j,v; i=0; char vowel[]={'a','e','i','o','u'}; while(array[i]!='\0') { for(v=0;v<5;v++) { if (array[i]==vowel[v]) { j=i; while(array[j]!='\0') { array[j]=array[j+1]; j++; } i--; break; } } i++; } } in function removeVowels() if I don't include i=0; and just say int i; why does it give segmentation fault? Isn't it automatically...

Is there Predefined-Macros define about byte order in armcc


c,armcc,predefined-macro
Is there Predefined-Macros define about byte order in armcc. I am a novice on the armcc.and sorry for my English. In gcc these are macros: __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__ __ORDER_BIG_ENDIAN__ __ORDER_PDP_ENDIAN__ ... Now I have to use armcc, Is there same like these with armcc? Thank a lot. by the way,the armcc...

Is i=i+1 an undefined behaviour?


c,increment,undefined-behavior
I'm using codeblocks and it is giving a different output to other compilers and I can't find a solution to it.What's the undefined behaviour in this program and is there any solution to avoid it? This is the code to print the nth number in a number system with only...

How convert unsigned int to unsigned char array


c++,c
I just need to extract those bytes using bitwise & operator. 0xFF is a hexadecimal mask to extract one byte. For 2 bytes, this code is working correctly: #include <stdio.h> int main() { unsigned int i = 0x7ee; unsigned char c[2]; c[0] = i & 0xFF; c[1] = (i>>8) &...

fread(), solaris to unix portability and use of uninitialised values


c,linux,memory,stack,portability
Valgrind found the following error and I, after reading the documentation, the code and other questions in here couldn't figure it out why. Valgrind: first warning ~$ valgrind --vgdb=yes --vgdb-error=0 --read-var-info=yes --leak-check=yes --track-origins=yes debitadmin* debitadmin ==20720== Conditional jump or move depends on uninitialised value(s) ==20720== at 0x4013BC6: initialise (dbg.c:199) ==20720==...

Heroku RAM not increasing with upgraded dynos


ruby-on-rails,ruby,ruby-on-rails-3,memory,heroku
I have a massive function i have been calling manually through the heroku rails console. I have been receiving the error rapid fire in my logs: 2015-06-22T14:56:42.940517+00:00 heroku[run.9877]: Process running mem=575M(112.4%) 2015-06-22T14:56:42.940517+00:00 heroku[run.9877]: Error R14 (Memory quota exceeded) A 1X dyno is suppose to have 512 MB of RAM. I...

Good approach to clean application data programmmatically in Android


android,android-fragments,memory,android-activity,picasso
When I use RecyclerView and/or Picasso. It seems to cache and store alot of data in my application. However, I want to delete the stored data and the cache in a simple way whenever I am exiting my app. What is the simplest way to delete all data stored in...

Loop through database table and compare user input


mysql,c
I am trying to loop through the rows in a MySql table and compare the data in a certain column to some user input using C. Currently my code looks like this: MYSQL *cxn = mysql_init(NULL); MYSQL_RES *result; unsigned int num_fields; unsigned int num_rows; char *query_string; MYSQL_ROW *row; if (mysql_real_connect(cxn,...

Array breaking in Pebble C


c,arrays,pebble-watch,cloudpebble
I'm trying to create a simple dice-rolling application in Pebble using C on CloudPebble. I have an array of different die sizes you can scroll through using Up/Down, and you roll (currently just generate a random number, it'll get fancier later) using the middle button. There's also a label at...