Fatal error: iostream: No such file or directory in compiling C program using GCC

Question

Why when I wan to compile the following multi thread merge sorting C program, I receive this error:

ap@sharifvm:~/forTHE04a$ gcc -g -Wall -o mer mer.c -lpthread
mer.c:4:20: fatal error: iostream: No such file or directory
 #include <iostream>
                    ^
compilation terminated.
ap@sharifvm:~/forTHE04a$ gcc -g -Wall -o mer mer.c -lpthread
mer.c:4:22: fatal error: iostream.h: No such file or directory
 #include <iostream.h>
                      ^
compilation terminated.

My program:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <iostream>
using namespace std;

#define N 2  /* # of thread */

int a[] = {10, 9, 8, 7, 6, 5, 4, 3, 2, 1};  /* target array */

/* structure for array index
 * used to keep low/high end of sub arrays
 */
typedef struct Arr {
    int low;
    int high;
} ArrayIndex;

void merge(int low, int high)
{
        int mid = (low+high)/2;
        int left = low;
        int right = mid+1;

        int b[high-low+1];
        int i, cur = 0;

        while(left <= mid && right <= high) {
                if (a[left] > a[right])
                        b[cur++] = a[right++];
                else
                        b[cur++] = a[right++];
        }

        while(left <= mid) b[cur++] = a[left++];
        while(right <= high) b[cur++] = a[left++];
        for (i = 0; i < (high-low+1) ; i++) a[low+i] = b[i];
}

void * mergesort(void *a)
{
        ArrayIndex *pa = (ArrayIndex *)a;
        int mid = (pa->low + pa->high)/2;

        ArrayIndex aIndex[N];
        pthread_t thread[N];

        aIndex[0].low = pa->low;
        aIndex[0].high = mid;

        aIndex[1].low = mid+1;
        aIndex[1].high = pa->high;

        if (pa->low >= pa->high) return 0;

        int i;
        for(i = 0; i < N; i++) pthread_create(&thread[i], NULL, mergesort, &aIndex[i]);
        for(i = 0; i < N; i++) pthread_join(thread[i], NULL);

        merge(pa->low, pa->high);

        //pthread_exit(NULL);
        return 0;
}

int main()
{
        ArrayIndex ai;
        ai.low = 0;
        ai.high = sizeof(a)/sizeof(a[0])-1;
        pthread_t thread;

        pthread_create(&thread, NULL, mergesort, &ai);
        pthread_join(thread, NULL);

        int i;
        for (i = 0; i < 10; i++) printf ("%d ", a[i]);
        cout << endl;

        return 0;
}

Answer 1

Neither <iostream> nor <iostream.h> are standard C header files. Your code is meant to be C++, where <iostream> is a valid header. Use a C++ compiler such as clang++ or g++ (and a .cpp file extension) for C++ code.

Alternatively, this program uses mostly constructs that are available in C anyway. It's easy enough to convert the entire program to compile using a C compiler. Simply remove #include <iostream> and using namespace std; , and replace cout << endl; with putchar('\n'); ... I advise compiling using C99, C11 or C18 (eg. gcc -std=c99 , clang -std=c18 etc)

Answer 2

Seems like you posted a new question after you realized that you were dealing with a simpler problem related to size_t . I am glad that you did.

Anyways, You have a .c source file, and most of the code looks as per C standards, except that #include <iostream> and using namespace std;

C equivalent for the built-in functions of C++ standard #include<iostream> can be availed through #include<stdio.h>

1. Replace #include <iostream> with #include <stdio.h> , delete using namespace std;

2. With #include <iostream> taken off, you would need a C standard alternative for cout << endl; , which can be done by printf("\n"); or putchar('\n');
   Out of the two options, printf("\n"); works the faster as I observed.

   When used printf("\n"); in the code above in place of cout<<endl;

    $ time ./thread.exe 1 2 3 4 5 6 7 8 9 10 real 0m0.031s user 0m0.030s sys 0m0.030s

   When used putchar('\n'); in the code above in place of cout<<endl;

    $ time ./thread.exe 1 2 3 4 5 6 7 8 9 10 real 0m0.047s user 0m0.030s sys 0m0.030s

Compiled with Cygwin gcc (GCC) 4.8.3 version. results averaged over 10 samples. (Took me 15 mins)

Answer 3

I faced the same error : -

fatal error: iostream: No such file or directory
 #include <iostream>

After some time I found that, I'm making a very very noob mistake here. Actually I was running my program file by giving the extension ".c" only.... :-( So check whether you're saving your file with proper extension or not... In this case, it'll be .cpp for C++ program file. So check that once and maybe you'll be good to go.

Answer 4

So now first check if you have downloaded the MinGW and have added it to the env variables

to check use g++ --version on your cmd/terminal

now if the squiggly lines still appear install the c++ Intellisense extension you will mostly get an error in the c_cpp_properties.json file because the compiler path may not be correct

so all you have to do is put the correct path and the swiggly lines will dissappear