[SOLVED] How to solve "undefined reference to `main'" error in gitbash terminal (possible duplicate)

I am getting to know C and I have encountered an issue I am not sure how to solve. I have tried to look at other questions on here, but I do not think that my issue is the same, as I do have an int main()-class.

I have three files with several doubles separated by new line. They each look something like this:

...doubles
7540.39
-7741.01
-6349.83
-7670.89
1357.22
-9087.59
-2514.53
1896.5
5708.62
8541.45
7948.56
-8000.89
4115.98
6805.74
9513.29
-3964.84
-7187.46
945.873
-2032.98
-4842.12
-174.058
-2470.14
-1055.73
-8893.61
2755.39
...more doubles

The code I am trying to run is only going to take in the three files and sum over them:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char* argv[]) {

    // File variable (saved as fp)
    FILE * fp;

    // Character variable (saved as line)
    char * line = NULL;

    size_t len = 0;
    ssize_t read;

    // path to files
    char filename[40] = "../input/file";

    // The # of the file (file0, file1, file2)
    int rank;

    // Check that we get correct # for file
    if (argc <=1 ) {
        printf("Missing argument, an integer in the range 0 to 2.\n");
    } else {

        // atoi() converts numeric strings into an integer value
        rank = atoi(argv[1]);

        /* Format filename as "../input/file" + rank */

        // Create space for rank and
        char numchar[2];

        // Write rank into char array
        sprintf(numchar,"%d",rank);

        // Appending rank to "filename"
        strcat(filename, numchar);

        // Open file in read mode
        fp = fopen(filename, "r");

        // Check for error when opening file
        if (fp == NULL) {
            printf("Could not open file \"%s\".\n",filename);
            // printf("Could not open file: ", filename);
        } else {
            printf("Will sum numbers from file \"%s\".\n",filename);
            // printf("Summing numbers from file: ", filename);
            double sum=0;
            while ((read = getline(&line, &len, fp)) != -1) {
                if (read > 0) sum += atof(line);
            }
            printf("Sum is %f.\n",sum);

            fclose(fp);
            if (line) free(line);
        }
    }

    return 0;
}

I have checked that the path is correct, and I have compiled the code using the command gcc mySum.c -o mySum and g++ mySum.c -o mySum2, both works for compiling and creating an executable program, I am not sure why.

When I try to run with the comman gcc mySum or gcc mySum -o mySumOutput, I get the following error

/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o: In function `_start':
(.text+0x24): undefined reference to `main'
collect2: error: ld returned 1 exit status

And when I try to run g++ mySum or g++ mySum -o mySumOutput, I get this error:

mySum: In function `_fini':
(.fini+0x0): multiple definition of `_fini'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crti.o:(.fini+0x0): first defined here
mySum: In function `data_start':
(.data+0x0): multiple definition of `__data_start'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.data+0x0): first defined here
mySum:(.rodata+0x8): multiple definition of `__dso_handle'
/usr/lib/gcc/x86_64-redhat-linux/8/crtbegin.o:(.rodata+0x0): first defined here
mySum:(.rodata+0x0): multiple definition of `_IO_stdin_used'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.rodata.cst4+0x0): first defined here
mySum: In function `_dl_relocate_static_pie':
(.text+0x30): multiple definition of `_dl_relocate_static_pie'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.text[.text.group]+0x0): first defined here
mySum: In function `_start':
(.text+0x0): multiple definition of `_start'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.text+0x0): first defined here
mySum: In function `_init':
(.init+0x0): multiple definition of `_init'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crti.o:(.init+0x0): first defined here
/usr/lib/gcc/x86_64-redhat-linux/8/crtend.o:(.tm_clone_table+0x0): multiple definition of `__TMC_END__'
mySum:(.data+0x8): first defined here
/usr/bin/ld: error in mySum(.eh_frame); no .eh_frame_hdr table will be created.
collect2: error: ld returned 1 exit status
[student1@mpi1 cprog]$ g++ mySum -o mySumOutput
mySum: In function `_fini':
(.fini+0x0): multiple definition of `_fini'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crti.o:(.fini+0x0): first defined here
mySum: In function `data_start':
(.data+0x0): multiple definition of `__data_start'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.data+0x0): first defined here
mySum:(.rodata+0x8): multiple definition of `__dso_handle'
/usr/lib/gcc/x86_64-redhat-linux/8/crtbegin.o:(.rodata+0x0): first defined here
mySum:(.rodata+0x0): multiple definition of `_IO_stdin_used'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.rodata.cst4+0x0): first defined here
mySum: In function `_dl_relocate_static_pie':
(.text+0x30): multiple definition of `_dl_relocate_static_pie'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.text[.text.group]+0x0): first defined here
mySum: In function `_start':
(.text+0x0): multiple definition of `_start'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o:(.text+0x0): first defined here
mySum: In function `_init':
(.init+0x0): multiple definition of `_init'
/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crti.o:(.init+0x0): first defined here
/usr/lib/gcc/x86_64-redhat-linux/8/crtend.o:(.tm_clone_table+0x0): multiple definition of `__TMC_END__'
mySum:(.data+0x8): first defined here
/usr/bin/ld: error in mySum(.eh_frame); no .eh_frame_hdr table will be created.
collect2: error: ld returned 1 exit status

I have been trying to run the program but I am at a loss as to how manage it. All help is appreciated!