When I try to assign char *infile to argv[2] all that I infile ends up being is "". In that first for loop it prints fine but it won't work with the infile. The outfile also doesn't have any problems whatsoever. I have been trying to run it with ./resize 100 large.bmp copy.bmp.

// Copies a BMP file

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

#include "bmp.h"

int main(int argc, char *argv[]) {
    for (int i = 0; i < argc; i++){
        printf("%s\n", argv[i]);
    }

    char *tmp = argv[1];
    tmp[strlen(argv[1]) + 1] = '\0';

    // ensure proper usage
    if (argc != 4) {
        fprintf(stderr, "Usage: ./resize n infile outfile\n");
        return 1;
    }
    for (int i = 0, n = strlen(tmp); i < n; i++) {
        if (!isdigit(tmp[i])){
            fprintf(stderr, "Usage: ./resize n infile outfile\n");
            return 1;
        }
    }
    //int n = atoi(tmp);

    // remember filenames
    char *infile = argv[2];
    char *outfile = argv[3];

    // open input file
    FILE *inptr = fopen(infile, "r");
    if (inptr == NULL) {
        fprintf(stderr, "Could not open %s.\n", infile);
        return 2;
    }

    // open output file
    FILE *outptr = fopen(outfile, "w");
    if (outptr == NULL) {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.\n", outfile);
        return 3;
    }

    // read infile's BITMAPFILEHEADER
    BITMAPFILEHEADER bf;
    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);

    // read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);

    // ensure infile is (likely) a 24-bit uncompressed BMP 4.0
    if (bf.bfType != 0x4d42 || bf.bfOffBits != 54 || bi.biSize != 40 ||
        bi.biBitCount != 24 || bi.biCompression != 0) {
        fclose(outptr);
        fclose(inptr);
        fprintf(stderr, "Unsupported file format.\n");
        return 4;
    }

    // write outfile's BITMAPFILEHEADER
    fwrite(&bf, sizeof(BITMAPFILEHEADER), 1, outptr);

    // write outfile's BITMAPINFOHEADER
    fwrite(&bi, sizeof(BITMAPINFOHEADER), 1, outptr);

    // determine padding for scanlines
    int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

    // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++) {
        // iterate over pixels in scanline
        for (int j = 0; j < bi.biWidth; j++) {
            // temporary storage
            RGBTRIPLE triple;

            // read RGB triple from infile
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

            // write RGB triple to outfile
            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
        }

        // skip over padding, if any
        fseek(inptr, padding, SEEK_CUR);

        // then add it back (to demonstrate how)
        for (int k = 0; k < padding; k++) {
            fputc(0x00, outptr);
        }
    }

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // success
    return 0;
}