在 c 中使用 fscanf 从文件获取分段错误

Question

int main() {
    FILE *matrix_r;
    matrix_r=fopen("matrix.txt","r");
    double x;char c;
    while(feof(matrix_r)){
        fscanf(matrix_r,"%lf%c",&x,&c);
        printf("%lf%c",x,c);
    }
    fclose(matrix_r);

    return 0;
}

Trying to read float values from the file but getting segmentation fault core dumped error. matrix.txt stores a matrix of floats.

contents of matrix.txt are below.

0.000000,876.671546,448.879717,1349.827396
876.671546,0.000000,1319.195209,964.193445
448.879717,1319.195209,0.000000,1741.628261
1349.827396,964.193445,1741.628261,0.000000

Answer 1

fopen() failed and feof(NULL) caused the segfault. If fopen() was successful then feof() would return false and the loop wouldn't run but your program wouldn't segfault.

Check the return value of fopen() & fscanf() . You only need to call feof() if you need to find out why fscanf() failed to read 2 items.

#include <stdio.h>

int main() {
    FILE *matrix_r = fopen("matrix.txt", "r");
    if(!matrix_r) {
        perror("fopen");
        return 1;
    }
    for(;;) {
        double x;
        char c;
        int rv = fscanf(matrix_r, "%lf%c", &x, &c);
        if(rv != 2)
            break;
        printf("%lf%c", x, c);
    }
    fclose(matrix_r);
}

Here is the output:

0.000000,876.671546,448.879717,1349.827396
876.671546,0.000000,1319.195209,964.193445
448.879717,1319.195209,0.000000,1741.628261
1349.827396,964.193445,1741.628261,0.000000

Answer 2

1. You do not check if fopen was successful. Calls to feof or fscanf if the file pointer is NULL invoke Undefined Behaviour

2. while(feof(matrix_r)){

while(.feof(...))) is always wrong ( Why is “while(?feof(file) )” always wrong? ), but your one has no logical sense at all (as you want to scanf if it is the end of the file).