使用 fscanf 时 C 中的分段错误（核心转储）

Question

I'm working in C and gettting segmentation fault (core dumped). The code compiles fine. inputfile is the output of the top command, and I only want to return the rows by the root user.

#include <stdio.h>
#include <stdlib.h>
void main(){
FILE* file = fopen("C:/Users/danie/OneDrive/Desktop/inputfile.txt", "r");
if (file == NULL){
        printf("Error opening file");
}
char* user[10];
char* ptr[10];
char* ptr1[10];
char* ptr2[10];
char* ptr3[10];
char* ptr4[10];
char* ptr5[10];
char* ptr6[10];
char* ptr7[10];
char* ptr8[10];
char* ptr9[10];
char* ptr10[10];
while (fscanf(file, "%s %s %s %s %s %s %s %s %s %s %s %s", ptr, user, ptr1, ptr2,ptr3, ptr4, ptr5, ptr6, ptr7, ptr8, ptr9,ptr10)==1){
        if (strcmp(user,"root")==1)
printf("%s %s %s %s %s %s %s %s %s %s %s %s\n", ptr, user, ptr1, ptr2, ptr3, ptr4, ptr5, ptr6, ptr7, ptr8, ptr9, ptr10);

}
}

Edit: Thank you all for your help! I'm super new to C. Clearly I had a lot of misunderstandings. I no longer receive the segmentation fault, but my output file is a bunch of nonsense. https://imgur.com/a/XjdYZXY

Answer 1

A segmentation fault is a memory access error. Replace all the char* name_of_array[10]; into char name_of_array[10]; as arrays are pointers themselves. This char* name_of_array[10]; is an array of pointers.

Then I see another error: according to the documentation , fscanf returns the number of items successfully filled. So instead of if(fscanf(file, ...) == 1) write if(fscanf(file, ...) != 0) (or better if(fscanf(file, ...)) ).

A similar error appears in if(strcmp(user, "root") == 1) : it should be if(strcmp(user, "root") == 0) ( documentation ).

I hope this helps you.

---

EDIT: I managed to make it work, although it probably can be improved.

 #include <stdio.h> #include <string.h> int main() { char pid[10]; char user[10]; char pr[10]; char ni[10]; char virt[10]; char res[10]; char shr[10]; char s[10]; char cpu[10]; char mem[10]; char time[10]; char command[10]; FILE* file = fopen("C:/Users/danie/OneDrive/Desktop/inputfile.txt", "r"); if (file == NULL) { printf("Error opening file"); } char line[150]; while (fgets(line, 150, file) != NULL) { sscanf(line, "%s %s %s %s %s %s %s %s %s %s %s %s", pid, user, pr, ni, virt, res, shr, s, cpu, mem, time, command); if(strcmp(user, "root") == 0) { printf("%s %s %s %s %s %s %s %s %s %s %s %s", pid, user, pr, ni, virt, res, shr, s, cpu, mem, time, command); } } return 0; }

Answer 2

The *ptrs[10] need to be just ptr[10] else you are allocating an array of pointers rather than an array of characters.

Not sure about the Windows top program but the linux one puts out a blank line at times. I consider it a bad idea to have scanf() a file in a while() loop.

char buffer[256];
while (fgets( buffer, sizeof(buffer), file ) ) { 
        if ( ! strstr( buffer, " root " ) ) 
                continue;

Now you can inspect input before processing it. This root test is in addition to the one looking at "user" to filter out any odd lines from we know are not to be printed, including strangely formatted lines.

Same idea to avoid using sscanf() return values within an as I like to pop the field return count into a variable:

int slen = sscanf(buffer, "%s %s %s %s %s %s %s %s %s %s %s %s",
                ptr, user, ptr1, ptr2,ptr3, ptr4, ptr5, ptr6, ptr7, ptr8, ptr9,ptr10);
if ( slen >= 10 )

This defends against short lines without a , say a blank line.

If you never print anything it may be because your program is putting out a bunch of terminal control codes for colors, etc., before each line potentially messing up your fields. I had to had edit out of my data file to get the code to work.