c线程并发和内存混合

Question

我有一个 C 语言的多线程程序。只有 1 个线程函数 (thread_read_file)，它将从结构中将文件内容读入变量。该函数有一个自动变量thiscontent，其中应使用fread存储文件的内容。

主程序正在执行4个线程，定期读取4个不同的文件，并将内容更新到内存中。

无论我尝试什么，一段时间后我都会得到结果，然后会出现一些垃圾。我尝试在阅读后强制添加“\0”，但是不行，它无论如何都会显示垃圾。

这是代码:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <pthread.h>
#include <string.h>

#define WAIT_TIME 2

/* change_it:
 * 8: picture for texture 0 finished to load into ram, needs to be set to 2 afterwards
 * 7: picture for texture 0 loading
 * 6: picture for texture 1 finished to load into ram, needs to be set to 2 afterwards
 * 5: picture for texture 1 loading
 * 4: pre stage 1, initialize time variable for time constant blending
 * 3: initial -> allows initial loading of textures
 * 2: texture has finished changed, dont change it anymore
 * 1: initiate texture change
 * 0: don't change any texture */
int change_it = 0;
signed int offset = -1;
int check_id = WAIT_TIME*100+1;

typedef struct {
    pthread_mutex_t read_mutex; // synchronize access to reading flag
    int reading;                                // 0 = not reading , 1 = reading , 2 = finished reading
    char *content;                          // content of file
    char *file;                              // file to read
} struct_file_content;

struct_file_content struct_data_file[4];

char *file_content[4];

// temperature in abu dhabi
#define file1 "/home/tias/repository/data/temp_ad.txt"

// temperature in irsch
#define file2 "/home/tias/repository/data/temp_ir.txt"

// change EUR / AED
#define file3 "/home/tias/repository/data/change.txt"

// monitoring data used to color cubes
#define file4 "/home/tias/repository/data/summary.txt"

void change_or_not(signed int* offset, int* check_id, int* change_it) {

    if ( *offset == -1 ) {
        *offset = (signed int) ( time(NULL) ) % WAIT_TIME;
    }

    int delta       = (int) ( ( ( (long) time(NULL) ) - *offset ) % WAIT_TIME );
    int delta_id = (int) ( (long) time(NULL) % (WAIT_TIME * 100) );

    if ( *check_id > WAIT_TIME*100 ) *check_id  = delta_id;

    if ( delta == 0 && *check_id != delta_id && *change_it == 0 ) {
        //*change_it = 4;
        *change_it = 1;
        *check_id   = delta_id;
    }

    if ( delta == 0 && *check_id != delta_id && *change_it == 2 ) *change_it = 0;

}

void pretend_texture_blending ( int* change_it ) {
    static old_change_it = 0;
    static int ori = 0;
    if ( old_change_it == 0 && *change_it == 1 ) {
        ori = time(NULL);
    }
    if ( time(NULL) > ori && *change_it == 1 ) {
        ori = time(NULL);
        *change_it = 2;
    }
    old_change_it = *change_it;
}

void *thread_read_file(void *data) {

    struct_file_content *thisdata = data;
    int mutex_res;
    long length;
    char *thiscontent = NULL;

    // lock mutex
    mutex_res = pthread_mutex_lock(&thisdata->read_mutex);

    if (mutex_res != 0) {
        fprintf(stderr, "thread_read_file() failed to acquire mutex: %s\n", strerror(mutex_res));
        pthread_exit(NULL);
    } else {
        fprintf(stdout, "thread_read_file: mutex locked for file: %s\n",thisdata->file);
    }
    // end locking procedure

    // let's open the file now
    FILE *f = fopen(thisdata->file, "r");

    // let's get the size of the file and bounce if it is too large
    // if size is acceptable, read the file into memory
    if (f) {
        fseek(f, 0, SEEK_END);
        length = ftell(f);
        fseek (f, 0, SEEK_SET);

        if (length > 39) {
            fprintf(stderr, "file %s is too big\n", thisdata->file);
            pthread_exit(NULL);
        }

        thiscontent = (char*)malloc((length+1)*sizeof(char));

        if (thiscontent) {
            fread(thiscontent, 1, length, f);
        } else {
            perror("malloc(3) error");
            pthread_exit(NULL);
        }

        fclose(f);

        // puzzling result is here :
        // first run with the 4 threads always work fine
        // next runs will randomly display mixed values / other memory locations

        fprintf(stdout,"thiscontent: %s length was %d for file %s\n",thiscontent,length,thisdata->file);

    } else {
        fprintf(stderr, "cannot open file %s\n", thisdata->file);
        pthread_exit(NULL);
    }
    // file read and fd closed

    // free old allocation of this content
    free(thisdata->content);
    thisdata->reading = 2;
    // assign new result to newly freed char*
    thisdata->content = thiscontent;

    // unlock mutex as everything has been updated

    mutex_res = pthread_mutex_unlock(&thisdata->read_mutex);

    if (mutex_res != 0) {
        fprintf(stderr, "thread_read_file() failed to release mutex: %s\n", strerror(mutex_res));
        pthread_exit(NULL);
    } else {
        fprintf(stdout,"thread_read_file: mutex unlocked for file: %s\n", thisdata->file);
    }
    // end unlock mutex procedure

    return NULL;
}

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

    static int init = 1;

    int p,q,r,curtime;
    float X,Y;
    static int data_update_done[4] = { 0,0,0,0 };

    static pthread_t thread_file[4];
    static int thread_res[4] = { 0,0,0,0 };

    static char *files[4];
    files[0] = (char*)malloc(sizeof(char)*strlen(file1));
    files[1] = (char*)malloc(sizeof(char)*strlen(file2));
    files[2] = (char*)malloc(sizeof(char)*strlen(file3));
    files[3] = (char*)malloc(sizeof(char)*strlen(file4));

    int mutex_err[4];

    // init = 1
    // => launches the 4 threads for reading the 4 files
    // 
    if ( init == 1 ) {

        strcpy(files[0],file1);
        strcpy(files[1],file2);
        strcpy(files[2],file3);
        strcpy(files[3],file4);

        for ( p = 0 ; p < 4 ; p++ ) {

            mutex_err[p] = pthread_mutex_init(&struct_data_file[p].read_mutex, NULL);

            if (mutex_err[p] != 0 ) {
                fprintf(stderr, "pthread_mutex_init(3) error: %s\n", strerror(mutex_err[p]));
                exit(EXIT_FAILURE);
            }

            struct_data_file[p].reading = 1;
            struct_data_file[p].content = NULL;
            struct_data_file[p].file = files[p];

            thread_res[p] = pthread_create(&thread_file[p], NULL, thread_read_file, &struct_data_file[p]);

        }

    init = 0;

    }

    // main loop
    while (1) {

        change_or_not(&offset, &check_id, &change_it);
        usleep(50000);
        fprintf(stdout,".%u.",change_it);
        fflush(stdout);
        pretend_texture_blending(&change_it);

        for ( p = 0 ; p < 4 ; p++ ) { // updating data if ready, destroying mutex when done, etc...

            // lock mutex
            mutex_err[p] = pthread_mutex_lock(&struct_data_file[p].read_mutex);

            if ( mutex_err[p] != 0 ) {  
                fprintf(stderr, "pthread_mutex_lock(3) error: %s\n",strerror(mutex_err[p]));
                exit(EXIT_FAILURE);
            }
            // end lock mutex

            // data update routine
            // is data ready: check variable 'reading' from the struct_data_file struct
            // 2            -> yes : update + destroy mutex 
            // 1 or 0 -> no : unlock mutex

            if ( struct_data_file[p].reading == 2 ) { // data is ready
                if (file_content[p]) free(file_content[p]);
                file_content[p] = (char *)malloc(strlen(struct_data_file[p].content)*sizeof(char));
                strcpy(file_content[p],struct_data_file[p].content);
                struct_data_file[p].reading = 0;
                mutex_err[p] = pthread_mutex_unlock(&struct_data_file[p].read_mutex);
                if (mutex_err[p] != 0) {
                    fprintf(stderr, "Warning: Error destroying mutex: %s\n", strerror(mutex_err[p]));
                }
            } else {
                mutex_err[p] = pthread_mutex_unlock(&struct_data_file[p].read_mutex);
                if ( mutex_err[p] != 0 ) {
                    fprintf(stderr, "pthread_mutex_unlock(3) error: %s\n",strerror(mutex_err[p]));
                    exit(EXIT_FAILURE);
                }
            }
            // end checking if data is ready
        }

        if ( change_it == 1 ) {
            for ( p = 0 ; p < 4 ; p++ ) {
                if ( struct_data_file[p].reading == 0 ) {
                    if ( data_update_done[p] == 0 ) {
                        thread_res[p] = pthread_create(&thread_file[p], NULL, thread_read_file, &struct_data_file[p]);
                        data_update_done[p] = 1;
                    }
                }
            }
        }

        if ( change_it == 2 ) {
            for ( p = 0 ; p < 4 ; p++ ) {
                data_update_done[p] = 0;
            }
        }

    // end main loop
    }
// end main
}

输出为

thiscontent: abu dhabi:35.6 length was 14 for file /home/tias/repository/data/temp_ad.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/temp_ad.txt
thread_read_file: mutex locked for file: /home/tias/repository/data/summary.txt
thiscontent: AED/euro:4.09534 length was 16 for file /home/tias/repository/data/change.txt
thiscontent: 11111 length was 5 for file /home/tias/repository/data/summary.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/summary.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/change.txt
.1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..0..1.thread_read_file: mutex locked for file: /home/tias/repository/data/temp_ir.txt
thread_read_file: mutex locked for file: /home/tias/repository/data/temp_ad.txt
thread_read_file: mutex locked for file: /home/tias/repository/data/summary.txt
thread_read_file: mutex locked for file: /home/tias/repository/data/change.txt
thiscontent: AED/euro:4.09534 length was 16 for file /home/tias/repository/data/change.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/change.txt
thiscontent: abu dhabi:35.6 length was 14 for file /home/tias/repository/data/temp_ad.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/temp_ad.txt
thiscontent: irsch:14.6.09534 length was 10 for file /home/tias/repository/data/temp_ir.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/temp_ir.txt
thiscontent: 1111 length was 5 for file /home/tias/repository/data/summary.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/summary.txt
.1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..1..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..2..0..1.thread_read_file: mutex locked for file: /home/tias/repository/data/temp_ad.txt
thread_read_file: mutex locked for file: /home/tias/repository/data/temp_ir.txt
thread_read_file: mutex locked for file: /home/tias/repository/data/summary.txt  
thread_read_file: mutex locked for file: /home/tias/repository/data/change.txt
thiscontent: irsch:14.6.09534 length was 10 for file /home/tias/repository/data/temp_ir.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/temp_ir.txt
thiscontent: abu dhabi:35.6 length was 14 for file /home/tias/repository/data/temp_ad.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/temp_ad.txt
thiscontent: AED/euro:4.09534 length was 16 for file /home/tias/repository/data/change.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/change.txt
thiscontent: 1111 length was 5 for file /home/tias/repository/data/summary.txt
thread_read_file: mutex unlocked for file: /home/tias/repository/data/summary.txt

这 4 个文件的内容是恒定的。

前 4 个结果始终是正确的(4 个线程读取 4 个文件，据我所知并且我已经做了很多测试，第一组结果始终是正确的)。

但是，当显示变量 thiscontent 时(第一次运行后)，我会得到额外的输出甚至更少的输出(文件 summary.txt 包含 11111)

例如:我得到了这个内容:irsch:14.6.09534我应该得到这个内容:irsch:14.6

还有我应该一直得到:此内容:11111 长度为 5但有时我得到此内容:1111 长度为 5

我不明白为什么，也无法修复它。我对变量 thiscontent 是自动的这一事实感到困惑，因此应该在每次函数调用时进行初始化。但输出显示不同。

这可能是内存分配问题，但我目前不知道。

非常感谢任何帮助。

PS:你可能会发现一些无用的代码，这是因为主程序是我写的opengl屏保，有1000行长。

我发布的代码是此屏幕保护程序的摘录，重现了该问题，但没有大量无用的代码用于此故障排除。

谢谢

PS no2:您可以将此代码复制/粘贴到名为threads.c的文件中并使用以下命令进行编译:

gcc -o threads threads.c -lpthread -D_REENTRANT

Answer 1

您正在将文件内容打印为字符串，但尚未向其中添加 nul 终止符。因此，字符串不会终止 - 您看到的额外输出是内存中超出内容的垃圾。

你所在的地方:

        fread(thiscontent, 1, length, f);

您还需要添加 nul 终止符:

        thiscontent[length] = 0;